Inverted Magnetron-Injection Electron Gun for
the Relativistic Gyroklystron at the University of
Maryland
M. E. Read1*, R. L. Ives*, G. Miram*, G. S. Nusinovich**,
W. Lawson**, B. Hogan**, and V. L. Granatstein**
*Calabazas Creek Research, Inc., Saratoga, CA
** IREAP, University of Maryland, College Park, MD
Abstract. As is known, coaxial circuits can provide a better mode selectivity than cylindrical
ones. They also allow for eliminating so-called 'cross-talk' between gyroklystron cavities
operating in high-order modes. Therefore, in present 100-MW scale relativistic gyroklystrons,
which are under development at the University of Maryland, coaxial structures are used.
Presently, inner conductors of these structures are supported by radial metallic pins that intercept
the electron beam. The use of inverted electron guns allows for avoiding this interception. In
the present paper a preliminary design of such an electron gun is described.
INTRODUCTION
There is a strong interest in developing high-power, millimeter-wave drivers for
future TeV-scale electron-positron linear colliders [1]. One of the main candidates for
delivering high-power, millimeter-wave radiation is the gyroklystron [2].
It is known that, in order to either increase the power of any microwave source
operating at a given wavelength or shorten the wavelength while maintaining a given
power level, it is necessary to operate in higher-order modes. In high-power
gyroklystrons, it is preferable to use coaxial microwave circuits, which not only allow
one to generate higher power, but also provide much more stable operation than
cylindrical circuits [2]. Such structures also mitigate the problem of voltage depression
in intense electron beams.
In present coaxial structures used in relativistic gyroklystrons developed at the
University of Maryland, an inner conductor is supported by thin radial pins, which
intercept the electron beam. This interception causes erosion of pins and, hence, limits
the operational time. Such an interception can be avoided when an inverted electron
gun is used.
1
Author contact: [email protected]
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
416
Anode
Mod Anode / Center conductor
FIGURE 1. Schematic of an inverted magnetron gun, where the modulating anode is at ground
potential and can be used to support the inner conductor of a coaxial cavity.
INVERTED GUN:
HISTORY AND SPECIFIC FEATURES
A schematic of an inverted magnetron-injection electron gun is shown in Fig. 1.
Here the emitter is located at the surface of a cathode whose dimensions are larger
than those of an inner electrode, which plays the role of an anode. This inner
electrode can be extended into the interaction region where it can play a role of an
inner coaxial conductor. As shown in Fig. 1, this electrode is grounded while a highvoltage pulse of negative polarity is applied to the cathode. In principle, this electrode
can also be supported from the collector side.
Although such inverted guns were used in Russian high-power coaxial gyrotron
oscillators developed for electron cyclotron plasma heating in tokamaks for a long
time, possibly, these guns were first described in detail in Ref. 3. Presently, 2 MW,
165 GHz coaxial gyrotron oscillators with inverted guns are under active development
for plasma applications in Europe [4]. Such coaxial gyrotron oscillators are also under
study in the U.S. [5,6].
Such coaxial devices allow selective excitation and stable operation even in such
high-order modes as TE3i5i7 [4]. With such modes, the alignment of the inner
conductor is critical. The ability provided by the inverted MIG for support of the inner
conductor at both ends aids in this alignment. Also note that, due to the inverted
geometry, the electric field strength at the emitter surface is weaker than in the gun of
a 'normal' geometry with the same diameters of the two electrodes.
PRELIMINARY DESIGN OF AN INVERTED GUN FOR
RELATIVISTIC GYROKLYSTRON
Relativistic gyroklystrons (GKLs) for driving future accelerators utilize intense
electron beams having operating voltage about 500 kV, beam current up to 600 A and
orbital-to-axial velocity ratio in the range from 0.9-1.0 to 1.4-1.5 [2]. These voltages
and currents are much higher than those used in gyrotron oscillators developed for
417
plasma heating and current drive. (Typically, these oscillators operate at voltages
below 100 kV and currents not exceeding 50-60 A.) Therefore, a design of inverted
magnetron injection electron gun (MIG) for a relativistic GKL was a real challenge,
particularly in that it was impossible to use simple scalings of existing inverted guns.
In addition to the standard goal of designing any MIG, which is to provide a desired
orbital-to-axial velocity ratio and minimize electron velocity spread, there were, at
least, two important restrictions. These are the electric field strength, which at the
emitter surface should not exceed 150 MV/m [7], and relative compactness of the gun
whose overall dimensions should be consistent with dimensions of the existing MIG.
Also, it was assumed that the cathode current density should not exceed 5-6 A/cm2,
which is the density in present emitters used in relativistic GKLs.
In order to meet all these requirements, the decision was made to design a triodetype MIG, in which an inner conductor plays a role of the modulating anode at ground
potential. The desired beam was 540 A at 460 keV, with an orbital-to-axial velocity
ratio about 1.5 and an axial velocity spread about 7.5%. Electron trajectories of an
initial design are shown in Fig. 2. The modulating anode is at ground potential. The
radius of inverted cathode is 8.8 cm and the mod-anode radius is 3.7 cm. The cathode
current density required from this cathode is about 4.6 A/cm2, the cathode slant length
is 2.1 cm and the cathode slant angle is 25°.
10
30
40
70
Axial Coordinate (cm)
FIGURE 2. Trajectories for the inverted MIG, as calculated using EGUN. Results are preliminary.
A 3-D model of the gun is shown in Fig. 3. The modulating anode is supported
through the cathode via the (grounded) shell of the gun. External positioners (not
shown) will be used to insure alignment.
The electric field magnitudes, as calculated using Maxwell 3-D are shown in Fig. 4.
As expected, the highest field is on the modulating anode / inner conductor, and has a
value of about 150 kV/cm. An effort will be made to reduce this in the final design.
418
Ground Shell
Mod Anode
Cathode
Support
Anode
Mod Anode
Support
Cathode
FIGURE 3.
3. Inverted
Inverted MIG
MIG for
for aa KU-band
KU-band gyroklystron.
gyroklystron. The
The inner
inner conductor
conductor of
FIGURE
of the
the coaxial
coaxial
gyroklystron will
will be
be connected
connected to
to the
the mod
mod (intermediate)
(intermediate) anode
anode for
gyroklystron
for support.
support.
l,5QQQe+OQ7
1.3500e+007
1.2QOOe+pQ7
l..Q500e+QQ7
9.0000e+006
7.5 00 Qe+0.0 6
6.0000e+006
4,5DOOe+006
3.0000e+006
1.SOOOe+006
O.OOOOe+000
.
FIGURE 4. Electric field magnitudes, in V/m, for the initial design of the inverted MIG.
FIGURE 4. Electric field magnitudes, in V/m, for the initial design of the inverted MIG.
419
ACKNOWLEDGMENTS
This work is supported by the U.S. Department of Energy.
REFERENCES
1. Wilson, P.B., "Applications of High-Power Microwave Sources to TeV Linear Colliders", Chapter 7
in "Applications of High-Power Microwaves", Eds. A.V. Gaponov-Grekhov and V.L. Granatstein,
Artech House, Boston, 1994.
2. Granatstein, V.L. and Lawson, W., IEEE-PS 24, 648 (1996).
3. Lygin, V.K. et al., Int. J. Electron 79, 227 (1995).
4. Kuntze, M. et al. ICOPS-2002, May 26-30, 2002, Banff, Alberta, Canada, IEEE Conference RecordAbstracts, Paper 1B0102, p. 96.
5. Read, M.E. et al., IEEE-PS 24, 586 (1996).
6. Advani, R., Hogge, J.P., Kreischer, K.E., Pedrozzi, M., Read, M.E., and Temkin, R.J.,
"Experimental Investigation of a 140 GHz Coaxial Gyrotron Oscillator," IEEE-PS 29, 943 (2001).
7. Phillips, R.M. and Sprehn, D.W., Proc. IEEE 87, 738 (1999).
420