SOME ELECTRON OPTICAL
DEVICES
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNI-FICUS Ir. H. J. DeWIJS HOOGLERAAR IN DE AFDELING DER MIJNBOUWKUNDE VOOR
EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP
VRIJDAG 21 DECEMBER 1962 DES NA MIDDAGS TE 4 UUR
DOOR
AUGUSTE BADlH EL-K AREH
ELECTROTECHNISCH INGENIEUR GEBOREN TE BAABDA, LIBANON."
) -t, CDit proefschrift is goedgekeurd door de promotor Prof. Dr. Ir.
J.
B. LePooleTo EDWARD G. RAMBERG fo, his g,eat mind and g,eat heart
A considerable portion
oE
the work described in this thesis was performed in theReA
Laboratories, Princeton, New Jersey,U.S.A.
For the technical and financial assistance received Erom these Laboratories, the author hereby expresses his thanks.CONTE.NTS
CHAPTER I. THE DEVELOPMENT OF THE HIGH SPEED OSCILLOSCOPE FROM THE EARL Y STAGES TILL THE PRESENT TIME
Introduction ... ... ... ... .... ... .... 1
Cold Cathode Oscillographs ...
3
Hot Cathode Types... 4
The Dufour Type... 6
Sealed OH High Vacuum High Speed Oscillograph ... 16
Recurrent Sealed OH High Speed Oscillographs ... 18
Cathode Ray Tubes with Post Deflection Acceleration ... 22
Oscillographs with High Voltage Deflector Plates ... 29
The Micro-Oscillograph... 32
Oscillographs with Twin Wire Deflector Systems ... 37
Traveling-Wave Tubes... 38
CHAPTER 11. HIGH SPEED OSCILLOSCOPE WITH ELEC-TRON OPTICAL MAGNIFICA nON USING FOUR-POLE LENSES Introduction ... ... 45
General Considerations ... ... ... .... ... .... 46
Performance with Rotational Symmetrical Systems ... 47
Performance with the New Type of Oscillograph... 48
Design of me New Oscillograph... 48
Choice of Pole Pieces... 50
Equations of Motion ... 52
Experimental Verification ... 53
Design of the Lenses... 57
Conclusions... 58
Analysis of the Spherical Aberration... 58
CHAPTER lIl. LOW DRIVE GUNS FOR KINESCOPES IntroductÏon ... 63
Single Mesh Guns ... 64
Amplifier Gun ... ... ... .... ... ... 66
CONTENTS (Continued)
CHAPTER IV. ENHANCED EMISSION BY SPACE CHARGE NEUTRALIZATION
Introduction ... ... ... 70
Space Charge
Neutralization...
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Experimental Set
Up.
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72Details About me
Compound..
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74Results
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74Conclusion
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14
CHAPTER V.AN ELECTRON BEAM MACHINE Introduction ... 75
General Considerations .. ... ... ... 78
Diffraction Limitation ... 79
Space Charge Limitation ... ... 79
Thermal
Limitations.
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87The Choice of Cathode ... ... 90
Field Emission ... 91
Thermionic Emitters.... ...... ... 94
Choice of Gun .... ...
96
Design of the Machine ....... 98
Description of the Machine ... ... ... 105
Power
Supply...
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110 Vacuum Installation ... 111Alignment.
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112 Illumination ...... 115 Deflection ... _. 115 · PulseSystem
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116Results ...
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123 SUMMARY ...... :... 127 SAMENV A rrlNG ... ... 128 REFERENCES ...... 130CHAPTER I
THE DEVELOPMENT OF THE HIGH SPEED
OSCILLOSCOPE FROM THE EARL Y STAGES TILL
THE PRESENT TIME
I ntroducti on
The study of many electrical problems involving transient phe-nomena is greatly eased if permanent records of the transients can be obtained for analysis.
The earliest instrurnents that were used for registecing an unknown vacying quantity were mechanical devices. These generally consisted of a moving paper on which a stylus or pen made permanent recordings of the variations or oscillations. In order to increase the sensitivity of the oscillographs, devices were later used in which a beam of light recorded oscillations on a strip of moving photosensitized film. How-ever, these types of oscillographs, in view of their inherent high inertia and hence frequency !imitation, were only app!ied for the study of rather slow phenomena. It was soon found out that a means for recording rather rapid variations far beyond the response of me-chanical oscillographs was badly needed.
For the measurement of transients, sphere gaps and klydono· graphs· were first used. However, the first instruments measuced only maximum values, and the second gave no ~ore than an approximate idea of the general charactecistÏcs and wave shapes of the surges. A solution to the problem was found in the cathode ray oscilloscope which was the only device capable of completely delineating the voltage-time relationships.
Essentially, the cathode ray tube is a visual indicator produced by means of an electron beam having negligible inertia, impinging on a phosphorescent screen. The beam, having an electcical charge and mass, is capable of def!ection at very great speeds by the action of electcic and or magnetic fields. The tube decives Ïts name from the fine beam of electrons originally called "cathode rays" since they are emitted in approximately straight lines from the cathode of the tube.
• For a description of the klydonograph, reference can be made to a paper by Keinath 1, References appear at the end.
The tracing back of the events that led to the development of the cathode ray tube is not a simple task.One of the first mentions of the cathode beam was made by Gassiot2 in 1859. However, neither Gassiot nor Plücker3 who published a paper in 1858 on the effect of magnets on the electric discharge, seems to have taken any notice of the negative discharge. The situation changed when in 1869 Hittorf4 published his series of papers on the conductivity of gases. His contdbution was mainly concerned with the negative discharge (now caUed cathode rays) and he noticed also the focusing effect of a magnetic field on a beam of the rays and the high potential gradient in the neighborhood of the cathode. Considering these achievements, it seems right to caU Hittorf the discoverer of cathode rays. Perrin5 in 1895 showed that the charge was negative. J. J. Thompson6 demon-strated in 1897-98 that the particles had a mass of 1/1850 th. of that of the H at om and that their charge was equal to that carded by the H ion in liquid electrolysis. The particles that form the beam of the oscilloscope are now known by the name "electrons" suggested by Johnstone Stoney.
The notabie work of the above mentioned, together with that of many others, paved the way for the application of cathode rays to e lectdc al engineering measurements.
The cathode ray oscilloscope is possibly the most versatile electron tube which has yet been devised. It owes this distinction to its inherent capability of faithfully recording electrical transients of extremely short duration. Since the beam may be detlected in two dimensions as a function of time and since the intensity of the beam and size of the spot may be controlled, a wide range of possible applications exist. The cathode ray osciUograph wiU serve admirably for the investigation of all electrical quantities such as voltages, currents and frequencies, their waveforms, distortions and phase relationships, dielectdc losse.s, magnetic measurements, atmospheric electricity and electrical discharges. With the existence of suitable transducers, that is devices which will convert a mechanical or any other non-electrical phenomenon ioto an equivalent electrical voltage or current, ie is also applied for medical purposes, nuclear physics, acoustics, electronics, strength of materiais, illumination measure-ments, etc. 7,8 Among these vast applications, we have not mentioned the television tubes, radar, electron microscopes of which the cathode ray tube forms an essential part.
There seems to be a considerable disagreement as regards the name of the instrument. It is called both "oscillograph" and "oscillo-scope" and both terms are frequendy used interchangeably by many
authors. Some argue that the suffix scope me ans to view whereas the suffix graph implies a device for making permanent recordings. AIso,
that oscilloscopes can be abbreviated to sc.ope without conflict with
other existing terms. A counter argument in favor of oscillograph is that the method of presentation of information on the screen is still in the form of graph whether or not oscillograms are recorded.
Since its birth, the cathode ray oscilloscope has maintained a position of unchallenged and ever increasing importance in the realm
of problems relating to high speed electrical phenomena. It would
take a considerable space here to give a complete account of all the important work that has been achieved contributing to the evolution
of the oscilloscope from Ïts early deve lopment to its various forms
which exist today.
A historical background of the principal steps is given in the following pages which the author hopes will be of some interest to many who are actually working with oscilloscopes and indeed any electron device. The material included here serves merely an academic purpose. For a detailed history of the oscillograph up to 1925 together with a complete bibliography readers are referred to a paper by MacGregor..,\forris and Mines 9•
Cold Cathode Oscillographs
The use of cathode rays for measuring purposes was first
sug-gested by Hess 10 in 1894. In his monumental paper entitled: "On The
Application of The Cathode Rays to The Study of Variabie Magnetic Fields" he desccibed a method of photographing the deflected cathode rays emanating from a discharge tube through a Lenard window. How-ever he nHow-ever did realize his idea in practice.
Braun 11 in 1897 built the first oscillograph of this kind and
was probably the first individual CO employ the cathode ray tube as an
indicating device. The earliest cathode ray oscillograph which he invented consisted of a sealed evacuated tube, a cold cathode, an anode removed to a side tube so that it is entirely clear of the path of the rays, a diaphragm with a small hole at its center and a fluo-rescent screen. Electromagnetic deflection of the beam was used. He was also the first to use a fluorescent mineral in an electron beam measuring instrument. The materÎal he used was willemite (anhydrous
crystalline zinc silicate, Zn2Si04 ) which in his tube showed a bright
green color under bombardment. The material was in the form of a fine powder laid upon a disc of mica which was (lIounted in the tube.
electrostatic deflection plates brought inside the tube, but he did not apply it as an oscillograph •.
In 1898, Ebert 13 used electrostatic deflection plates outside an oscillograph.
A year later, Zenneck 14 made some improvements by using two separate diaphragms in place of one, and used a metal plate with a hole in it as the anode, instead of the small electrode in a side tube. In 1903, Wehnelt 15 applied electrostatic deflection in his oscillograph with the deflection plates inside the vacuum tube.
In these early tubes, it was found experimentally that to produce
a satisfactory beam it was necessary to use a relatively high vacuum and a high accelerating voltage. This decreases the collision of the electrons with the ions present due to the gas, and in the meantime decreases the time spent by the electrons to pass through the tube. However, it was found that even with these precautions there was still much to be desired, not merely in the size of the spot indi-cator, but in its intensity and sharpness;
Wiechert and othersl6 • 17,18 in 1898, found that by applying a fairly strong magnetic field to the tube parallel to its axis, the spot could be concentrated into a smaller area.
Several improvements have been made to the Braun tube among which must be mentioned the work of Milham 19 in 1901, MacGregor-Moreis 20 in 1902, Ryan 21 in 1903, Rankin 2 2 in 1905 and Roschansky 23 in 1911.
All tubes produced until that time used cold cathodes which meant that high voltages had to be applied and that there were no possibilities of sealing off the tubes. Thus until the first decade of the 20th century, all experimenters using cathode ray tubes were re-quired to use extremely high voltages to obtain electrical discharges within the tubes, and to incorporate a vacuum installation, although only relatively slow phenomena were studied.
Hot Cathode Types
In 1905, Wehnelt24 introduced the "hot cathode" in his version of oscillograph. The "hot cathode" term is applied whenever a dis-charge is stimulated by means of thermionic emission. The outstanding advantages gained by the use of this apparatus are the production of a very intense electron beam, combined with a much greater sensitivity to deflection due to the low beam velocity. Much lower voltages could be used and there was also the possibility of sealing off the tube.
oscillo-graph, several investigators brought many improvements to its original shape. Willows and Picton in 191025, Knipp and Welo in 191526 , Crooker in 191827 , Lübcke in 191828 , Keys in 192129 , HuIl in 192130 , and many others, all contributed to the improvement of the hot cathode oscillograph.
In his ver sion of the hot cathode type,
J
ohnson in 192231 suc-cessfully applied a neat method of gas focusing which he states was originaIly suggested by van der Bijl. Inert gas filled the tube under low pressure. Under the impact of the electrons in the beam, the gas molecules are ionized and form a positive core of ions which draw the electrons inward .. The point of focus is made to coincide with the screen by varying the electron density in the beam and hence the amount of ionization. This can be achieved by either altering the cathode temperature or the potentialof the cylinder surroundingthe cathode.The great advantage of this focusing method is the fact tha~ the act ion on the beam is continuous throughout its length, not interfering with arrangements for deflection of the beam. There is a definite time factor in the establishment of the positive ionization wÏthin the beam, and this leads to the disadvantagc that if the beam is def!ected too rapidly through the gas space, the ionization fails to build up to the proper value. This fixes a frequency limit in the tube. Despite the drawback that the ion bombardment on the cathode shortens the life of the tube considerably the early gas focused tube reached practical application as a measuring device for the analysis of electrical wave-forms and was made in small quantities for this purpose.
In 1931, von Ardenne 3 2 made the important improvement of sur-rounding the cathode with a negatively charged cylinder which served to pre-concentrate the electron be am before it passed through the hole in the anode, or accelerating electrode. This greatly improved the focusing properties and the efficiency of the beam, since before that time the simple apertures inserted in the path of the beam did no more than cut off a part of the electrons traveling up the tube. With the introduction of the Wehnelt cylinder - as it is now called - these waste electrons were guided into the main stream and contributed to the intensity of the spot. The focusing effect of the gas was con-trolled by the cathode emission and by the potential applied to the negative cylinder.
Even when other focusing means were proposed, electricalor magnetic, the sealed oH, hot cathode, low voltage osciIlograph could not record phenomena which occurred in a microsecond or less since the oscillogram which would be visible in conditions of dim external
lighting was only of the repetlt1ve form. Using a cold cathode tube with 30 kv. Zenneck33 photographed fluorescent screen traces of electrical events of one millisecond duration. He applied the moving plate principle. The photographic plate was carried in a dark slide which was traversed horizontally across the back of the camera by a wire wound up by a drum. The drum was driven through self·disengaging gears byan electric motor. By this means the drum,once started, made one revolution and then stopped. The drum shaft also carried a commu· tator which served to set in action the transient phenomenon to be investigated. The speed of the photographic plate was 1 m /sec.
Because of its recording speed limitations, the modified Braun tube was found inadequate to satisfy a growing demand for higher and higher recording speeds.
The Dufour Type
The kinedc energy, and consequ~ntly the square of the velocity of the cathode rays, is directly proportional to me accelerating po· tenda!. The depth of penetration of the rays ioto a photographic layer is proportional to the fourth power of the velocity. This led to a radically new avenue of attack since it is obvious that one of the most powerful and direct methods of increasing photographic sensitivity is the use of high accelerating potentials, letting the electrons impinge straight on the emulsion itself. This was appreciated by Dufour34 who, using the above mentioned technique, produced the first high speed oscillograph. In a way, it seems appropriate to caU Dufour Uthe father of high speed oscillography." He obtained a recording speed of 4000 km per seconde The early Braun tube had a recording speed of something like 1 km per seconde
8ased on his earlier experiments, Dufour constructed me first practical oscillograph of the. so·called internal photography type, a
description of which he published in 1920.
A diagrammatic view of the Dufour oscillograph is shown in Fig.!. In his type of electron gun, there is no radical departure from the practice of his predecessors. As with the Braun tube, the electron beam is produced by means of a cold cathode, a voltage of 60 kv being employed. It consists of a concave aluminum di sc e which is the cathode, and a diaphragm
I
which is connected to the body of the tube and which forms the anode. This limits the beam to a fine pencil of rays. Inv,
suitable electrostatic deflecting pl!ltes or electromagnets are fixed outside the tube. The beam then enters the camera house a and strikes the cylindrical photographic film g or the glass plates h..C
d , {
Fig.
1.
Dufour's OscillographThe cylindrical drum supporting the film can be rotated by means of an extern al magnetic device
p
controlling the spindie n through the glass cap o. A time scale can be optically recorded on the edge of the film by way of the window s and totally reflecting prism t. The side b of the camera house is removable.In
order to get rid of the water vapour which is emitted in the vacuum from the gelatine emul-sion of the film or plate, a vessel k containing P20s is fitted insidethe camera.
For low frequencies, the rotating drum which is 15 cm wide and has 48.5 cm circumference is used. At higher frequencies, since the necessary speeds of deflection are greater than any mechanical speed conceivable with the rotating drum the latter is removed and is replaced by a pile of photographic plates h. The cathode rays are in this case deflected at a known speed across the plate by means of
an electromagnet in which the current is increasing at a known rate. The apparatus was constantly connected to vacuum pumps to obtain and hold the necessary operating vacuum af ter insertion of the photo-graphic film, and to adjust the pressure in order to obtain a fine and intense trace.
Using this oscillograph, Dufour obtained remarkable records of transients of high frequency. Figure 2.1 shows the make and break of direct current in inductive circuit with a condenser across the break. Figure 2.2 shows damped oscillations of 150 x 106
cycles per second.
(ll (2)
Fig. 2. Dufour's Results
Dufour states that the deflection sensitivity of his oscillograph is 100 volts per cm for electrostatic deflection and 10 ampere-turns per cm for electromagnetic deflection. However, as he does not give any dimensions of his deflecting plates or magnet coils, the transit time distortion can only be estimated in his oscillograph.
Numerous investigators were stimulated to follow Dufour' s lead soon af ter his successful production of a practical high speed oscillo-graph. Among them figure Rogowski, Tamm and Fleger35, Gabor36,
Norrinder37 , Wood38 and KnolI, Matthias and Knoblauch39 who made in turn several improvements to the Dufour type oscillograph.
Focusing improvements were made by the addition of a second focusing cai!. Since cold cathodes we re used, a pressure of about 10-2 mm Hg had to be maintained in the discharge tube to produce
the necessary electrons. However, at this pressure, the beam scatters due to the colli sion of the electrons with the ions of the gas present in the tube.It was thus found necessary to divide the vacuum chamber into two sections having different degrees of vacuum. The electron
gun consisted of a cathode insulated on a glass stem and an anode in the form of a metal block with a capilliary connected to the main part of the tube where a higher vacuum is maintained. A leak was intro-duced in the cathode side, and the vacuum pumps were connected to the main body of the tube. It was thus possible to maintain a relatively high vacuum in the main part of the tube, and yet keep a pressure of 10-2 mm Hg which is necessary for the discharge in the cathode region. The diffusion pump was by·passed to allow the body of the oscÜlograph to be opened to the atmosphere and roughlyevacuated again without the necessity for cooling off the diffusion pump. Some oils are now available which may be exposed when hot to the atmos-phere without being damaged.
When the cathode is kept energized for immediate recording, some means must be adopted to prevent the beam from lingering on the photogr~phic plate and blurring it. In cold cathode types, since no modulator can be applied, a beam trap consisting of two additional deflector plates is used. The beam is blanked by applying a voltage to the plates, and is switched on by removing the blanking voltage. The technique of beam trapping was due originally to Gabor40 • Figure 3 shows the basic circuit of his arrangement. On the appli-cation of a tripping pulse to the middle sphere, the initiating spark gap conducts and the voltage between the beam trap plates rapidly faUs allowing the beam to pass freely. An instant later, the time base plates become charged.
Heofing
conrlenser
Bl'om-Iropping
lniliofing
eleclrotles
gup
Time -circuit
conrlenser
T,;ne-sweep
eleclrorles
Fig. 3. Gabor's Beam Trap
In 1928, Berger41 introduced a beam trap consisting of four deflector plates connected as shown in Fig. 4. This will prevent
secondaries from reaching the screen. On the other hand, with this arrangement and with pecfectly balanced plates, voltages insufficient to trap the beam wiU not deflect it.
Fig. 4. Berger's Beam Trap
Of particular interest is the beam trap used by Norinder 37 in making cathode ray oscillograms of actual lightning voltages. This was an ingenious improvement over previously known forms of cathode ray oscillographs by which operation is made entirely automatic on the arrival of the cransient. It has been widely used by many investi-gators, among whom Fortescue, Atherton and Cox"2 and Ackermann"3 can be mentioned. The intern al construction is shown dï'agrammatically in Fig. 5 •. When a record is to be made of an investigated disturbance, the necessary potential is applied between the anode and cathode to produce the beam, the useful porti on of which passes through the anode. With no voltage on the upper set of deflecting plates, the beam is intercepted by the target which consists of a narrow bar of con-ducting materÎal. Stray electrons are prevented from reaching and fogging the film by the lower diaphragm which has a hole of about 6 mm diameter in its centre. When the transient arrives, the beam is deflected past the target. The system of plates between target and diaphragm is connected to the top plates and is dimensioned 50 that
the beam on ce deflected is again bent back by the reversed potential. By adjusting the length and spacing of the two pairs of deflecting plates, the beam is caused to pass through theorifice for all applied voltages. lts deflection is proponional to the applied voltage.
O"yIoNIIJ'"
r±-:::J-
r,;",iI,1".'"
Fig. 5. Norinder's Beam Trap
an oscillograph is produced by energizing the cold cathode for a very
short time. The beam is only momentarily flowing through the tube, and is gene rally made to precede the phenomenon co be studied by a
fracdon of a second. A beam trap under these conditions is obviously
not necessary. On the other hand, this permits a relatively high beam
current to be used which is a fundament al requirement of high speed
oscillographs, and chis without any excessive hearing of the discharge
tube. However, these advantages are more than offset by the fact that
errors arise due to the non-uniformity of the speed of the electrons.
An impulse voltage applied to the cathode will always have a rise and decay time. The ideal voltage would be rectangular. The authors
have shown these errors to be very small and of the order of 1 to 2%
of those with
D.e.
conrinuous excitation.lntemal photography types of hot cathode oscillographs were
made in modified foems for practical use in laboeatory and in field work by George45 and Ackermann43•
George used an electric immersion lens at the cathode and an electric final focusing lens, and thus one may regard his oscillograph
as the forerunner of the practical sealed oH type which began to emerge in subsequent years.
Improvcments were also made by loading the camera with roUed films in daylight. Many exposures could then be made before it became necessary to break the vacuum to reload. The trouble due to the gas given off by the film was overcome by storing the film in a vacuum desiccator at apressure of about 10-2
mm Hg with phosphorous pent-oxide as a drying agent, for a few hours before use.
In 1929, Hochhäusler46 applied a method of avoiding the opening of the tube by introducing the film into the vacuum eh amber through a mercury fi1led tube.
As a culmination in the quest for high recording speeds, Rogowski and his collaborators47 succeeded in 1930 in obtaining the enormous speed of about 60,000 km per second which is 1/5th the speed of light. According to them continuously evacuated oscillographs with souree voltages varying up to 95 kv wiU provide writing speeds up co 100,000 km/sec. Also the performance limitatÏon of the high voltage tube was likely to be imposed by the finite velocity of the electrons in the beam and not by the lack of photographic sensitivity.
At this stage, the writing intensity being ample, more con-sideration was given to the simplification of the apparatus which in many cases had become complicated. Increasing numbers of investi-gators studied the possibilities of obtaining high speed oscillograph records wÏth films outside the vacuum.
The solution was sought first by using a Lenard window48• This is a very thin vacuum tight membrane supported internaUy by a rein-forcing grid against the extern al atmospheric pressure, and is fixed in the position normally occupied bythe fluorescent screen. It is thick enough to maintain the vacuum within the tube, but thin enough to allow electrons to pass through. The camera can then be at atmos-pheric, pre$sure, and this overcomes the difficulty of braking the vacuum and then re-exhausting the tube whenever the photographic material has to be changed.
ThC'mpson49 has shown theoretically that the dep th of pene-tration
ot
electrons in matter is given byt
=
1.25 x 1011 (V~ - ViJ/a I-I where t is the depth in cm ,V
0 andV I
are the initial and finalelec-tron velocities expressed in volts, and a is a constant depending on the materia!.
Whiddington and others50,5 1. Using an aluminum foil 0.001 cm thick, and taking a for Aluminum = 732 x 1040
as given by Whiddington, the voltage for which the electrons will just pass, will be of about 25 kv. However, it is not enough that the e lectrons should just pass since they still have to penetrate the emulsion layer of the photographic film which is held in contact with the window. This necessitates the use of higher initial voltages which in a way offsets the advantage of the Lenard window. Another disadvantage is the fact that the passage through the foil tends to cause a scattering of the electrons in the beam.
Even the fact that a Lenard wind ow will overcome the diffi-culties of having co break the vacuum, the oscillograph has to be continuously pumped since the foils have fine pin holes in them and will not keep the tube vacuum tight.
Despite these above disadvantages, Knoll 52 constructed a tube of this kind and reported a writing speed of 5000 km per second with an accelerating voltage of 75 kv.
Due to the progress made in the attainment of sharply focused beams and exceedingly high fluorescent spot intensities, and because of the existence of commercial high speed photographic lenses, a return to simple extern al photography as us.ed in connection with Braun's original tube was reconsidered. The earliest attempts in this direction made by Rogowski and Fleger53 and later by Buss and Pecnick54 were successful. Following their experiments, DoddsS5 reporced in 1933 the recording, by extecnal camera, of speeds of 30,000 km per second at 95 kv cathode excitation with cold cathode tubes. Dodds thus claimed that with extecnal photography everything could be done [hat was formerly possible only with intecnal pho-tography. However, the rapid development of continuously pumped cold cathode tubes operating with high accelerator voltages and intecnal photography culminated in many practical constructions. Examples ofthese types have been described by Burch and Whelpton 56 in 1932, Miller an::! Robinson57 in 1934, in an E.R.A. report in 194158 and by Wade, Carpenter and MacCarthy59 to mention only a few.
The oscillograph described in the E.R.A. report has the same features as the instrument reporced by Burch and Whelpton but the discharge tube axis was inclined to that of the main body of tbe oscillograph. The reason for this was to avoid the considerable number of particles - the so-called "retrograde rays" - originating from the discharge tube in addition to the main electron stream. Park60 has recently described a methad in which the Ïntensity of the electron beam current is momentarily increased up to 50 times its
normal value in a high voltage cold cathode discharge tube. By synchronizing the intensification with the observing period, he ob-tained writing speeds equal to about 3/4 of the speed of light using a 50 kv cathc;>de ray oscillograph.
The intensification is caused by superposing a steeply rising voltage pulse of ab out 800 volts on the normal steady voltage of 50 kvacross the electrodes of the discharge tube serving as the electron beam source. The intensification lasts for about 2 micro-seconds. Park also attempts to explain the intensification mechanism.
Modern cold cathode internal photography types of high speed oscillographs have been described by Induni61 in 1953. A cross section of a two beam oscillograph described by him and made by Trûb-Täuber is shown in Fig. 6. It is an all metal tube, pumped with
Fig. 6. A Modern Trüb Täuber Oscilloscope
a Holweck rotary molecular pump. A number of beams originate from a common cold cathode in which, Induni says, practically no damage occurs, and which can produce a beam sharpness and brightness that
does not change with time. Spark gaps are used for the time base. The writing speed of this oscillograph is about 10,000 km per second, and the spot diameter is about 0.03 mmo The high voltage is applied in the discharge chamber 16 through the bushing shown in 18. Af ter the anode, adjusted with 17, the beam passes through the aper-tures 15,the beam trap 11 and 14 aodisfocused bythe coils 13 before being deflected in the horizoncal and vertical direction by 8 and 9 and finally reaches the photographic chamber 1. A film cassette 2 of dimension 14 x 14 cm is inserted through 4, and is normally covered by a lid coated on its upper surface with a fluorescent material, and which is operated by means of 5. The screen cao be observed through 6. A door
3
is also provided. The vacuum pumps are connected to7.
The beam can be adjusted by means of 12.10 are the deflectingplates bushings, 19 the time relay, aod 20 the needie for admitting air to maintain the discharge.The chief advantage of the continuously pumped tubes is due to the fact that records are produced by the direct action of the beam on the photographic emulsion giving greater photographic sensitivity, high writing speed and excellent resolution. Moreover, it eliminates the distortion introduced by photcgraphing at short distances with large aperture lenses. Since the oscillographs can be constructed almost entire ly of metal and built up in demountable sections, they c an be made very flexible as far as intern al configuration is concerned. The size, shape and separation of the deflecting plates can be changed without difficulty and the separation cao, if necessary be adjusted while the oscillograph is in operation.
On the other hand, the initial cost is rather high. lts bulk produced by the large size of the tube, together with associated dif-fusion and backing pumps and the high tension supplies increases the technical difficulties encountered in maintaining and operating it. These disadvantages make such ao oscillograph unsuitable for many applications.
Transit time distortion and the lumped impedances presented by the plates limit the frequency response of such oscillographs.
Af ter Dodds had demonstrated that very high recording speeds can be obtained by photographing the fluorescent screen from the outside, more work was concentrated on the sealing oEf of the tube and the resulting elimination of vacuum pumps. Rogowski aod Szeghö6 2,6 3 deve loped a permanently sealed glass oscillograph tube with cold cathode and zinc sulphide fluorescent screen with an accelerating voltage of 40 to 50 kv. In order to maintain the discharge, aod for protection of the cathode, the tube was filled with Hydrogen.
The loss of Hydrogen through clean up action was replenished by diffusing it into the tube. A loosely mounted spherical cathode was used which can be turned co expose a fresh surface by lightly tapping it, since cold cathodes are subject to crater formation af ter some hours of use. A complete oscillograph embodying a tube of this type was described by Parker-Smith, Szeghö and Bradshaw in 193564•
In 1934, Gondet and Beaudouin6S described a hot cathode oscillograph with extern al photography in which they obtained a writing speed of 30 to 40 km per second approximately. According to them the use of cold cathodes is ideal if the oscillograph is to work intermittently, it is difficult however to apply it for permanent use for two reasons:
(a) The production of a beam suitable for recording re quires the maintenance of a weIl determined degree of vacuum which is difficult in a metallic tube.
(b) Even if the stability of the vacuum is guaranteed (e.g. auto· matic valve governed by the beam current) the surface of the cathode changes af ter a fe w hours of operation and renders the spot unswtable for photographic recordings.
However, even wieh external recording they preferred to use a continuously pumped tube in order to be able to replace the damaged filament and to alter the position and shape of the deflector plates.
Sealed OH High Vacuum High Speed Oscillograph
A high vacuum sealed glass cathode ray tube, if able to carry out satisfactorHy the work that was once considered to be in the province of the continuously pumped tube, would grel;ltly reduce the co st and increase the portability of that instrument.
Samson66 in 1918 was the first to make a sealed olf cathode ray oscillograph with hot cathode. However, the unsatisfactory design of the electron gun made ie unsuieable for any oscillographic work.
Low voltage high vacuum sealed off cathoçle ray tubes were made in small quantities in which relatively low writing speeds could be obtained. A step towards narrowing this gap was made in 1937 when Ku!!hni and Ram067 introduced a sealed oEf tube running at accelerating voltages up to 15 kv and with recorded writing speeds as high as 250 km /sec.
A detaHed' descripcion of a commercially available hot cathode sealed oEf oscillograph was given by McGillewie68 in 1938.
He used an electrostatically focused tube, with an accelerating voltage of 5000 volts. The method of operation of his tube can best
be explained with reference to Fig. 7. The gaps between the 3 elec-trode spark-gap S are adjusted to the minimum distances sufficient to
CO=>---~--~-~~ H.T+
"1
S?
Ct Rt-l800VO>---~~---lI~I.-R-2
It---=..G
-I K1
nl
~
_____ '-1t:_C-_1-_-_-_ .._-:-~-
..."""'~
p HT-Fig. 7. McGillewie's Circuitprevent sparkover. The outer electrodes are charged to about 1800 volts positive and negative with respect to earth, the centre electrode is connected to earth via a resistance of 1 megohm. By applying a pulse of about 500 volts of either polarity on the centre electrode, breakdown takes place and the negatively charged electrode rises suddenly to earth potential and a positive pulse is applied thcough C" Rl, R2 and C2 to the cathode, and so via the decouplirig condenser
Cl to earth. Rl and R2 are adjusted in such a way that the voltage generated between grid and cathode is sufficient to cancel the nega-tive cut oH voltage supplied by the potentiometer P, and the beam is brought to fuU brilliance in a fraction of microsecond. In the meantime the spark gap initiates the time-sweep and the time marking device. The writing speed obtained was of the order of 20 km per second and the deflection sensitivity at an accelerating potentialof 5000 volts was 0.15 mm per volt.
Considerable improvements were subsequendy made by Katz and Westendorf69 in
1939
and later in1941
who made a sealed ofttube with electric final focusing speciaUy constructed for high speed single stroke transient osciUograph work. A writing speed of 50,000 km per second with 20 kv acceleration voltage was obtained. The sensi-tivity was 0.05 mm /volt.
Goldstein and Bales70 described a cathode ray tube for periodic recording of single fast traces at rates up co 4000 per second. Speeds as high as 700 km /sec could be recorded without sacrifice in de-flection sensitivity and with commerciaUy available tubes and films. The film moves continuously at high speeds in the camera. By initiating
the transients synchronously with the film speed, it was possible to project the film cinematographically.
Recurrent Sealed Off High Speed Oscillographs
Whenever the steady state of a circuit is changed, as for ex-ample, when a switch is operated, a traveling wave or surge of voltage and current is produced. Sometimes, a lightning stroke collapses a cloud field over a power line or even strikes the line itself and intro-duces surges in the circuit. The surges produced are frequently of such a magnitude as to damage unprotected apparatus or flash over insulation and thereby cause service interruption. In order to guard
against those undesirable effects, a surge is generated artificially
and is applied to the circuit to be tested. A study of the voltage produced by the surge in some parts of that circuit enables the design
of equipment that withstands such stresses. Cathode ray tubes are_
invariably used for such measurements. If no puncture of insulation
or flashover occurs due to the passage of a surge, the continual repe-tition of the applied surge will cause a reperepe-tition of the trace on the screen of the oscillograph. The recording of a single trace on the oscillograph is obviously unnecessary in this case. By lening the trace superimpose several times, the light output from a low voltage sealed oH oscillograph is enough for visual observation or for photo-graphic purposes. Moreover, recurrent test pulses are very convenient since the oscillograph trace is more easily examined than with single strokes of the time base.
Rohats71 seems to be one of the first to have appreciated the
above considerations, and to produce a recurrent surge oscillograph. The schematic circuit of his oscillograph is shown in Fig. 8. The operation of his oscillograph can be explained as follows:
The grid of the thyratron tube Tl is negatively biased so that it
ionizes at the ere st of the positive half cycle from Cs' C4 begins to
charge through R4 th us giving a deflection on the time axis of the
cathode ray tube. At the same time a positive impulse is transmitted
through Cs to the grid of Ts which fires. At this instant, Cl being
previously charged in the negative half cycle through the rectifier
diode T2 , begins to discharge through L Rl R2 Rs C2 from which the
impulse applied to the apparatus under test is taken and displayed on the oscillograph screen. The time base speed may be regulated by
changing R4'
Rohats shows the use of his oscillograph by giving a series of about 36 oscillograms illustrating the passage of a traveling wave
along a 20~section artificial line. He also desccibes various uses of his instrument such as in machine windings and transformers.
]
Fig. 8. Rohat's Circuit
Wilkinson72 used similar techniques in 1936, and in a later paper in 193873 he gives a more detailed description of his apparatus. He used higher sweep recurrence rate which enabled him to photo-graph fa st transients in a shorter time than was possible in the earlier work of Rohats. The time base and impulse generator which are syn-chronized can be operated at a recurrence fre quency of 1000 c /second with a sweep speed of 30 mm per microsecond and a minimum time sweep duration of ab out 5 microseconds. Electromagnetic scanning was used. His oscillograph included a circuit for suppressing the beam in the intervals between sweeps.
In the same year, Scoles74 desccibed a recurrent surge oscillo-graph which differs in certain respects from those already described and with which impulses varying from 1/50 to 1/500 microsecond and time base durations varying between 50 to 2000 microseconds are obtained.
In 1934, White 7 5 gave a description of an oscillograph similar to th at built by McGillewie and which can be used to record a single transient or recurrent transients from an external source. The recur-rence pul se frequency was 100 cydes per second and the highe st speed gives a complete traverse of the screen in less than 1
micro-second. Thyratrons were used although the circuits differ from those due to Wilkinson and Scoles.
A recurrent oscillograph was reporced recently by White and Nethercot 76 of the Electrical Research Association and is probably the most highly developed yet described in the literature. It is sug-gested to give in what follows a description of their instrument.
Figure 9 shows the sweepcircuit oftheir recurrent surge oscHlo-graph. This uses hard valves in place of mercury WIed thyratrons.
Trig~
Fig. 9. Sweep Circuit Used by White and Nethercot
V lt an argon fiUed thyratron, is arranged to give negative pulses to the grid of Vz , normally at a recurrence frequency of -lOO cydes per second. The time constant C, RI is adjusted so that V2 remains blocked byeach of these pulses for the duration of the longest sweep time required. While Vz is conducting, the anode current of Vz which is supplied from a souree of 600 volts through a diode V, produces voltages across resistances Rz and R, which are applied to the screen and grid of V4 and block it. When Vz is blocked by the pulses, the screen voltage of V. is maintained by the charge of capacitor C4 • but the voltage across R,. which was normally blocking V •• collapses owing to the relatively small capacitance associated with R,. The ficing of V4 causes Cllwhich was previously charged to approximately 2 kv. to discharge into Cz• at a rate set by the value of R4 so that
variation of the latter, or of Cl and CH sets the time base sweep. The
diode V, prevencs current flow back into the 600 volt supply. When
V2 again becomes conducting, capacitor C2 is discharged via resistor R, and V2J and che restoracion of anode current in Va causes V. to be
blocked again, whilst Cl becomes recharged through resistance R ••
The voltage changes across Cl and C2 are equalized by balancing the
stray capacitances across chem. Capacitor Cs and resistor R6 form a differentiacing circuit such thac che linear voltage swing across C2 is
accompanied by a con sCant positive voltage across R6 which is trans-mitted through a blocking capacitor C6 to the modulating grid of the
cathode ray tube. In order co trip the impulse generator, positive voltage pulses developed across R, are used. A wave front of less than 0.1 microsecond was obtained, as compared with 0.5 microseconds which was the value obtained with mercury filled thyratrons. The cathode ray cube voltage varied between 2 to 6 kv.
An ingenious system for displaying very fast recurrent transients on a conventional cathode ray oscillograph has been described by McQueen77 • The mechod consists of measuring the instantaneous amplitude of a selecced point in the waveform and displaying it on the screen. E ach coordinace is made co persist for a considerable porcion of the time interval between recurrence By causing the selecced point co occur at a slightly different instant during each recurrence, the graph is finally craced out.
The waveform co be displayed is applied to the grid of a valve which is normally cut off. At the instant selected for waveform ampli-tude measurement a very narrow negacive pulse about one millimicro-second wide is applied to the cathode of the valve causing a short pulse ofcurrent to flow in the valve. A pulse output is obtained whose amplitude is a function of the instantaneous amplitude of the input waveform. This pulse is finally converted into a suitable Y deflettÏon. The outstanding characteristic of this method is the fact that the waveform which is being observed is not used to deflect an elec-tron beam in a cathode ray tube, but is instead applied to a circuit capable of measuring its instantaneous magnitude at a predetermined point. The advantages claimed with this method are the following: (a) The high speed waveform is confined to the grid circuit of the probe input valve.
(b) The probe unit may conveniently be placed very close to the circuit pcoducing the waveform.
(c) The input impedance of the probe unit is high.
(d) Very low amplitude waveforms may be viewed. Transient amplitudes of less than 0.1 volt may be measured, and not more than
0.5 volt peak-to-peak signal is required for fuIl deflection on a 12 cm cathode ray tube screen.
(e) Due to the very narrow gating pulse, waveforms having com-ponent frequencies up to 300 Mc/s can be faithfully reproduced.
(f) If two probe units are used, instantaneous values of con-current waveforms may be measured.
(g) The oscillograph time scale can be much longer than the transient, by a known amount.
(h) There is no necessity to produce a steep edged brightening waveform.
The limitation in frequency response is due to two major draw-backs. The first one is caused by the low pass filter formed by the probe input lead and the grid of the valve which has been found to have a cut off frequency of 380 Mcl s. The second one is due to the width of the gating pulse. As the gating pulse is of finite width, it is the mean value of the amplitude of the waveform during the gating pulse which IS measured. A simple calculation shows that with a gating pulse of width Tand a waveform of V sin wi, the resulting waveform is
sin 17fT
17fT x input waveform 1-2
For T
=
1 miIlimicrosecond andf
=
300 Mc/s the ratio of output to input is 0.86, or an attenuation of 1.3 db. The oscillograph has been used for the study of s~ort pulses in radar re ce ivers and in the moni-toring of T. R. cell voltage spikes.Cathode Ray Tubes with Post Deflection Acceleration
The light output of a phosphor increases approximately as the square of the volt~ge through which the electrons striking the screen have been accelerated. A high brilliance of the screen af ter the impact of the electrons is one of the principal factors underlying the high writing speed performance of transient oscillograph tubes. It is thus clear that the use of as high a total voltage as possible is advantageous. However, an increase of the accelerating voltage means a decrease in deflection sensitivity. In order to secure the advantages of higher brightness without a corresponding sacrifice in deflection sensitivity, and thus combining the otherwise conflicting conditions, a method has been used in which the beam is deflected at low velocity and is then accelerated af ter deflection in order t.o increase the writing speed.
Post deflection acceleration or intensification, as it is some-times called, was first suggested by Schelle/8 in 1920. He used an accelerating grid in the immediate neighborhood of the screen, thus giving the electrons a final increase in energy before they hit the phosphor.
This method was applied in 1928 by Sommerfeld 79 in a continu-ously pumped tube. However, it was not possible under norm al circum-stances, to totally avoid an image of the accelerating grid on the screen.
In 1938, Bigalke80 developed a sealed-off acceleration tube in which the final increase in velocity of the electrons is given by a group of three to four conducting rings in which increasingly higher potentials are applied. These rings are fixed in the immediate neigh-borhood of the screen.
Another system put forward by Schwartz81 consisted of a high resistance which was wound jn a spiral form around the tube filling the space between the deflection plates and the fluorescent screen. In thts way, by applying a potential across the ends of the resistance, he obtained a uniform increase in potential required for the final acceleration.
Schwartz also suggested the use of either a planar double layer or a spherical double layer brought in the im~ediate neighborhood of the deflection plates. However, he overestimated the possibilities of such a scheme by neglecting the lens action of the double layer, or surface of potential discontinuity.
Besides the advantage of increased sensitivity and higher writing speeds, post deflection acceleration simplifies insulation problems in the glass finch and the base, in view of the fact that the final high tension side is applied to a contact on the side of the tube. However, certain disadvantages arise which limit the use of this type of tube. Rogowski and Thiele82 showed that the deflection sensitivity of a post acceleration tube not only diminishes but can become zero or negative due to the lens act ion of the final accelerating electrodes. In Fig. Wa the influence of a weak post acceleration lens is shown. Here the deflection of the electron beam is only slightly decreased. In Fig. lOb, the influence is so strong that the def1ection has entirely disappeared. In Fig. Wc, the strength of the post deflection voltage makes the deflection even negative. The absolute value of the voltage is not the determining factor of the strength of the lens.It is the ratio of the fin al accelerating voltage to the anode voltage or the ratio of the voltages of the first and last coating in the case where rings are used, which determines the strength of the lens action and the dis-tortion of the deflection.
s
-
--(a's
C b ,
s
(c,Fig. 10. Reduction in Deflection Sensitivity Due to Post Acceleration
An accelerating field which does not deflect the electron beam is one with equipotential surfaces perpendicular to the direction of the beam at every point. The cross sections of these equipotential surfaces would be circles having their centre in the middle of the horizontal deflection plates for the horizontal plaoe, and the middle of the vertical plates for the vercical plane. Ic has not yet been pos si-bIe to produce such an ideal field. The reduction in deflection sensi-tivity with increasing post deflection acceleration is in a way compen-sated by the fact that the spot becomes sharper aod optical enlargement becomes possible.
The distortion cao become serious when one attempts to utilize the fuIl screen diameter. By careful design, it is possible to increase the deftection sensitivity by a factor of about four without serious distortion of the trace.
electrode of 5000 volts was used. He obtained writing speeds of 24 km
I
second with a beam current of 20 micro amps, thus an increase bya factor of 25 over the ordinary type of oscillograph. The conducting coating of his tube was split into two coatings, one connected to the anode and the other to a separate lead on the outside to which any desired voltage Erom 1 to 5 kv can be applied. The coatings consisted mainly of a deposit of graphite on the inside of the glass. Platinum was added on the edge of the first coating in order to in-crease the flashover voltage. Several platinum rings were also added between the two coatings. These were leEt floating and free to charge to any potential. In this way the desired potential difference was applied without any risk of flashover.DeGier remedied the distortion due to astigmatism and the barrel-shaped deformation, by providing paddle-shaped appendages which are perpendicular to the plates producing the extra focusing. The paddies are made large enough te eliminate the distortions.
An intens Wer type cathode ray tube having a rather high writing speed has been reported by Lempert and Feldt84 • The type of dis-tortion reported by DeGier due to the asymmetry of the field and the resulting astigmatism is avoided in the ie tube by beg inning the first intensifier gap at a sufficient di stance from the end of the deflecting plates. The bulb was also shaped so as to shield the deflecting plates from the intensifier field, and the intensifier potential was applied gradually over the length of the bulb body. Distortions caused by the nonuniform strength of the field act ion for different amounts of deflection were avoided by using a cylindrically shaped bulb body, thus keeping the beam at a maximum distance from the edges of the intensifier field. According to them approximately equal voltage steps in the intensifier electrodes gave best results. For maximum efficiency and stabie operadon the fluorescent screen was operated at a potential close to the final accelerating potential. In order to improve the second-ary emission characteristic of the screen, the fin al accelerating elec-trode had to be as near the screen as possible. On the other hand, the final accelerating electrode must be far enough from the screen so that its effect can penetrate sufficiently and reach the axis of the tube. In
order to satisfy the two requirements, the final accelerating electrode was made wide enough. With a total accelerating potentialof 25 kv and an fli lens and an image reduction of 5: 1, a maximum photographic writ-ing speed of 10,000
kmi
second was recorded. Lempert and Feldt say that the maximum frequencies at which their tube can be used arepd-marily determined by the transit time effect limitations and the resonance effects in leads to the deflecting plates. Special multiband tubes have
been designed by them with a deflection system built for operation at 1000 megacycles using coaxial connections to the deflecting plates. No data is given about such tubes.
Pierce85 compared the ideal post deflection acceleration tube with the ordinary tube working at the post deflection acceleration voltage and concluded remarkable results. He started by defining a new parameter which is the reciprocal of the deflecting voltage or current required to move the spot one spot diameter on the screen. He calls it the deflection "sensibility" of the device.
It has been shown by Langmuir86 and Pierce87 that in an elec-tron beam, because of the thermal velocities of the elecelec-trons leaving the cathode:
1-3 where
i
is the current density,i
1 is the "limiting current density,"i
is the cathode current density, T is temperature of the cathode ino .
degrees Kelvin, V is the potential at the point considered in volts, and
0
is the half angle of a cone within which the paths of all incoming electrons lie.This relation is valid for any point along the beam. In the region of the deflection, V will be large and
0
small. Equation 1-3 can then be approximated to:11600
- T -
1-4
In the limiting condition in which the electron flow fills a cone of peak angle a at each point of the beam cross sectio(l we have:
11600
- T -
1-5
where A is the area of the beam assumed constant throughout the region. Solving for a in Eq. 1-' we get:
a> al =
2(
}jo )~ {1l~00
. V dr~
1-6 Defining a quantity E which relates the beam current 1 to the limiting current 11 by:E = 1
~
we get
a= _ - 2
'(EAi
( I)~
o (
11600
)"11,
- T - V d 1-7
E is a measure of the quality of the electron-optical system, and will be large for good tubes and small for poor ones. Generally, E wiU have a limiting value less than uniey but, allowing complete freedom of design, theoretically, E cao be made to approach unity.
In magnetic deflection, the angular deflection which is assumed to be very smalI, can be written:
a=(~)
iV
d Ïa1-8
where i is the cucrent in the deflecting coils, V d is the potential in the reg ion of deflection and c is a constant depending on the size, shape and number of turns of the deflecting coils. 88 Combining Eqs. 1-7 and 1-8, we may write for the reciprocal of the cucrent required to move the spot one spot diameter, in other words for the deHection "sensibility"
a;,
l
=
~ (11600 EAio)
2.1-9
i 2 TI
It is thus obvious that for magnetic deflection, the deHection sensibility is independent of the potential in the region of deflection. Post deflection acceleration does not increase the deflection sensi-bility in the magnetic case.
Foe the electrostatic deflection, the situation is different as cao be shown in what follows.
The angular deflection, again assumed to be very smalI, can be written in the form:
1-10 where v is the deflecting voltage, V d the potential at the reg ion of deflection, and B a constant depending on the geometry of the de-flecting plates.
Combining this equation with Eq. 1-7 we get foe the deflection sensibility:
-L
=
-1.
(11600
EAio)J4 V
-~
Here it is dear that post deflection acceleration is of some advantage with electrostatic deflection.
These condusions derived by Pierce were discussed by White89 who finds that the line of argument does not seem justifiable. Accord-ing to him, the simplification made by Pierce in the question of focus is not always true in practice. Ic is not generally agreed that the deflecting forces and line width may always, or even frequently, be equated to one another.
White referred to an ideal post deflection acceleration tube which may be imagined to consist of an ordinary tube to which is added an infinitely thin electrode at the back of the fluorescent screen. The electrode would be maintained at a voltage higher than that of the final anode of the electron gun. According to him, such an elec-trode would not impede the flow of electrons through it or disturb the distribution of electric and magnetic fields over the path of the beam. However, a tube of this nature would suffer from defects similar co those outlined for the normal type of post deflection acceleration tubes since the accelerating field would still penetrate down the tube, thus introducing scan distortions.
An ideal tube with post deflection acceleration has been de-scribed by Allard90 • This tube is completely free from scan dis-tortions. Here, the fin al accelerating field was introduced between two parallel electrodes placed very near and parallel to the fluorescent screen. In this arrangement, the equipotential surfaces effectively form parallel planes between the accelerating electrodes. However, this gave no fruitful results, since the beam, when passing through the first mesh electrode, liberated electrons by secondary emission which were in turn accelerated towards the screen and thus reduced the contrast range. On the other hand, the resolving properties of the tube were also affected since these secondaries gave an apparent increase in spot size in the central region of the tube face, while at the edges they formed a second trailing spot. Ideally for th is system to work satisfactorily, a very fine mesh, or in the limit, a very thin continuous metallic film is required for the Hrst parallel electrode. This must allow the beam to pass through and yet liberate no
second-ary electrons. This condition has not yet been fulfilled in practice. Robinson and Allen 91 have described how errors due to the sensitivity being nonuniform over the screen can be reduced. This is done by photographing a series of orthogonal equipotentials and using these to calibrate the oscillograph recordings. However, their method is only accurate for low frequencies. A method available for cali-bration at the higher frequencies has been suggested by Hollmann92•
