An RF transmitting tube for the 20 GHz to 500 GHz range comprises a gyrotron and a multistage depressed collector. A winding provides a magnetic field which acts on spent, spinning or orbiting electrons changing their motion to substantially forward linear motion in a downstream direction. The spent electrons then pass through a focusser into the collector. Nearly all of the electrons injected into the collector will remain within an imaginary envelope as they travel forward toward the end collector plate. The apertures in the collector plates are at least as large in diameter as the envelope at any particular axial position

Kosmahl, H. G.

English

NASA Technical Reports Server

tes Patent [191 11 11 
WI  
[54] GYROTRON 'BTRANSMIITING TUBE 
[75] Inventor: Henry G. Kosmahl, Olmsted Falls, 
Ohio 
[73] Assignee: The United States of America as 
represented by the United States 
National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 220,212 
[51] Int. Q.3 .............................................. WQ19 25/00 
[52] US. 6;1. ........................................... 315/4; 315/3; 
315/5; 315/5.35; 315/5.38 
[58]  Field of Search .................. 315/3, 4, 5,  5.35, 5.38 
P61 References Cited 
[22] Filed: Dec. 24, 1980 
U.S. PATENT DOCUMENTS 
3,302,053 VI967 Udelson ......................... 315/5.38 X 
3,433,992 3/1969 Tancredi et al. .............. 315/5.38 X 
3,463,959 8/1969 Jory et al. ............................... 315/4 
3,614,516 10/1971 Phillips ............................... 3W5.35 
3,764,850 10/1973 Kosmahl ............................ 315/5.38 
4,207,495 6/1980 McDowell ......................... 3W5.38 
Primary Examiner-Saxfield Chatmon, Jr. 
Attorney, Agenr, or Firm-Norman T. Musial; John R. 
Manning; James A. Mackin 
~571 ABSTMrn 
An improved R.F. transmitting tube for the 20 GHz to 
500 GMz range comprises a gyrotron (IO) and a multi- 
stage depressed collector (11) as shown in FIG. I. A 
winding (19) provides a magnetic field which acts on 
spent, spinning or orbiting electrons changing their 
motion to substantially forward linear motion in a 
downstream direction. 
The spent electrons then pass through a focusser (18) 
into the collector (11). Nearly all of the electrons in- 
jected into the collector will remain within an imagi- 
nary envelope designated by idashed lines 8 and 9 as 
they travel forward toward the end collector plate (26). 
The apertures in the collector plates (22-25) are at least 
as large in diameter as the envelope (8,9) at any particu- 
lar axial position. 
FIG. 2 illustrates magnetic field strength from the cath- 
ode (13) of FIG. B to the collector entrance in focusser 
18. 
8 Claims, 2 Drawing Figures 
FIG. 2 
https://ntrs.nasa.gov/search.jsp?R=19830023681 2020-03-22T01:21:08+00:00Z
.s. Patent JUP. 26, 1983 
4,395,656 Lh 1 
GYRQTRON TRANSMITTING TUBE 
DESCRIPTION 
ORIGIN OF THE INVENTION 
This invention was made by an employee of the 
United States Government and may be manufactured or 
used by or for the Governmen without the payment of 
any royalty thereon or therefor. 
TECHNICAL FIELD 
This invention relates to radiofrequency transmitting 
tubes operating in the 20 GHz to 500 GNz range and is 
directed more particularly to a gyrotron type oscillator 
of amplifier tube. 
Since the beginning of the space age, many communi- 
cation satellites have been placed in geosynchronous 
orbit above the earth. Recent evaluations of satellite 
communications indicate that in the coming decades 
there will be such an increasing demand for satellite-to- 
earth communications that the capacity limits of the 
frequency bands of presently used satellites will be ex- 
ceeded. 
In order to transmit increasing amounts of informa- 
tion, it will be necessary to go to higher radio frequency 
(R.F.) transmission bands. Satellite transmitters operat- 
ing in the 30/20 GHz range are presently under devel- 
opment. However, it is expected that frequencies will 
eventually reach the 100 GWz to 500 GHz range and 
even beyond. 
A device capable of generating electromagnetic radi- 
ation in the 20 GHz to 500 GNz range is the gyrotron. 
Gyrotrons are presently used for R.F. heating; for ex- 
ample in fusion and plasma work and for military appli- 
cations. 
In a gyrotron, electrons emitted by a cathode are 
forced by a magnetic field to follow generally helical 
paths as they move through the field in a direction away 
from the cathode. As the electrons leave the field with 
which they have interacted to generate R.F. electro- 
magnetic waves, the diameters of the paths of their 
angular rotation or orbits increase dramatically making 
them very difficult, if not impossible, to collect with 
reasonable efficiency. 
The reason it is impractical to conyert an electron’s 
azmuthal kinetic energy T= 8. m(r.O)2 into electrical 
energy efficiently is because 8 which is the angular 
velocity of a given electron is determined entirely by 
the local magnetic field. However, the energy of the 
particular electron also depends strongly on the radius 
of its orbit. Thus, for a given 0 azmuthal kinetic energy 
T is proportional to the orbit radius squared at Z=a 
constant where Z is a given magnetic potential. Conse- 
quently it will be seen that it is impractical to place 
electrodes for extracting rotational energy from spent 
spinning electrons at a given magnetic potential 
Z=constant. 
BACKGROUNDART 
U.S. Pat. No. 3,702,951 to Kosmahl incorporated 
herein bv reference discloses a high efficiencv multi- 
L 
plate being conical and having a spike extending toward 
the source of charged particles. 
U.S. Pat. No. 3,764,850 to Kosmahl also incorporated 
herein by reference discloses an electron beam control- 
5 ler in which a magnetic field of predetermined intensity 
and shape is applied to a spent electron beam to refocus 
the spent electrons prior to their injection into a collec- 
tor. 
U.S. Pat. No. 3,463,959 to W. R. Jory et a1 discloses a 
10 charged particle accelerator apparatus including means 
for converting a rotating helical beam of charged parti- 
cles having axial motion into a non-rotating beam of 
charged particles. This is accclmplished by utilizing 
crossed electric and magnetic fields. The energy of the 
l5 beam is not changed while the type of motion is substan- 
tially altered. 
U.S. Pat. No. 4,199,709 to Jean-Louis Alirot et a1 
discloses an injector for an annular beam of monokinetic 
electrons in helical orbits having a high inclination 
2o angle relative to the axis of the helix of the type having 
an annular electron gun in a revolving vacuum enclo- 
sure. 
None of the above patents are concerned with con- 
verting the azmuthal energy of spiralling electrons into 
25 longitudinal kinetic energy and injecting the electrons 
into a collector at high efficiency. 
DISCLOSURE OF THE INVENTION 
In accordance with the present invention, there is 
30 provided a gyrotron device having a multistage de- 
pressed collector. The magnetic field imposed on elec- 
trons in the gyrotron causes them to take generally 
helical paths with the diameter of the orbit of each 
35 electron being relatively constant in the magnetic field. 
As the spiralling electrons leave the magnetic field, they 
encounter a refocusing magnetic field which tapers 
from the B value of the first magnetic field to substan- 
tially zero over a distance of from 1 to 3 wavelengths. 
40 The orbital or spiralling motion of the electrons is con- 
vered into substantially linear motion in order that they 
may be injected into a multistage depressed collector at 
high efficiency. 
BRIEF DESCRIPTION OF THE DRAWINGS 
A C  .vJ 
The details of the invention will be described in con- 
nection with the accompanying drawings in which 
FIG. 1 is a schematic, longitudinal section of a gyrotron 
embodying the invention. FIG. 2 is a graph of the mag- 
50 netic field strength along the center of the gyrotron in a 
direction moving from left to right as viewed in the 
drawings. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
Referring now lo FIG. I, there is shown in accor- 
dance with the invention a schematic drawing of a 
gyrotron transmitting tube IO and a multistage de- 
pressed collector ll. The gyrotron tube 16) is comprised 
61) of a cylindrical tube 12 having an electron emitting 
cathode I3 at one end. A current carrving solenoid 
55 
stage depressed collector for use with a s&ce of winding I4 establishes a magnetic fieli &h which 
charged particles such as spent electrons. Each elec- electrons emitted by cathode 13 interact to produce 
trode plate of the collector has a central aperture with radio frequency energy which is transferred to a suit- 
the apertures increasing in diameter in a downstream 65 able antenna (not shown) by means of an RF output 
direction from the source of charged particles. The coupler 16. The double ended arrow 15 defines an inter- 
electrode plates are generally bowed towards the action region which is the axial length of the magnetic 
source of charged particles with the final electrode field provided by solenoid 14. 
4,395,656 
At the other end of the gyrotron tube 10 is an end 
wall 17 which includes a ring of magnetic material 18 
through which electrons are injected into the collector 
11. A focusing coil 19 disposed around the gyrotron 
tube 10 between the solenoid 14 and the collector 11 
provides a magnetic field, the strength of which tapers 
from a value B equal to that of the solenoid field to 
substantially zero at the focusing ring 18. 
The refocusing field is maximum at the upstream end 
of the solenoid field nearest the cathode as indicated by 
line 20 and declines to zero over the axial distance indi- 
cated by the double-ended arrow 21. The strength and 
shape of the refocusing field is in accordance with the 
teachings of U.S. Pat. No. 3,764,850 for a beam control- 
ler. The taper of the refocusing field depends on mag- 
netic field strengths and electron velocities and trajecto- 
ries to name a few parameters. Thus, the taper can be 
computed according to U.S. Pat. No. 3,764,850. 
The collector 11 is similar to that shown in U.S. Pat. 
No. 3,702,951 and includes bowed collector plates 22, 
23,24 and 25, each of which has a central aperture. The 
end collector plate 26 is of conical shape and includes a 
spike 27 extending toward cathode 13 along the central 
axis of the tube 10 and the collector 11. 
The required electrical potentials for electrodes 22 
through 26 may be supplied from a voltage divider 28 
by means of leads 29, 30, 31, 32 and 33, respectively. 
Negative voltage V, is applied to the voltage divider 28 
at terminal 34 while the other end of voltage divider 28 
is grounded as at 35. The cathode 13 of the gyrotron 
tube 10 is supplied with a negative potential via a lead 
36 connected to a point on the voltage divider near its 
most negative end. The voltage divider 28 is shown 
only for purposes of clarity. It would be preferable to 
use taps on a transformer winding for greater efficiency. 
Referring now to FIG. 2, there is shown a graph of 
the magnetic field strength in a direction from the cath- 
ode 13 of FIG. 1 to the focusing ring 18. The line Bo 
representing magnetic field strength increases as shown 
between the cathode and the upstream end of the sole- 
noid field over the distance represented by the double- 
ended arrow 37 and remains constant over the length of 
the solenoid field which is represented by the double- 
ended arrow 38. 
The magnetic field of the refocusing coil 19 decreases 
from the solenoid field Bo value to substantially zero 
between the solenoid field and the focusing ring, which 
distance is represented by double-ended arrow 39. The 
distance represented by arrow 39 is from one to three 
wavelengths as determined by the cyclotron frequency 
generated by the gyrotron tube 10. 
Referring again to FIG. 1, the dashed lines 8 and 9 in 
collector 11 represent an envelope within which ideally 
all but generally 90 percent of the spent electrons in- 
jected into collector 11 will be contained as long as they 
are moving forward toward collector plate 26. Accord- 
ingly, it will be seen that the central aperture in each of 
the electrode plates 22 through 25 should be slightly 
greater than the envelope so that spent electrons do not 
impinge on the cathode-facing surfaces of those elec- 
trodes. 
As the electrons fall back toward the focusing ring 
18, they will take paths outside of the envelope repre- 
sented by lines 8 and 9 and be captured on the back 
surfaces of the electrodes 22 through 25. The operation 
of the above-described embodiment of the invention 
may be more fully understood from the following dis- 
cussion. 
4 
4 
The potentiakenergy of a magnetic dipole h4 in a 
magnetic field B issiven by: U=G.$. When the field 
changes slowly, M is adiabatically invariant, e.g. 
I MI =consta$. Now the_*force on a dipole in a mag- 
5 netic field is F= VU = V(M.@ = GV.3. For an electrqn 
in a field B, M=emr2;Tf where 7=(2~r/rO)=(2~/0) 
l/fc, T being the circulation time of an electron for one 
orbit and n' representing a vector normal to the plane of 
orbit. If Bo decreases slowly to B-0 predominantly i z  
10 the Z direction Bz>>Br then almost all of U=G.B 
will be converted into 4 m.Z2 if the decrease is over a 
distance Z equals from Ih to 3A. 
Thus, by imposing a tapering magnetic field on spent, 
spiraling electrons over a distance of from lh to 31, the 
15 motion of the electrons becomes generally longitudinal 
along an axis centered in an imaginary envelop within 
which nearly all the spent, spiralling electrons lie. 
It will be understood that changes and modifications 
may be made to the above-described invention without 
20 departing from its spirit and scope, as set forth in the 
claims appended hereto. 
I claim: 
1. An electromagnetic wave generator operating in 
a gyrotron tube having an electron emitting cathode 
at one end; 
an electron collector disposed at the other end of said 
gyrotron tube, said other end having disposed 
thereat a focusing ring through which electrons are 
a solenoid disposed around said gyrotron tube estab- 
lishing a first magnetic field which causes electrons 
emitted by said cathode to follow helical paths; and 
means for establishing a second magnetic field be- 
tween said first magnetic field and said focusing 
ring, said second magnetic field being character- 
ized by decreasing from the B value of said first 
magnetic field to substantially zero over an axial 
distance of from Ih,to 3A,in a direction away from 
said cathode, and at such a rate as to establish opti- 
mum conversion of spiral energy of the spent elec- 
trons into forward motion. 
2. The generator of claim 1 wherein said focusing 
ring is a coil having a d-c current flowing therethrough. 
3. The generator of claim 1 wherein said means for 
establishing a second magnetic field comprises a coil 
disposed around said gyrotron between said solenoid 
and said collector. 
4. The generator of claim 1 wherein said collector is 
5. The generator of claim 4 wherein said multistage 
depressed collector comprises a plurality of centrally 
apertured plates bowed toward said cathode and a coni- 
cal end electrode having a spike extending from its apex 
6. The generator of claim 5 wherein the aperture 
diameters increase at a rate at least as great as the rate of 
expansion of an imaginary envelope within which 90 
percent of the electrons injected into said collector 
60 would be contained before falling back onto the collec- 
tor plates. 
7. The generator of claim 1 wherein said means for 
establishing a second magnetic field comprises a perma- 
nent magnet surrounding said gyrotron. 
8. The generator of claim 1 wherein said means for 
establishing a second magnetic field comprises a super- 
conducting magnet. 
the 20 GHz to 500 GHz range and comprising: 
25 
30 injected into said collector; 
35 
40 
45 
50 a multistage depressed collector. 
55 toward said cathode. 
65 
* * * * *  


Gyrotron transmitting tube

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