DESIGN
OF A 170 GHz, 4 MW COAXIAL SUPER GYROTRON
WITH DUALBEAM OUTPUT
M. Thumm^{1,2}, J. Jin^{1}, M. V. Kartikeyan^{3 },B. Piosczyk^{1},
T. Rzesnicki^{1}
^{1}Forschungszentrum Karlsruhe,
Association EURATOM FZK, Institut fuer Hochleistungsimpuls und
Mikrowellentechnik (IHM), Postfach 3640, 76021 Karlsruhe, Germany
^{2}Universitaet Karlsruhe,
Institut fuer Hoechstfrequenztechnik und Elektronik,
Kaiserstr. 12, 76128 Karlsruhe, Germany
^{3}Dept. of Electronics & Computer
Engineering, Indian Institute of Technology Roorkee (ITER)
Roorkee 247 667 (UA); India
email: manfred.thumm@ihm.fzk.de
Recent experiments at FZK
and IAP Nizhny Novgorod suggest that coaxial cavity gyrotrons delivering in
excess of 2 MW power at frequencies ranging from 140170 GHz operating with
very high order volume modes can successfully be realized. In this work, the
feasibility of a super power coaxial cavity gyrotron at 170 GHz capable of
giving power around 4 MW, CW, operating in the ultra high volume modes TE_{44,26},
TE_{50,30} or TE_{54,32 }is presented as a small step towards a
big leap from 2 to 4 MW power levels. These modes are capable of giving a
perfect dualbeam output through two CVD diamond windows with a suitable
dimpledwall quasioptical launcher. This will reduce the technical
complexities connected with high diffraction losses (stray radiation) inside
the tube. The realization of such an ultra high power gyrotron will drastically
reduce the number of gyrotrons and corresponding superconducting magnets required
in ECRH systems of fusion reactor installations.
In the mode
selection procedure, such modes have been chosen, which will give an ideal dual
beam focussing at the quasioptical launcher (that is with helical Dm_{1} = 1 and Dm_{2} = 5 wall
perturbations for which m_{2}/2 = 360^{o}/f = 2.5, where f is the azimuthal spread
angle). In this selection procedure only three wellqualified modes, namely, TE_{44,26},
TE_{50,30} and TE_{54,32} have been picked out. Calculations
have been performed both with a singlemode selfconsistent code (SELFC) and
with a multimode timedependent and selfconsistent code (SELFT). In Table 1
the dimensions of the optimized coaxial cavities (L_{cav}, R_{cav},
R_{ins}) together with the beam electron radius (R_{b}), the
corresponding Qfactor, the operating parameters (U_{b}, I_{b},
B_{cav}), the electronic efficiency (h_{el}), the generated
microwave power (P_{RF}) and the peak Ohmic wall loading (r_{cav}, r_{in}) are summarized. The cavities
have linear input (q_{1} = 3°, L_{1}
= 22 mm) and output
(q_{3} = 2.5°, L_{3}
= 22 mm) tapers with rounded transitions.
Table 1: Cavity geometry and
theoretical results

TE_{44,26} 
TE_{50,30} 
TE_{54,32} 
L_{cav} (mm) 
16 
20 
20 
R_{cav} (mm) 
39.75 
45.69 
49.03 
R_{in} (mm) 
10.6 
12.2 
13.4 
R_{b} (mm) 
12.88 
14.6 
15.74 
Q_{D} 
1595 
2625 
2666 
U_{b} (kV) 
117 
130 
140 
I_{b} (A) 
100 
96 
104 
B_{cav} (T) 
7.17 
7.27 
7.38 
h_{el} (%) 
34.2 
34.5 
34.3 
P_{RF} (MW) 
4 
4.3 
5 
^{*}r_{cav} kW/cm^{2} 
2.0 
1.8 
1.8 
^{*}r_{in} kW/cm^{2} 
0.08 
0.06 
0.10 
As a result of the SELFT
simulations it has been found that all the three modes considered are
independently oscillating over a wide range of nominal parameters without
problems with other competing modes. The TE_{44,26} and TE_{50,30}
modes are capable of delivering powers up to 3.53.8 MW, CW only, if we
consider a tolerance limit of about 10 % of the computed output power. However,
the TE_{54,32} mode is well capable delivering around 4.5 MW, CW,
power at 170 GHz.
(^{*}enhancement
factor of 2.0 included).