PROCEEDINGS OF THE MEETING ON MOLECULAR BEAM TECHNIQUES held at the Laboratoire Méditerranéen de Recherches Thermodynamiques Nice, France 1 July 1962 J.J. SMOLDEREN Editor RHODE-SAINT-GENESE, BELGIUM March 1Q63
held at the
Laboratoire Méditerranéen de Recherches Thermodynamiques Nice, France
1 July 1962
J.J. Smo1deren Editor
In the development of a new field of study a propitious moment arrives when progress can be given a new impetus by a conso~idation of information, bet ter understanding of the several paths of study being followed, and a close personal contact of scientists working in the field.
For molecular beams such a moment was adjudged. to have arrived during the summer of 1962. In addition to the fruifful state of development of molecula.r beams most of the peowle interested in the field were gathered in Paris for the 3rd Rarefied Gas Dynamics Symposium. Thus the idea was born for a Meeting on Molecular Beams simultaneously at the Laboratoire Méditerranéen de Recherches Thermodynamiques (LMRT), Nice, France, and the European Office, Aerospace Research (EOAR), Brussels, Belgium.
With the initiative of Dr. F. Marcel Devienne, Director, LMRT, the cooperation and support of Mr. Roland A. Willaume, Director of the International Exchange Program, AGARD, Lt. Colonel Harold A. Ellison, Executive of the Fluid Dynamics Panel, AGARD, and the coordination of Major Orlando J. Manci, Jr., EOAR, this meeting was organized for 1 July
1962. The Laboratoire Mediterranéen de Recherches Thermodynamiques was chosen as the site of the meeting on the cordial invi~ation of Dr. Devienne, Professor J.J. Smolderen, T.C.E.A., Brussels, was selected secretary on the mutual recommendation of all concerned.
The technical success of this meeting was due to the active
participation of all attendees, the excellent organization and arrangements made by Dr. Devienne, and now the complete report consolidated and written by Professor Smolderen. The warm hospitality extended by Dr. Devienne
contributed also in very large measure to the lasting memory of this assembly and deserved fully the sincere gratitude expressed by all attendees.
ORLANDO J. MANCI, Jr. Major, USAF
I. CLASSIFICATION OF MOLECULAR BEAMS. (Dr DEVIENNE)
In order to introduce the discussion, it seems useful to establish a short list of the various types of molecular beams.
1. Thermal Molecular Beams.
This is the classical and oldest type of molecular beam (ref.l). It appears to be limited in performance and possibilities, although aL improvement might be obtained through an increase of the oven temperature.
2. Aerodynamic Moleeular Beams.
There are three different techniques available to provide a molecular beam by aerodynamic means.
a) Use of the flow produced by a de Laval nozzle.
b) Use of a steady state Kantrowitz-Grey scheme (ref. 2).
c) Use of an intermittent Kantrowitz-Grey scheme using a shock tube flow.
The second teehnique is used, for instanee, by the groups of prof. Seott and prof. Fenn and the third technique is used by Dr.Skinner's group.
3. Neutralized Ion Beams.
This technique, based on the neutralization of an eleetrically accelerated ion beam has been used by prof. Hurlbut, Dr Medved and
Dr Devienne.
Dr Hurlbut has also suggested that a satellite could be con-sidered as a moleeular beam generator but this eannot, of course, be considered as anormal laboratory tool.
DISCUSSION AND SUGGESTION.
Dr Hurlbut understands there might be a slight difference between the techrtique used by Dr Skinner and the conventional Kantrowitz scheme, using a shock tube as a driver, and requires some clarification about this
difference.
Dr. Skinner. Our device differs from the Kantrowitz-Grey system. In fact we ran into difficulties with such a system, designed according to the
theory, in high stagnation enthalpy flows. The trouble is probably associated with the large difference between stagnation temperature a.nd wall temperature, and possibly it will be necessary to revise the
Kantrowitz-Grey theory to take into account at least first-order
collisions near the skimmer entrance. One might expect that a more severe criterion has to be satisfied than the present theory requires.
Our present system (see ~ef. 3) is essentially made up of a two -stage expansion nozzle, similar to the type used in hypersonic shock tunnels. The gas is expended until the mean-free-path becomes large enough to enable a collimating orifice to opera te in the near-free-mole-cule regime. The arrangement is shown schematically in Fig. 1.
The gas is prepared in a shock tube. Generally the tube is operated in the tailored interface condition; that is to say the shock, af ter reflection at the end wall, returns through the interface between driver and driven gases without producing aIlY reflected shock or expansion wave. The interface is stopped, leaving a stationary pocket of hot gas in
the end of the tube. Conditions are arranged to produce this hot gas at atmospheric density, which then determines the density at all points of the nozzle system. An equivalent statement is that the initial mean-free-path is always the same, in order to obtain the correct mean-free-mean-free-paths throughout the nozzle system.
regime. How~ver, as the second stage flow expands into the vacuum dump tank, the mean-free-path grows rapidly, and at the collimator position it is estimated to be larger than the collimator diameter. Some attenuation
I
does take place as a result of collisions with particles rebounding from the collimator cone. Thermal accommodation at this surfaee slows down the particles and there will be a cloud_of gas upstream of it. However, our measurements indicate that the effect is not serious. The flow conditions
in the collimator region ean be estimated by the methods outlined by Lord (see Ref. 4).
In the first moleeular beam we operated, the first nozzle expanded the gas to a Mach number of 10.5. The second nozzle then could have an attaehed shock, and an interior eone angle large enough to allow completely free expansion into vacuum. That is to say the Prandtl-Meyer expansion at the lip from M=lO.S to M=OO requires less turning angle than that provided for by the slope of the wall.
The two most important features of this system are first that the hypersonic expansion reduces the random motions to a negligible
I
fraction of the mean velocity, and secondly that? by proper design and operation, dissociation products can be frozen in the first nozzle. We are particularly interested in the latter feature which will allow us to obtain beams of oxygen with about 12% of the gas dissoeiated. The
intensities we can obtain at present are approximately 1019 molecules -2 -1
cm sec
The limit of intensity to which we can go is not elear at the moment. The upper limit may be set by scattering in the thermally
aceommodated gas upstream of the collimator, or by scattering in the beam itself. A recent Russian paper deals with the latter (Ref. 5) and there is the possibility of carrying very intense beams for considerable distanees if the velocity distributions are narrow enough, making the mean-free-path in the beam much greater than that in the gas ahead of the collimator.
"r. Souguet. How are the density and energy of the beam measured?
Dr. Skinner. No direct measurements were made so faro We obtain our data from the nozzle flow calculations which are expected to be reliable because of the small boundary layers.
Dr. Skinner suggests the discussion of the following question. What would be the velocity distribution in a neutralized ion beam if it
could be slowed down to, say, 10 eV.? Would it be, say 10 eV
±
0.5 eV, the last term being due to emission velocities?Dr. Hurlbut mentions that the technique of slowing down a neutralized ion beam was used by the Systems Research group at Stanford, under Dr. Hahn.
Dr. Devienne. This question is difficult to answer. We made some initial tests with a low electrical acceleration (lOV) and very large intensities but could make no measurement of distribution up till now, because of difficulties of detection.
From results obtained with accelerated beams, it appears that the velocity spread in the lew energy beam was important.
Typically we obtained velocities between say 30 and 70 krols for large accelerations of the order of 1000V. This spread was observed in the case of large intensity beams (500~A-8mA at the outlet of the ion source). The spread in molecule velocity depends on the spread in ion velocity.
At present, we have less spread for the same accelerating voltages: i.e. about 10% in speed (20% in energy) (see fig. 2).
Spatial distribution at 1 m. from the exchange section is shmm on fig. 3. The spatial spread is more important for the ions than for the molecules. The beam is concentrated in a section of about 2.5 cm in
Dr. Devienne expects the absolute spread to he about the same af ter slowing down, which would result in an enormous relative spread.
Dr. Hurlbut. The dispersion in energy seems to be a characteristic of the souree. The Stanford group used an R.F. discharge ion souree and an
extraction voltage of 5 KV and large currents, in the mA range, were
obtained. The beam was focussed for containment, bent by a magnetic field to sepa~ate doubly charged argon ions and other extraneous species, th en decelerated to the energy level required. The spread in energy was found to be the same, whatever the degree of deceleration used, and turned out to be typical of the ion souree. One cannot of course define a souree temperature here, beeause the distribution in the souree is far removed from equilibrium, but energy spread was of the order of 40 to 50 eV.
Major Manei. The heam was thus accelerated, then the extraneous species were extracted hy a magnetic field, af ter which the beam was decelerated
to the original speed.
Dr. Skinner. I thought that the result obtained by this group were better. What was the intensity in neutral partieles?
Dr. Hurlbut. The intensity was the same as would have obtained by extracting at 50 eV, i.e. a fraction of ~A (at 50 eV).
The principal difficulty lies in the use of R.F. excitation.
Dr. Walsh. From the point of view of spread, the eritical parameter would 2
appear to be the flux, in Amp. per cm .
Dr. Skinner wonders whether the situation regarding space charge in an ion beam is comparable to the case of the mass spectrograph, where there is usually a limit of about 10-10 Amp, irrespective of the cross section.
Dr. Hurlbut. One can operate a mass spectrograph at many different energies and the total current one can have in the ion beam is a function of the energy.
Dr. Skinner. There should thus be no limit?
Dr. Fenn. In a mass spectrograph, there is an additional requirement about the resolving power, which implies a limitation in the current.
Dr. Devienne. We intend to use the acceleration-deceleration scheme but we fear that the absolute spread will be the same af ter deceleration or even be inc~~sed by the acceleration-deceleration process.
Dr. Hurlbut agrees with these expectations.
Dr. Skinner. Is it possible that one might gain by converging at lower energies?
Mr. Souquet. Af ter the beam has been through the converging field, space charge effect becomes significant. The focussing field ends before the charge exchange section and one gets spatial separation at low energies.
Major Manci. Would it not be useful, then, to reduce the physical dimensions of the charge exchange section?
Dr. Skinner. These are already very small and i t would prove very difficult to reduce them.
Dr. Fenn. Does the focussing change the energy level of the ions?
Mr. Souguet. We use a triplet focussing lens (decelerating~accelerating
decelerating). There is no change in energy, theoretically, if the exit potential equals the inlet potential.
Dr. Hurlbut points out that there is a slight advantage in having an energy in the beam slightly in excess to the final required energy in order to carry the beam through the apparatus and through the weak magnetic fields produced by iron pieces in the apparatus, to the transfer chamber. This principle is examplified in the sputtering apparatus at Berkeley.
Dr. Devienne. It seems to me that the intensity one requires depends on the possibilities of detection: i.e. if a sensitive detector is available, no large intensity will be needed.
If one wishes to reproduce aerodynamic conditions, however, beams with large fields and intensities will be necessary. All this depends, of course, on the purpose of the research. There are two problems, therefore, which are somewhat interdependent: to increase the intensity of the beam and to increase the sensitivity of the detection. The latter might weil be the easier to solve.
Mr. Souguet. During a visit at the General Dynamics Co. (San Diego), I was told of some attempts to neutralize the space charge by injecting slow electrons in the beam, in the hope of keeping the ions in focus.
Having had no recent information on this technique, I would welcome any comment or recent data.
Dr. Smith does not think that the results were successful because the method is not used by Dr. Medved's group at the Astronautics Division of
the General Dynamics Co. at present.
report on their activities up to 1961.
The scheme of introducing electrons in a beam has been tried in several laboratories (cfr NASA Abstracts journai), but no useful results were óbtained in connection with molecular beams to this point. The beam ean be somewhat neutralized ~lobally. But there is significant recombina- . tion, which makes the beam hard to work with. Furthermore, the process is not understood at all. The focussing becomes difficult and one cannot increase the intensity.
There does not appear to be much promise for the future of the method.
Mr. Souguet. Does the presence of electrons, mixed in the beam, interfere with charge exchange processes?
Dr. Hurlbut. No.
Major Manei. The C.S.F. company appears to have been successful in applying the method.
Dr. Skinner. What are the problems involved in decelerating and focussing such beams?
II. PERFORMANCE OF MOLECULAR BEAMS." (Dr SKINNER)
In "the field of molecular beams, as in any other, it is usually difficult to discern the weak points of the various devices.
The au thor feels that the first criterion of performance for a molecular beam is to produce experimental results that can be interpreted statistically. This requires, for the steady state devices, that the
signal-to-noise ratio be large enough, and, for the non-steady state devices (such as the one used by Dr. Skinner) , th at the number of molecules involved in a test be large enough to allow for the application of a statistical method. It seems that every facility satisfies this criterion, at least theoretically.
We are all mainly interested in two subjects: the interactions between gas molecules and the interactions between a gas molecule and a surface. The author is mainly concerned with the first subject and his group intends to carry out enough experiments to cover some of the practical cases, and the first thing, therefore, will be to study interactions between molecules in the ground state. There is some doubt about the state of
molecules in a charge exchange beam and such a beam may not be weIl suited for such experiments. This is an example of how the choice of the molecular beam system is conditioned by" the purpose of the research.
Would it appear reasonable to characterize the performance of the mol ecu lar beams by discussing what one can hope to do with them?
Dr. Hurlbut. Yes. I hope to get an idea on the performance of the existing beams.
Dr. Skinner. I suppose we should include the molecular beams which ne~rly
exist and those which we have in mind. Ideally, in addition to some re-quirements on signal-to-noise ratio, we should have complete control over
the following items: mean energy, energy distribution, type of species and state of molecules.
Prof. Scott. As pointed out earlier by Dr. Devienne, a beam is designed for a particular experiment. I can therefore imagine some tests where no high intensity will be required, particularly if classical counting tech-niques are used in a stationary state device, and other tests, where very high intensities will be required, such as the measurement of scattering from a surface, etc.
Dr. Skinner. Some experimenters intend to study the state of the scat'tered molecules. Very large intensities will be needed for this purpose, even more than the intensities provided by the shock tube technique we plan. So far, such studies have proved inpracticable because of the insufficient statistical data produced by the existing beams.
Dr. Devienne. Could Dr. Skinner describe the exact performance of his system.
Dr. Skinner. The apparatus is not yet completely developed. The perfor-mance, however, can be estimated from available shock tube data. Taking the Cornell Aeronautical Laboratory shock-tunnel performance as a possible limit, we could, in principle, hope to get energies of SeV as an upper limit. At present, in this tunnel, we obtain 3 to 4 eV. Heated hydrogen is used as the driver gas. The maximum pressure of the driver gas is, at present, about 26,000 p.s.i. but will ultimately go to the design maximum of 30,000 psi at 9000
F. The theoretical flow veloeities are never quite reached in practice.
A favorable feature of a shock tube as a beam souree is that the gas used is at equilibrium so that the usual techniques (spectroscopy, etc.) are available to study the gas state. Furthermore, at the proper pressure
levels, substantial dissociation is present in oxygen and other gases such as carbon dioxide (but not in nitrogen), and this is useful for our purpose.
:
Dr. Muntz. The 3 to 4 eV referred to ar~ not all translational energy. The translational energy would be closer to 1 eV.
Dr. Skinner. The 3-4 eV become translational 'energy af ter exparision through the nozzle.
Dr. Muntz. If the conversion is complete. (1)
Dr. Skinner. Without driver heating, a diaphragm pressure ratio of 500,000 is attainable and enèrgies close to 2 eV can b.e obtained, if the gas is not dissociated. The nozzle is designed so as to obtain frozen flow, which is an advantage in the molecular beam application, as opposed to what is desirabie for conventional wind tunnel operation. Dissociation and vibra-tion will be frozen but usually not the rotavibra-tion. There is, however, a problem in obtaining complete knowledge of the state of the molecules in our beam.
Prof. Fenn. In molecular beam work, small geometrical dimensions are used and there is accordingly mare likelihood for the freezing of states to occur.
Dr. Skinner. Yes, and one also has less difficulties with boundary layers for the same reason. The energy and energy spread in our beam ean be deduced immediately from the energy equation for an ideal gas flow:
;2 :t ~
3..
+
~:.
ë1..
o:2
(-1
)(=1
(1) Comment added in proof-reading by Dr. Skinner: Generally vibrational energy will be frozen in.
where~ is the gas flow velocity, a the local velocity of sound and ao the velocity of sound at stagnation condition. ~ is the ratio of specific heats.
The seeond term in the first memher is made negligible by the hypersonie expansion and also characterizes the veloeity spread to be ex-pected.
For instanee, for J:.1
=
14, the spread would be about 15% of the mean velocity. Neglecting this second term, we get1.(:z,
a..
:t-
~---E
:2..
r-1
and immediately obtain the beam energy in terms of the stagnation conditions. For instance, if the driver gas is hydrogen at 26,000 psi and
9000K, as in the present operation of öur wind tunnel, we should be able
to get nearly 5 eV in N
2. However, in other gases, if dissociation is appreciable, we would get less.
Prof. Scott. Is the data about freezing deduced from experimental obser-vations or from theoretical calculations? A large number of collisions is obviously.required to convert thermal intO kinetie energy, and if such a conversion is completed in the nozzle, there should be ample opportunity to reach close to equilibrium. It will be difficult to ascertain the state of the gas at the entrance of the beam apparatus.
Dr. Skinner. Only a few (3 or 4) collisions are required to obtain equi-librium for translational degrees of freedom, and rotational equiequi-librium requires a few more collisions. However many more collisions are required
to reach vibrational equilibrium. The number of collisions required for recomhination is very large.
Dr. Muntz. The experimental observations of non equilibrium effeets in nozzles substantiate the theoretical ealculations at thistime, although a considerable amount of work remains to be done.
Dr. Skinner. We currently have a program at C.A.L. in which the kinetics of nozzle flow are being studied.
Prof. Scott. What is the validity of the adiabatic expansion in quasi-steady flow?
Dr. Muntz. The boundary layer is very small in the nozzle considered here.
Prof. Fenn. I think that the final enthalpy was very low in Dr. Skinner's tests.
Dr. Skinner. The situation is as follows: the thickness of the gas layer, cooled by the end-wall of the shock tube increases as vt:,(t being the time). The gas flowing through the nozzles maybe visualized as coming from the
interior of an hemisphere centered at the throat, the radius of which is proportional to
t~3
(the volume being proportional to t). The cool layer will therefore eventually catch up with the hemisphere. The dimensions ofthe throat and the nozzles must be chosen in order to get a useful running time, during which the influence of the cooled layer is not too strong. Our latest nozzle has a throat of 4 mmo diameter.
Dr. Hurlbut. What are the intensities of the beam obtained by this method and what detectors do you use?
18 2
Dr. Skinner. In our first beam we had 2.5xlO molecules per cm and sec. and we expect to get 1019. We are currently using thin-film resistance thermometers as detectors.
Dr. Devienne. What is the total duration of the flow?
tailored interface condition is obtained (ref. 6), the running time is of the order of 1 ms. With a cold hydrogen driver, this leads to energies of the order of 1 eV in nitrogen. If one requires the upper limit of energy, without driver heating, about 1.8 eV, the tailored interface operation becomes impossible and the arrival of a shock reflected from the interface reduces the useful running time to ab out 2xlO-4s. in Dur present shock tube.
Dr. Devienne. You obtain very large intensities, as compared to the inten-s1ties used by Prof. Hurlbut and myself, but during a very short time. Do you have suitable instrumentation to measure, say, scattered intensities, etc., in such a short time?
Dr. Skinner. This is, of course, the most difficult part in Dur system. We intend to study specific collisions between specific pairs of particles. There are t'-lO methods to make those experiments.
a) Chemical reaction. This is a possible but messy technique.
b) Use of a mass spectrometer of extreme sensitivity allDwing the analysis of high intensity beams during a very short time. This is the opposite of the usual situation of a \veak beam with long measurement time.
We thus have to tackle two problems in connection with the mass spectrometer: 1° The high intensity at the mass spectrometer inlet.
2° Apparent aperture effects, occuring when the velocity of the incoming particles is large.
There are toD many things to be considered to go into much detail here. For example, the stream of electrons in the ionizer is usually confined by a weak magnetic field. In swinging around a scattering center, the energy of the par~icles entering the spectrometer would vary fr9m nearly th~ full beam energy to zero, and it is very likely that deflections caused by this weak field would cause the efficiency, with which ions are collected, to vary with angle. Also, because of the high flux of particles it is possible to trap particles in certain areas of the spectrometer. Those which collided
with the slit plates in a conventional ion accelerator-lens system would become thermally accommodated and probably build up significant clouds in these regions, causing scattering which would vary with time. The latter problem could be alleviated by drawing off the ions at right angles to the incoming stream, but this would aggravate the first problem.
Prof. Fenn. Why cannot one run axially in the spectrometer?
Dr. Skinner. Let me give one example we did some calculations on. We considered an existing small satellite apparatus. The density in this device would increase, in time, up to a level which becomes enough to cause very appreciable scattering. We considered strong-focussing ion sources and the Paul-type filter, since these have more open structures and could probably be used with the stream entering axially.
Prof. Scott. One must be very careful in using such apparatus because they tend to be extremely energy-sensitive.
Dr. Skinner. The advantage, however, of such Paul-filters is that they require no magnet.
Prof. Scott and Prof. Fenn. They also have a very high transmission coefficient and one needs all the transmission one can get in the applica-tion discussed here.
Dr. Hurlbut. We have used such devices and our experience is that it is rather difficult to interpret the results correctly.
There is a limited region of field strength in which one has transmission.
We used i~ to analyze the impurities in an argon beam, and it proved easy to detect singly and doubly charged argon ions. But we felt
it was not a f1exib1e instrument and it proved more difficult to run over a range of potentials to obtain a spectrum than with a conventional
spectrometer.
Dr. Skinner. We intend to set i t up for a single species ('-Je have to separate nitrogen from oxygen). Could anyone provide data about the performance actually obtained with Paul-filters. Experimental results seem to be scarce.
Prof. Fenn. You will have no problem i f the "ratio of masses of the species is not too close to unity.
Prof. Scott. We built such an instrument at the University of Virginia, and obtained a resolution (M/AM) of around 80, with possibilities for improvement. It' was used on an unneutralized nitrogen beam to find im-purities, with very satisfactory res~lts. I don I t knO\I1 how one cou1d make
absolute cross section measurements with it, but the resolution is suffi-cient to make relative cross section studies.
A.
Kuppermann, at the Chemistry department of the University of Illinois, has des~gned an interesting filter of great flexibility, using different R.F. stages.A
wide range of resolut~ons and mass ratio is available. This apparatus was built in our machine shop.Dr. Hur1but. We build our O\-Jn device, using voltage scanning.
Prof. Scott. Voltage scanning is more advantageous because the pO\-Jer requirements increase less rapidly in function of frequency than with
frequency scanning (for which it increases as the fifthpower of frequency). Frequency scanning is used in Germany, but only in the case of leak de-tectors. The paul type spectrometer must be designed for a particular experiment.
Dr. Skinner. There is another type of filter using t,,,o sets of quadrupoles with.a square inlet. In one cross-section, it is a diverging-converging
lens~ and in the other section a converging-diverging lens. This is preceded by a linear accelerator. It has a very open structure.
Most filters require a slit and this is not acceptable in our particular application. One must also stress the fact that the Paul type filter is extremely sensitive to the direction of the input.
Dr. Hurlbut. The Lockheed Company designed our instrument. There was an attempt to use it in a satellite, which was unsuccessful because the device is highly direction sensitive and the satellite was unstable in orientation.
Prof. Fenn. What is the Mach number in the nozzle of Dr. Skinner's system
18 2
to obtain 2.5xlO molecules per cm and s., assuming atmospheric stagnation density.
Dr. Skinner. The Mach number is really not important, and is somewhat indefinite towards the end of the expansion.
Prof. Fenn. There is a very strong dependence of the intensity on ,,,hat one could call a Mach number in the nozzle, assuming given upstream density.
Dr. Skinner. The Mach number in question would be 30 if one computes it from the geometry, assuming an adiabatic expansion. This determines the local density. This value is, hm"ever, only indicative.
Prof. Fenn. Hhat is the mean free path, under the operating conditions yielding 2.5xl018 molecules per cm2 and sec.
Dr •. Skinner. We have used theoretical results describing a nozzle expansion into vacuum. We don I t, of course, know how to make flm" calculations on
the transitional regime. The mean-free-path is probably greater than 1 cm at the point where the beam is formed. (2)
Prof. Fenn. How many shots per day can one obtain from such a shock driven beam?
Dr. Skinner. This depends ,on whether or not one has to open the vacuum vessel. A fast shut-off device, with 0.8 ms. shutter speed, has been planned. The apparatus can be immediately brought back to low pressure if
-5 such a device is used. A few minutes only are needed to reach 2xlO Hg, using a small pump.
Dr. Devienne. I suppose that the pressure is measured by a short response time transducer.
Dr. Skinner. Pressure is measured in the shock tube by a fast rise-time gauge.
Prof. Scott describes the performance of his own apparatus at the University
of.~Virginia (fig. 4).
It us es a classical, steady state nozzle source, with nozzle, skimmer and collimator. The device is operated mostly with nitrogen. The length
between converging nozzle exit and skimmer inlet may be altered in the hope of.varying the local Mach number at the skimmer entrance (The Mach number is, of course, better defined in our applications, as indicating the ratio between ordered and disordered kinetic energy).
The detectors used are similar to those used by Prof. Hurlbut, and use t~'l0 Alphadyne gauges. One of the gauges is directed towards the
(2) Comment added in proof-reading by Dr. Skinner: The beam is formed well dO\Ynstream of the nozzle exit.
beam and the other, facing away, measured the background pressure. The collectors of the gauges are connected to suitable electroniccircuitry in order to measure the difference in collector current. Stagnation conditious are as follows (using nitrogen):
Temperature 3000K
Pressure: ranging from 5 to 200 mm Hg (so far).
Two independent parameters are thus available to adjust the operating conditions: the distance 1 from nozzle exit to skimmer inlet and the stagnation pressure. The collimation distance is 100 mm and the nozzle throat diameter ab out 1 mmo The Knudsen number~ based on skimmer diameter, ranges from 0.1 to 10.
Dr. Skinner. Do you get the Kantrowitz-Grey type flow over this range?
Prof. Scott. Dur measurements do not allow us to answer this question. Our results do not check with the theory and it is very unlikely that they
should.
Prof. Fenn. If one reaches Knudsen numbers of the order of 10, one should obtain agreement with the theory.
Dr. Skinner. I wonder if one would obtain the beam eharacteristics expected from the theory, when working at higher energies.
Prof. Scott. In order to obtain ordered motion, eollisions must oeeur. When the distanee 1 is increased, one expects to get more collisions and
therefore more conversion of disordered into ordered kinetie energy. We found three different regimes when inereasing 1 (fig. 5).
1. For low Knudsen numbers, it is likely that a shoek will form in front of the skimmer and the flux in the beam is much less than indieated by the Kantrowitz theory.
2. For Knudsen numbers between 0.5 and 1.5, the results are only slightly less than indieated by the theory.
3. For Knudsen numbers larger than 1.5 one gets only about 80% of the flux predicted from the theory.
The problem is now to decide what one should get.
Prof. Fenn. We apparently got 100% of the theoretical amount, but one
should 'kno,~ what Mach number one actually has in order to make a comparison. If the inviseid ~en-Thornhill expansion theory is used, one calculates a Mach number whieh .is probably much in excess of the real Mach number.
Prof. Scott. It is extremely difficult to measure Mach numbers and flow fields at the skimmer entranee because of the small sizes and low densities.
Prof. Chuan. What Mach number would you wish to get and at what density level?
Prof. Scott. A Maeh number around 10 would be satisfactory, at a density level for which the mean free path would be two or three times the skimmer diameter (1 mm).
Prof. Chuan and Dr. Smetana. Our experience is that a Maeh number of this order cannot be obtained at sueh a pressure level (i.e. l~ Rg or less) in a· nozzle expansion. Dr. Smetana even suggests that the flow obtained might turn out to be subsonic.
Prof. Scott and Prof. Fenn. Ma'eh numbers of 10 were actually measured in sueh nozzles, using Pitot statie probes.
Prof. Chuan. The measurement of Mach number using Pitot statie probes is completely unreliable at such pressure levels. A measurement of stagnation
and pitot pressure will yield the correct Mach number only if there are no entropy increases in the flow between stagnation and the Pitat probe. An extensive study of expansion processes at low densities was carried out at USCEC over the last three years. It is possible to get reliable information about the flow only if there is an isentropic core along the axis of the nozzle, characterized by a small region of flat velocity profile. Such a core is not always present. For instance, in the c~se of a nozzle.with a throat diameter of 1 cm, operating at 10 mm Hg stagnation pressure, no isentropic flow was found, even at the theoretical throat (which is not the actual throat in such a case). One of our Ph. D. students is making a theoretica 1 study of nozzle flow, taking into account the viscosity effects, which agrees very weIL with our experiments.
Prof. Scott. Your objections are probably entirely valid, and, in fact we donlt know what happens in the flow. It will probably prove to be impossible
to analyze the flow. We have naively applied the ~ven Thornhill results (ref. 4) in order to compare the actual flux with the theoretical flux (assuming a diabatic expansion). We get from 5 to 50% of the theoretically predicted values.
Dr. Smetana. Would the agreement with the experimental data be better if one assumes the flow at the skimmer to be subsonic?
Prof. Scott. On the contrary, the agreement would be a lot worse. For a constant source pressure and subsonic Mach number, the flux would be extremely large. It decreases strongly when the Mach number increases. However, the advantage in taking the beam at a higher.Mach number is that
the ratio of ordered to thermal motion is much larger, 50 that the charac-teristics of the beam are improved . . What is lost in intensity is thus gained in velocity distribution. We have not really attempted to study the nature of the flow and make a thorough compariso~ with theory. Such a
comparison is probably meaningless bacause we donlt know the value of the Mach number at the skimmer.
We have obtained rather large intensities at low energies: (from 0.05 to 0.08 eV) using a stagnation temperature of 300 oK. The objective here was to obtain a quantitative characterization of the beam in function of the way in which the beam is produced, i.e. as a function of the distance 1 and the supply pressure, without any attempt at studying the flow.
-4
The minimum pressure we got behind the skimmer was 2xlO mm Hg.
Prof. Chuan. The use of the ratio
f9()
Ifo
(poO being the local statie pres-sure) for the computation of the Mach number will yield the largest value for M. The use of the ratio p~/
Pc>
(plo being the local Pitot pressu:re) gives a smaller value for M, and the ratioPco
/
p' ,
the smallest value.Prof. Scott. We only used the ratio plO/Po and obtained a maximum calcu-lated Mach number of 5 (assuming isentropic flow and normal shock), but I don't believe comparisons would be useful because of our lack of knowledge about the flow.
Dr. Skinner.suggests that the use of Lasers for Schlieren observation of the flow might be helpful, for it may provide a gain of one order of
magnitude in the sensitivity, which might make such an observation possible.
Prof. Scott. The only data we are really interested in is the beam flux as a function of the parameters 1 and Po as shown on the curve (fig. 5) .. If 1 is small and Po large, the beam intensity becomes too large and the detectors are jatmlled (unstability, in electronic instrumentation).: We have
17 2 .
observed a beam with in-tensity exceeding 10 molecules per cm and 5 .
The detector gives no information on energy and indicated only total flux.
processes at the entrance section are assumed to be known?
Prof. Scott. Calculations of entrance flow are not used. The detectors are calibrated. The calibration is based on a well defined thermal molecular beam produced by an oven. The skimmer is removed and very low pressures are .
used. No extrapolation is involved in the use of this calibration. We make sure that the detector collects all of the beam. There is a linear relationship between flux and pressure for Knudsen numbers larger than one. For Knudsen numbers lower than one, the pressure increases less rapidly with
the flux. The detector constant is deduced from the flux-pressure curve. This calibration procedure is also checked by a theoretical calculation of the molecular efflux.
There are, however, two large uncertainties in this calculation. 10 the resistance of the gage to the molecular efflux and
20 the uncertainty about the gas temperature in the detector. The exact
knowledge of the gas kinetic temperaturelr in the detector is not very important because the results depend only on~. The temperature T lies between the cathode temperature and the chamber temperature.
The results obtained from the use of this detector are very reproductible, i.e., within 0.5%, whatever the geometry of the detector. It appears therefore to be a really valid instrument.
Typically the indicated ion gage pressure is about or below
la -7 mm Hg in the detection chamber (.occasionalJy 5xlO;"8 mm Hg), while:the -6
collimating chamber pressure is below la mm Hg. Ideally, the detection chamber background pressure should be as low as possible to get a large sensitivity in the detection.
We have also used a chopper to measure velocity distributions. The molecular velocity distributions measured in the beam do not compare with theory. HO\vever, the distribution curves are narrow and displaced
so that we know that we have ordered motion. But they do not agree with the Maxwellian theory. If one computes the Mach number from the most
probable velocity, one may compare the theoretical and experimental curves and one observes that the experimental curve is narrower, particularly on the low velocity side. This might be explained by differential scattering at the entrance which is more intense at low velocities. The main facts, however are that the most probable velocity is shifted so that ordered motion is present and that the velocity distribution is narrower than the
theoretical distribution so that the beam is closer to the ideal.
Typical intensity for the beam is 1017 molecules per cm2and
S.,
but this is defined as the total flux divided by the total cross section. The width of the beam varies along the beam axis and the aperture of the collimator. The angular distribution is within a diameter of 6 mm which corresponds to an angular divergence of about 5°.
Dr. Devienne. What is the absolute value of the molecular velocity in your beam? I am not interested in Mach number which, in my opinion is
meaningless in the field of molecular beams.
Prof. Scott. The ratio of the most probable speed to the most probable speed at normal temperature is about 2. This would correspond to a most
probable speed around 800 mis.
Dr. Devienne. Usually, the total intensity depends on the pumping speed. What is the pumping speed in your system?
Prof. Scott. The pumping speed in the collimation chamber is 25,000 lis. I think that we can gain bvo orders of magnitude in the intensity. We
intend to study the elastic scattering in the collimator by varying the density and the beam intensity and measuring their effect on the velocity distribution. Our present program is to characterize the beams in function of the way they are created. When this will be known, we will begin the
Prof. Fenn us es a system essentially similar to the one described by Prof. Scott, but without middle pumping chamber (fig. 6). The pressure behind the skimmer is of the order of 10-6 mm Hg in his apparatus. The pumping speed is between 30,000 and 40,000 l/s., using a large diffusion pump.
According to the Kantrowitz-Grey theory, the flux obtained is a function of the skimmer section. If the skimmer section is opened too far, in the hope to get more flux, it is feared that problems will arise in connection with molecular colllsions in front of the skimmer. It is not to be expected, therefore, that the system can provide a flux of one or two orders of magnitude in excess of what is obtained at present, because
17
the efficiency decreases very rapidly. At present, fluxes of 10 molecules per cm2 and s. with nitrogen and 1018 molecules per cm2 and s. with hydrogen.
Prof. Scott. When I speak ab out one or two orders of magnitude improvement, I mean the effectively usable beam flux. The efficiency is of no importance here and is only an academie point.
Prof. Fenn. What one needs is a high effective Mach number in front of the skimmer. If one opens the skimmer more, more flux is obtained but the usuable flux does not increase very much.
We have obtained the same sort of curves for flux versus skimmer position as Prof. Scott (fig. 7). We believe the explanation to be as follows:
In region 1, there is a swallowed shock at the skimmer. At some critical Mach number, the shock comes out of the skimmer. Further on (region 2,
fig. 5), the efficiency increases because the conditions in the skimmer approximate ~ore and more closely the conditions of the gas upstream of the skimmer. Further increase of the distance 1 probably leads to the formation of a boundary layer on the walls of the skimmer, this boundary layer taking away part of the flux that would reach the detector.
,Dr. Hurlbut. It appears that the highest efficencies are obtained in the case of the shock attached at the skimmer entrance. \.Jhy cannot one operate systematically at this condition.
Prof. Fenn. If one translates the data into ratio of effective flux to theoretica 1 flux, one finds a maximum there, but the absolute flux can still be increased. Another argument against operation at this condition is that the Mach number is low so that the velocity distribution in the beam will not be very good.
If we measure the intensity along the axis of the beam, we obtain an inverse square Law in function of the distance to the skimmer, so that the skimmer operates as a source and this results in good collimation.
We have almost stagnation at the shock and this is foilowed by an expansion in the skimmer.
Our detector is also of the manometric type, but is not as sophisticated as the instrument described by Prof. Scott.
This detector is calibrated in an oven beam and also in a water va por beam, which is condensed on a target cooled by liquid nitrogen. The c.ondensation weight data is then compared with the manometer data (we have measured the condensation coefficient to be 0.86 ~_0.05). The agreement is good and within the uncertainty about the ionization data for water vapor, which we obtained from the literature. The overall accuracy is
found to be wi thin 20%.
Finally, let us make a few remarks about the Mach number in front of the skimmer and the existence of isentropic expansion. We are not
aerodynamicists.and would therefore welcome any information or comments on this difficult subject.
We used the Owen Thornhill inviscid theory. The results from this theory are well known and yield a curve of Ma.ch number versus ratio of the distance x along the axis to the nozzle diameter a.
should, therefore, base the data not on the geometrical diameter of the
nozzle, but on an equivalent diameter based on the discharge coefficient of the nozzle, which can be measured. Futhermore, we use a Pitot tube to
calculate the Mach number, assuming isentropic flow upstream of the normal shock on the Pitot tube.
If we plot the results, using the equivalent diameter defined above, ~l7e obtain points on the o-l7en Thornhill curve up to an abscissa value dependent on the pressure level, nozzle dimensions and nature of the gas.
The breakdown occurs earl ier when the pressure is decreased and the points af ter this breakdown lie below the theoretical curve (fig. 8).
Now, if.one assumes that there is entropy generation upstreams
of the Pit,ot tube, one should get apparent Mach numbers above the Owen Thornhill curve, but this was never observed in practice, which leads us
to believe that we have isentropic flow on the axis up to the normal shock
on the Pitot tube.
Assuming this, and using the Pitot data to calculate the
theor-etical flux and compare with the total fluxes obtained, we obtain efficienëies up to 50% of the theoretical results. If we extrapolate these data., we
would expect to get efficiencies up to 100% if the Knudsen number at the
skimmer (based on skimmer diameter) were of the order of 10. But this
•
would lead to a very small skimmer and produce very small total fluxes.
Prof. Chuan thinks that the decrease of Mach number observed with respect
to the Owen Thornhill curve might be explained as follows. At these very
low densities, viscous effects become very important and one can then get
a recompression af ter the init'ial expansion. This compression, hml7ever,
ocçurs through a weak oblique shock system, (somewhat like in centerbody diffusers) and is therefore very nearly isentropic. As aresult, the Mach
numbers calculated from Pitot data might still approximate the correct
values and the .curves shown simply exhibit an expansion followed by the
Dr. Hurlbut describes the activities of the group at the General Dynamics/ Astronautics under Dr. Medved. Dr. Medved regrets not to be able to attend
the meeting personally.
A change transfer argon beam is used with an acceleration voltage of 1 KV, for the study of secondary election production at clean surfaces.
The currents to the target are very low, of the order of 10-9 to -8
10 Amp. The apparatus is very clean, for it can be baked. Clean surface operation can be obtained during 5 minutes. The degree of cleanliness is
-9 -10
confirmed by pressure data: 10 mm Hg and even 3x10 mm Hg have been
obtaine.d af ter bake out. These performances are reached by using all the
modern methods of vacuum technology. The results obtained are internally
consistent.
A technique has recently been developed for measuring neutral beam fluxes. Basically, a thermocouple probe is utilized, which has a sensitivity of' approximately 250~V per m~ of bèam power. The probe is
\
calibrated using ion beams of the same species. With such a movable probe in. the ultra high vacuum system, they are able to obtain measusrements of the secondary electron yield for neutral atom impact on clean, well defined, surfaces in an energy range of 500 to 2500 eV.
•
Another charge transfer beam is used at the University ofCali-fornia, in Berkeley, by Stein, for Sputtering tests on Na, K, KC1. The energy of the beam ranges from 50 to 400 eV and the beam intensity ranges
·from 10-9 to 10-8 Amp,
de~ending
on conditions.This is an atomic beam which uses charge transfer on K-atoms. More details will be indicated in chapter IV.
Dr. Devienne describes the performance of his own molecular beam, at the
Laboratoire Méditerranéen de Recherches Thermodynamique (Nice). The methods
of measurement on the instrumentation will be described under chapter 111. Many ion sources were tested, and the source used .at present
-6
The layout of the apparatus is shown on fig. 9 which shows the ion source, the focalization system, the low speed, neutral, molecular beam produced by a Laval nozzle, and the system used for the measurement of speed by a time of flight device. The ions are deflected and received on a trap.
We have observed that the charge exchange rate depends on the density of.the low speed neutral molecular beam from the nozzle. The rate is maximum, for a given density, .at the highest ion speed. For an acceler-ation voltage of 1 KV, we obtain, reproductibly, a total flux of about
1. 5xl0l4 molecules per s., which corresponds to 24xlO -6 Amp. This indicates the efficiency of the change transfer process (for a stagnation pressure of 700~ Hg for the de Laval nozzle).
-5
The pressure at the exit of the molecular beam is 2xlO .mm Hg. The distribution of the beam in space is shown on fig. 3, and the velocity distribution of the molecules, which is , in the range bet\o1een 55 and 60 Km/s is shown on fig. 2.
By changing the acceleration potential, molecular speeds between 30 and 50 Km/s may be obtained but the distributions are not exactly similar.
The effective diameter of the beam is 2.5 cm, so that the beam
-6 2
111. SPECIAL INSTRUMENTATION FOR MOLECULAR BEAMS.
(Prof. Scott)
If we rule out
R.F.
spectroscopy and the measurement of nuclearproper.ties, we may subdivide the molecular beam measurements in two main
categories:
1. Detection of molecular beams, i.e. methods for determination of the
total flux.
2:. Measurement of the distribution of speed, momentum and energy in the
beam.
1. Detectors. The most important types of detectors are the following:
a) Ionization gauges using metal atoms (cfr. the description by
Dr. Hurlbut).
b) Thermistor and heat transfer gauges (cfr. the description by Dr. Skinner) .
c) d)
Differential ionization gauge (as devised by Dr. Hurlbut).
Methodsbased on the modulation of a beam in order to g~t an
AC signal and aeeordingly improve the signal to noise ratio of the detector
(teehnique developed by the General Atomics Group). e) Method of Dr. Devienne.
2. Determination of the distribution of velocity. momentum and energy.
There are several ways to obtain these distributions, depending
on ·tOe energy level.
If Qne wants to study inelastie seattering (through solid surface
interaction or by gaseous targets or chemieal interaction), one has to know preeisely the distribution of energy as well as the energy levels and the way in which the partieles are distributed among the different levels of
excitation.
a gas in thermodynamic equilibrium at, say, 5,OOOoK, one ean, in princip.le, measure inelastic scattering cross sections by a molecular beam experiment. These cross sections can be worked back into a theory involving the
inter-action po.tential of the particular molecules or atoms of interest. However, this appears to be critically dependent on the internal state of the
scattering atom or molecule. A method is therefore required to classify the internal states of the molecules or atoms in the molecular beam.
Dr. Skinner describes the heat transfer gauge detector as requested by Dr. Hurlbut.
Our inserumentation is adapted to the unsteady character of the beam. Of course, a steady state beam may be made unsteady by chopping
(ramark from Dr. Hurlbut).
The heat transfer gauge us es a thin film on a glass s~bstrate. The operation may be studied using a one-dimensional model. The surface
temperature T is observed during a fraction of a ms, through the measurement of the film resistance. The temperature history T(t) enables one to compute the heat flux Q Ct). The relationship between Q and T is best expressed in terms of theLaplace transforms of these functions, q(s) ~
c..:J ' _~ ~r;: C')
qlSJ.'::' l'/(tp .. S T(S)
~
and T(s):
where s is the Laplace variabie, and k, Ci and
f
the heat conductivity, heat capacity and density of the substrate.An analog network converts the temperature signal into a heat fl ux signa 1.
In free molecule flow, we have, clearly
'1
r;".",-
- "":J - / . J . b r '-,l7fi.;~"3
'where
rJ..
is the accomodation coefficient and 1/2PU
3
the kinetic energy flux.Outside the free molecule flm.; regime, one has to use a few difficult theories which attempt to compute the ratio
Q/1
F.M. as a function of the Knudsen number, and these theories are used when the gauge is
,operated in regimes different from free molecule flow.
Prof. Fenn. Does cl.. refer to translational energy) so that, for instance}
~>1 if there is a significant amount of vibrational excitation?
Dr. Skinner. Yes. The gauge actually measures the difference between
stagnation and wall temperatures, the latter being therefore unimportant if the stagnation temperature is much higher than the wall temperature (in cases where both temperatures are comparable, the gauge would be of no use anyway) .
We use a large current (60 mA) in the gauge and the resistance is
of the order of 10011.
Here are a few figures describing the capabilities of the basic "
instrument and the corresponding electronic equipment.
A. Millivac VS64-A hushed-transistor amplifier is used, providing
exceptionally low noise level:
0.
6
)àl
RMS in the 2 cps-180Kcps range. Thisenables one to distinguish a lj.(V square wave on an oscilloscope!)
Since the publication of ref. 3, an improvement of one order of magnitude was obtained and it turned out to be possible to measure heat
2
fluxes dmvn to ab out 0.06 BTU per ft and s. with an accuracy of
±
7%.This represents the electrical capability of the system. It is useful to
19 2
remember that a flux of 10 molecules per cm and s. at leV corresponds
2
approximately to 1/8 BTU per ft and s.
The calibration procedure provides the value of the k cf factor
for the substrate. Bulk properties of the glass used cannot be relied
upon because of the change of surface conditions. In 'order to obtain the
yalue of this factor, the gauge is pulsed electrically_ in air and in water
-and the values resulting from this technique are more accurate than the
ones obtained by the usual method. The whole procedure is based on a
linear theory.
accomodation coefficientat is taken to be unity.
The metal layer is obtained by the use of Hannovia liquid platinum paint and is covered by a .silicon monoxide or magnesium fluoride coating. If such coatings are used, it is found that the resistance of the layer aqd its thermal coefficient remain constant in time.
Prof. Scott. Do you use the gaugeonly for total flux measurements or can you mask it and measure angular distributions?
Dr. Skinner. The gauge is very satisfactory for total flux measurements but measurement of angular distributions of scattered beam would be very difficult with it.
Major Manei. What is the relative size of the gauge compared to the beam dimensions?
Dr. Skinner. Our present gauges have a 1 mm x 0.6 mm section but there is rio problem in making them in any dimensions.
Prof. Fenn. Does the coating (with silicon monoxide for instanee) modify in anyway thé gauge response?
Dr. Skinner. No, the response is so fast (o.Olfi~) that any increase of this time will make very little difference. Even silver coating over the oxide coating will not modify the response enough to interfere with
measurements made over 1 m sec. (This type of gauge is used in catalytic recombination 'vork) .
Dr, Hurlbut describes the differential ionization gauge and the surface ionization gauge detectors.
were designed for entirely different purposes.
The differential manometer gauge we used four years ago, and which is probably much less sophisticated than Prof. Scott1s apparatus,
could resolve 2 parts in 105 of the background pressure, so that the -11 .
resolution 't-lould be of the order of 2xlO . mm Rg in pressure differential (between shutter opened and shutter closed), for a background pressure
-6
around 10 mm Rg.
The detector is made of two relatively inexpensive gauges. The gauge reservoir behaves as an isothermal reservoir so that there is no ambiguity about the temperature of the gas effusing into the exit cone towards the extractor.
In scattering measurements, we were always operating at the limit of resolution, beca~se a good angular resolution requires small apertures.
The long resportse time of the gaugesprecludes the use of a chopped beam.
We feel that all other total intensity detectors for energetic beams (say in the range from 1 eV to 100 eV),. and particularly the fast response detectors, should be calibrated against a gauge of this general character.
Dr. Skinner. Could you give a rough graph indicating the time constant versus signal to noise relationship for this device;
Dr. Rurlbut. The response time is of the order of one second. Usually we wait for about five seconds between successive shutter opening.
Dr. Skinner. Would one such cycle provide an adequate measurement?
Dr. Rurlbut. One has to apply statistical methods to such measurements, and the procedure depends on the noise spectrum. I think that an integra-tion over 20 cycles per point would provide satisfactory results. This
represents a test duration of about 100 seconds per point. The corresponding
angular resolution would be about 0.5°.
Dr. Devienne. What quality of voltage supply stability does one r(quire
to reach the sensitivity quoted by Dr. Hurlbut?
Dr. Hurlbut. The gauge stability is almost entirely conditioned by the émission current stability. This stability is essential to obtain a sensitivity of 2xlO-S.
Prof. Scott. The heart of the system therefore lies in the emission current stabilization. Batteries are not satisfactory (they provide only a constant
voltage, not a constant current). One has to use au elaborate feedback control system.
Prof. Chuan. Does one get equilibrium conditions at the mouth of the detector in onè second?
Dr. Hurlbut. The lay time is one second but we use a five second interval to ensure reasonable approach to equilibrium conditions.
Prof. Scott comments about the differential ionization gauge detector. The principle of the device, at least as we understand it, is as follows:
the first gauge, which we call the beam gauge, is facing the beam and
collects the total beam flux plus the background flux. There is a duplicate
gaug~, facing away from the beam, which collects only the background flux. One then measures the difference between the ion currents from the two gauges, and this difference is a unique function of the beam flux. There is no problem of dumping, etc. This is essentially a steady state device.
There is, however, a big objection against the calibration of a differential gauge detector by means of a low temperature oven beam (with
an energy of the order of 0.01 eV), namely, because it is not clear whether this calibration can be extrapolated to beams with energies of the order of several eV.
Dr. Hurlbut describes the surface ionization detector (Taylor detector). This gauge, which is well known, is a useful tooi but not the
ultimate tool. It is based on the surface ionization occuring \-lhen the ionization potentialof the incident particle is less than the work function of the surface it encounters. Potassium, sodium and cesium have been used.
An interesting property of this detector has been discovered
,
recently. ,If th.e incident particle has sufficient energy to overcome the
adsorption.forces at the surface it encounters, then it can rebound as an ion, whatever tbe temperature of the surface. An incident potassium neutral, for instance, may rebound as a potassium ion. It used to be thougbt that
tbe surface must be at a certain temperature before the detector would work, but this fs not true if the surface is sufficiently clean to be free of adsorbed ~ayer.
Tbe fact that the indicent atoms must bave suff!cient energy to ove,rcome the adsorption forces has had some utility in connection with the sputtering experiments. It enabled us to observe, in function of the angle, tbe difference of intensity between the stream of all particles and the stream of particles having energies in excess of this threshold energy. We donft know what the threshold energy is. We can only estimate it as
being in excess of the adsorption energy, i.e. about 2.5 eV. We have, here, a method for dis~inguishing between particles with energy below or above a certain energy.
The Taylor detector is essentially 100% efficient, at least for potassium. For sodium, the efficiency goes up somewhere. This phenomenon is goverened by the SARA equation. It is found that the cold detector is about as effective as the hot detector for sodium. The hot detector is some\.;rhat more efficient for potassium. It is not easy to understand the
