DESIGN OF A 170 GHz, 4 MW COAXIAL SUPER GYROTRON
WITH DUAL-BEAM OUTPUT

M. Thumm^1,2, J. Jin¹, M. V. Kartikeyan³ ,B. Piosczyk¹, T. Rzesnicki¹

¹Forschungszentrum Karlsruhe, Association EURATOM- FZK, Institut fuer Hochleistungsimpuls- und Mikrowellentechnik (IHM), Postfach 3640, 76021 Karlsruhe, Germany

²Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,
Kaiserstr. 12, 76128 Karlsruhe, Germany

³Dept. of Electronics & Computer Engineering, Indian Institute of Technology Roorkee (ITER)
Roorkee 247 667 (UA); India

e-mail: 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 140-170 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 dual-beam output through two CVD diamond windows with a suitable dimpled-wall quasi-optical 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 quasi-optical launcher (that is with helical Dm₁ = 1 and Dm₂ = 5 wall perturbations for which m₂/2 = 360^o/f = 2.5, where f is the azimuthal spread angle). In this selection procedure only three well-qualified modes, namely, TE_44,26, TE_50,30 and TE_54,32 have been picked out. Calculations have been performed both with a single-mode self-consistent code (SELFC) and with a multi-mode time-dependent and self-consistent 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 Q-factor, 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 (q₁ = 3°, L₁ = 22 mm) and output
(q₃ = 2.5°, L₃ = 22 mm) tapers with rounded transitions.

Table 1: Cavity geometry and theoretical results

	TE44,26	TE50,30	TE54,32
Lcav (mm)	16	20	20
Rcav (mm)	39.75	45.69	49.03
Rin (mm)	10.6	12.2	13.4
Rb (mm)	12.88	14.6	15.74
QD	1595	2625	2666
Ub (kV)	117	130	140
Ib (A)	100	96	104
Bcav (T)	7.17	7.27	7.38
hel (%)	34.2	34.5	34.3
PRF (MW)	4	4.3	5
^*rcav kW/cm^2	2.0	1.8	1.8
^*rin 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 3.5-3.8 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).