sixteen resonator anode structure used in the early work b u t including the features of “packaging,” axial cathode m ount, and wave guide output.
A num ber of vexing b u t nevertheless interesting problem s was encountered.
Principal am ong these were the tendency to operate in another mode under certain conditions and the problem of obtaining satisfactory m agnetic field uniform ity in a design which necessitated the removal of a considerable
M A G N E T R O N 4 5 G E N E R A T O R OE C E N T I M E T E R W A V E S 327
I t m ight be surmised th a t the trouble in the unbroken strap case was due to a com ponent of n = 7 which did not couple to the o u tp u t circuit. The phenomenon was studied in an operating m agnetron with small electro
static probes built into several resonators. The n = 7 com ponents were identified, their relative intensities being approxim ately those expected.
It is of some interest to note th a t the “m oding” encountered here differed become v e iy im portant in determ ining the over-all efficiency. A noticeable improvement in unloaded Q was effected when an atm osphere of prepurified
- \T2 was substituted for the CCfi-alcohol m ixture which had previously been
used to prevent oxidation during the final brazing. T he CCfi-alcohoi method had been abandoned for another reason, namely, th a t chemical analysis showed carbon deposited on the steel pole-pieces underneath the copper plating.
If one determ ines the circuit efficiency for m atched load by impedance measurements on a non-oscillating m agnetron [see equation (25) of P A R T I], one m ay then calculate its value for any load. From m easurem ents of
increased, the decrease in R F voltage in the ir mode places th a t mode a t a disadvantage in com petition w ith the n = 7 mode.
As a result of these considerations, a new resonator system was designed in which the electronic conductance a t the norm al operating point was to be two thirds of th a t in the first design. T o m aintain the pulling figure invari
a n t, it was necessary to reduce the to tal resonator capacitance. A lthough all of this capacitance could n o t be removed from the strap s w ithout reducing ratio would result in two bad effects: first, a loss of m agnetic field and, second, an antibarreling of the m agnetic field which results in an axial force acting necessary for a nearly uniform field over 80 p er cent of the gap and a focusing field over the rem ainder, were determ ined in electrolytic tan k experiments.
Since the perm endur pieces fill up a p a rt of the hole in the m agnet pole piece and have a separation less than the pole gap, they contribute substantially
M A G N E T R O N A S G E N E R A T O R OF C E N T I M E T E R W A V E S 329
to the magnetic field in the anode-cathode region. T he effective gap is reduced from 0.380 in. to 0.340 in. by their presence, resulting in ab o u t a 20 per cent decrease in m agnet weight. While their prim ary function is magnetic, the perm endur pieces also serve as normal cathode end disks pre
venting electrons from reaching the pole pieces. T heir smooth contour which fill up the end spaces of a m agnetron having radial cathode m ounting.
An upper lim it to the cooling is set by the fact th a t the cathode m ust be raised to 1050°C in activation, using a heater which can be contained w ithin the cathode sleeve w ithout encroaching too much upon its wall thickness.
The high tem perature needed during activation sets certain lim its upon th e materials which m ay be used in the cathode structure. Since the cathode is mounted from one end only (this being dictated by assem bly considera
tions), mechanical strength is exceedingly im portant. A cathode, once off center, is subjected to m agnetic forces on the perm endur ends which glass and K ovar construction sufficiently rugged to make unnecessary any protective housing.
In the wave guide output designed for the 4J50 m agnetron, the necessary
impedance transform ation is accomplished by a quarter wavelength section opening directly into the o u tp u t wave guide a t one end and into the outside wall of one resonator a t the other (see Fig. 30 and discussion in P A R T I).
The small height of the resonator system from pole piece to pole piece makes it necessary to use a loaded line, in this case of H-shape cross section. The section. The circular opening, preferable for glass sealing, is a compromise, the critical diam eter being determ ined by experiment. The relatively large size of the window m akes it capable of w ithstanding R F voltage breakdown even a t very high power.
Control of the output coupling is most readily effected by varying the width of the slot in the H-section. T he over-all transform ation properties of the H-section have been analyzed theoretically and the results confirmed by m easurem ents on o u tp u t circuit models. By means of this analysis
M A G N E T R O N ,15 G E N E R A T O R OF C E N T I M E T E R W A V E S 331
milled into this piece on either end. This center slab is “capped” on each end by a slab of copper, in which are contained the m agnet pole pieces and appropriate surfaces on which to build the rest of the m agnetron. Some of the details of this structure m ay be seen in the cutaw ay model of Fig. 75.
D espite the large impedance transform ations involved, the o u tp u t cir
cuit is quite frequency insensitive. As it appeared th a t these m agnetrons might be made tunable, considerable atten tio n was paid to this characteris
tic. N um erous tests of the o u tp u t circuit and its com ponent p arts were
Fig. 73—An external view of th e d jS O "packaged” m agnetron (280 kw., 9375 m c/s).
Note the circular glass window in the wave guide o u tp u t circuit and rugged axial in p u t lead which requires no external p rotecting boot.
made over the S500-9600 m c/s band. T he iris and q u arter wave length guide section were studied theoretically in this regard as well. The trans
former properties of the window and choke coupling com bination is the most frequency sensitive p a rt of the entire o u tp u t circuit. By adjustm ent of the distance from the H-section to the window, it was found possible to cancel some of the sensitivity of the two parts.
External views of the 4J50 (4J78) and the 4J52 m agnetrons are shown m Figs. 73 and 74 respectively. The internal view of the 4J52 is shown in Fig. 75.