High efficiency gyrotron oscillator and amplifier

An electron cyclotron maser high-frequency generator/amplifier having a cavity resonator positioned in an axial magnetic field. A spiralling beam of relativistic electrons is injected into the cavity and stimulated emission of radiation by the electrons takes place at the frequency of a wave mode supported by the cavity resonator. The transverse efficiency, defined as the average electron energy loss divided by its initial transverse energy, is maximized by disposing a shaped iron collar about the cavity resonator to provide an axial magnetic field in the cavity resonator whose amplitude increases in the beam direction-of-travel. In an alternative embodiment, the transverse efficiency is maximized by tapering the inner wall of the cavity in the axial direction to provide a wave-mode in the cavity resonator whose electric-field amplitude increases in the beam direction-of-travel.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for generating microwave and 
millimeter wave radiation by stimulating the coherent emission of 
cyclotron radiation from a beam of free electrons. 
The relativistic electron cylotron master (commonly called a gyrotron) is a 
microwave device based on the cyclotron maser interaction between an 
electromagnetic wave and an electron beam in which the individual 
electrons move along helical trajectories in the pressure of an applied 
magnetic field. In the gyrotron oscillator the electron beam sustains a 
constant-amplitude normal-mode oscillation in an open-end cavity. 
The primary motivation for achieving a high-efficiency gyrotron is 
connected with its application in controlled-fusion research. To reach the 
fusion ignition temperature, a great amount of energy (many megajoules) 
has to be injected for plasma heating. Furthermore, this should be done 
with the maximum efficiency in order to alleviate the energy break-even 
condition. A highly efficient gyrotron has been recognized as one of the 
most promising sources to meet these requirements. 
Two commonly used definitions of efficiency need to be distinguished. The 
overall efficiency (.eta.) is defined as the average electron-energy loss 
divided by its total initial energy, and the transverse efficiency 
(.eta..perp.) is the same quantity divided by the initial transverse 
energy. 
Many methods for efficiency enhancement have so far been considered. 
However, in the past, the maximum transverse efficiency was limited to 
about 40 percent. 
SUMMARY OF THE INVENTION 
It is therefore one object of the present invention to improve a 
relativistic electron cyclotron maser. 
It is another object of the present invention to enhance, in a relativistic 
electron cyclotron maser, the efficiency of energy-transfer from the 
transverse motion of the electrons to an electromagnetic wave. 
The object of the present invention are achieved by incorporating 
efficiency-enhancement means in a relativistic electron cyclotron maser. 
The cyclotron maser has a cavity resonator positioned in an axial magnetic 
field to receive a spiralling beam of electrons. The cyclotron maser 
produces stimulated emission of radiation by the electrons at the 
frequency of a wave mode supported by the cavity resonator. The beam of 
electrons passes through a phase-bunching region and an energy extraction 
region in the cavity resonator. The efficiency enhancement means functions 
to set a bunching parameter .alpha. much less than unity in the 
phase-bunching region of the cavity resonator to maximize the 
phase-bunching of the electrons in the beam, and to cause the bunching 
parameter to increase in the energy extraction region of the cavity 
resonator so that the bunched electrons in the beam reach their minimum 
energies at the same time. In the disclosed relativistic electron 
cyclotron maser the maximum efficiency of energy transfer from the 
transverse motion of the electrons to an electromagnetic wave mode within 
the cavity can be as much as 100 percent. 
The foregoing, as well as other objects, features and advantages of the 
present invention will become more apparent from the following detailed 
description when taken in conjunction with the appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1a is a diagrammatic view of a conventional relativistic electron 
cyclotron maser. Cyclotron masers of this type are well known to those 
skilled in the art. The particular cyclotron maser illustrated in the 
drawing is disclosed in U.S. Pat. No. 3,398,376 which was issued to J. L. 
Hirshfield on Aug. 20, 1968, although the present invention is applicable 
to virtually any type of relativistic electron cyclotron maser having the 
general configuration illustrated. 
Referring again to FIG. 1a, the cyclotron maser shown employs an evacuated 
tube 18 surrounded by means such as solenoidal windings 19 for producing 
an axial magnetic field B whose direction is indicated by an arrow 3. FIG. 
1b shows the variation of the magnetic field B along the axis of the tube 
18. A relativistic electron beam source is axially disposed within the 
tube 18. While the source may take a variety of forms, it may conveniently 
take the form illustrated of an electron gun 10 and a twisted transverse 
magnetic field whose direction is indicated by arrows 11, called a 
corkscrew, for producing an axial beam of spiralling electrons. The 
electrons cross the magnetic hill shown in FIG. 1b into a microwave cavity 
resonator 12 with most of their kinetic energy transverse to the axial 
magnetic field. In the resonator 12, the electrons drift through a uniform 
region of the axial magnetic field, shown in FIG. 1b, whose value is 
selected such that the cyclotron frequency of the electrons entering the 
cavity, i.e., the frequency of rotation of the individually spiralling 
electrons in the beam, is slightly lower (by a few percent) than the 
frequency of the electromagnetic wave mode within the cavity. In a 
classical picture, a net transfer of energy from the transverse motion of 
the electrons to the electromagnetic wave mode results from phase-bunching 
of the electrons in their cyclotron orbits. The electrons, which are 
initially randomly phased upon entering the resonator, become bunched in 
phase after drifting through a phase-bunching region of the resonator. 
Phase-bunching occurs because the cyclotron frequency depends on the 
relativistic electron mass. Electrons that absorb energy from the cavity 
fields become heavier and slip back in phase; conversely, those that lose 
energy advance in phase. Following the phase-bunching region, the 
electrons pass into an energy-extraction region of the resonator where the 
phase of the electrons and that of the cavity fields favors deceleration 
of the bunched electrons. The bunched electrons are kept decelerating by 
the cavity fields, and, losing their kinetic energy, they transfer this 
energy to the electromagnetic wave mode within the cavity. Following the 
resonator 12, the electrons are collected at a water-cooled electrode 13. 
The apparatus illustrated in FIG. 1a can also be employed as an amplifier. 
For this purpose, the microwave cavity 12 is connected to one arm of a 
circulator 14. Amplification of microwave power from a signal source 15 
reflected from the cavity resonator 12 is then observed in the other T 
leg. FIG. 1c illustrates this modification. 
In the prior-art relativistic electron cyclotron masers as typified by the 
above-mentioned patent, the maximum intrinsic efficiency of energy 
transfer from the transverse motion of the electrons to an electromagnetic 
wave (.eta..perp.) is at most 40 percent. 
Before entering into the detailed description of preferred embodiments of 
the present invention according to accompanying FIGS. 2a, 2b and 3 of the 
drawing, the theory of the present invention will be explained 
hereinafter. A bunching parameter .alpha. can be defined as a function of 
axial coordinate in the resonator: 
##EQU1## 
wherein: .epsilon.=the dimensionless ratio of the cavity-wave-mode, 
electric field amplitude E to the magnetic field amplitude B at the 
entrance to the resonator. 
c=speed of light (2.998.times.10.sup.10 cm/sec) 
r.sub.o =Larmor radius of the electrons at the entrance to the resonator, 
in cm. 
.DELTA..omega.=frequency mismatch between the Doppler-shifted cavity wave 
mode and the cyclotron frequency of the electrons, in sec.sup.-1. 
Reference may be had to NTIS Publication ADA 069461 entitled "The 
Non-Linear Theory of Efficiency Enhancement in the Electron Cyclotron 
Maser", herein incorporated by reference, where it is shown that the 
maximum intrinsic efficiency of energy transfer from the transverse motion 
of the electrons to the electromagnetic wave mode in the cavity 
(.eta..perp.) can be dramatically increased by: 
(1) making .alpha.&lt;&lt;1 in the phase-bunching region to maximize the bunching 
of electrons in phase space; and then 
(2) increasing .alpha. in the energy-extraction region to a value of the 
order of unity so that all of the bunched electrons reach their minimum 
energies at the same time. This can be accomplished either by: 
(1) providing an axial magnetic field B whose amplitude in the cavity 
resonator increases in the beam direction-of-travel across the resonator 
(to make .DELTA..omega., which depends on B through the cyclotron 
frequency, a decreasing function of axial coordinate); or 
(2) providing an electromagnetic wave mode supported by the cavity 
resonator whose electric-field amplitude increases in the beam 
direction-of-travel across the resonator (to make .epsilon. an increasing 
function of axial coordinate). 
FIG. 2a is a diagrammatic view of a first embodiment of a relativistic 
electron cyclotron maser according to the principles of the present 
invention. A number of the structural components depicted in FIG. 2a may 
be identical to elements previously described above. For ease of 
comparison, these components are identified in FIG. 2a with the same 
reference numerals as employed above. These components include evacuated 
tube 18, magnetic field producing means such as solenoid windings 19, a 
relativistic electron beam source such as electron gun 10 and corkscrew 
field 11, microwave cavity resonator 12, and water-cooled electrode 13. It 
is noted that the features and structure of these components form no part 
of the present invention and further details thereof can be found in the 
above-mentioned U.S. Pat. No. 3,398,376 whose disclosure is herewith 
incorporated by reference. In accordance with the invention, the axial 
magnetic field B is caused to increase in the cavity in the beam direction 
by known methods such as the interposition of iron circuits or by varying 
the number of solenoid windings per unit length. A specific illustrative 
structure for accomplishing this is shown in FIG. 2a and comprises a 
shaped iron collar 20 that encircles the cavity resonator 12. Iron collar 
20 reduces the magnetic field B in the resonator due to the magnetic 
shielding provided by the permeable material of the collar. The inner 
radius of the collar 20 is tapered so that B is small near the beam 
entrance plane and large near the beam output plane. The plot of FIG. 2b 
of the drawing shows the axial variation of the DC magnetic field B of 
FIG. 2a. The magnetic field profile is an example taken from the 
above-referenced NTIS publication ADA 069461 and is merely illustrative of 
one manner of varying the magnetic field B. It is to be understood that 
the magnetic field can be varied in any manner such that B increases in 
the beam direction-of-travel across the cavity resonator. 
FIG. 3 is a diagrammatic view of a second embodiment of the relativistic 
electron cyclotron maser according to the principles of the present 
invention. Again, structural components which are identical to elements 
previously described are identified with the same reference numerals as 
employed above. In accordance with the invention, the electric-field 
amplitude E of the wave mode supported by the cavity resonator 12 is 
caused by known methods to increase in the beam direction-of-travel across 
the resonator. A specific illustrative structure for accomplishing this is 
shown in FIG. 3 and comprises substituting for resonator 12 a cavity which 
has an inner wall whose radius is tapered in the axial direction such that 
E is small near the beam entrance plane and large near the beam output 
plane. 
The specific details of the manner of otherwise constructing the iron 
collar 20 or the tapered cavity 12 form no part of the invention itself 
and are within the level of ordinary skill in the art. 
In the embodiments of the present invention illustrated in FIGS. 2a, 2b and 
3, the variation of the magnetic field B and the wave mode electric-field 
amplitude E are such as to make .alpha.&lt;&lt;.vertline. in the phase-bunching 
region, and to increase .alpha. in the energy-extraction region, so that 
the maximum intrinsic efficiency of energy transfer from the transverse 
motion of the electrons to the electromagnetic wave mode within the cavity 
can be dramatically increased to as much as 100 percent. Reference may be 
had to the above-referenced NTIS Publication ADA 069461 for further 
mathematical analysis. In particular, it is shown in that reference for a 
model where .alpha. is made equal to 0.077 in the phase-bunching region, 
and increased to a value of 0.31 in the energy extraction region, one 
obtains a theoretical transverse efficiency of 75 percent. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.