Development
of Advanced High Power Gyrotrons
for
EC H&CD Applications in Fusion Plasmas
B. Piosczyk1a, A.
Arnold2, E. Borie1a, G. Dammertz1a, O.
Dumbrajs3, R. Heidinger1b, S. Illy1a, J. Jin1a,
K.
Koppenburg1a, G. Michel4a, T. Rzesnicki1a, M.
Thumm1a2, D. Wagner4b, X. Yang1a
1Forschungszentrum
Karlsruhe, Association EURATOM-FZK,
(a) Institute for Pulsed Power and Microwave
Technology, (b) Institute for Materials Research I;
Postfach
3640, D-76021 Karlsruhe, Germany,
2University Karlsruhe, Institute of
High-Frequency Techniques and Electronics,
D-76128 Karlsruhe, Germany
3Department
of Engineering Physics and Mathematics, Helsinki University of Technology,
Association
EURATOM-TEKES, FIN-02150 Espoo, Finland.
4Max-Planck-Institut
fuer Plasmaphysik, Ass. EURATOM-IPP,
(a)
D-17491 Greifswald, Germany; (b) D-85748 Garching, Germany,
e-mail:
bernhard.piosczyk@ihm.fzk.de
The R&D activities at
Forschungszentrum Karlsruhe (FZK) on advanced high-power millimeter (mm)-wave
gyrotrons for future use in electron cyclotron heating and current drive
(EC H&CD) in magnetically confined fusion plasmas consist of: (1) the
development of a coaxial cavity gyrotron capable of delivering 2 MW continuous
wave (CW) at 170 GHz and (2) investigations on tunable multi-frequency
gyrotrons.
In the case of the coaxial cavity gyrotron
the feasibility of manufacturing multi-megawatt gyrotrons in continuous wave
(CW) operation has been demonstrated in proof of principle experiments at
pulses around 1 ms. Problems specific to the coaxial arrangement have been
investigated and information relevant for an industrial realization of a high
power coaxial gyrotron has been obtained. Based on these results the
development of a coaxial cavity gyrotron with an RF output power of 2 MW,
CW at 170 GHz, as could be used for ITER, started recently in cooperation
between EURATOM Associations (CRPP Lausanne, FZK Karlsruhe and HUT Helsinki)
together with European tube industry (Thales Electron Devices, Velizy, France)
as reported in another contribution at this conference (J.P. Hogge et al.).
To verify experimentally the design of the
main components (electron gun, cavity and RF output system) of the industrial
prototype, the previously used 165 GHz coaxial gyrotron at FZK has been
redesigned for operation at 170 GHz. The maximum magnetic field of the
superconducting magnet at FZK of only 6.667 T requires a reduction of the
operating voltage from 90 kV (as foreseen for the industrial prototype) to
80 kV. The cavity and the RF output coupler are identical as foreseen for
the industrial tube. The new design of the electron gun avoids regions in which
electrons can be trapped in order to avoid the built up of a Penning discharge.
All components are under fabrication. The experiment which is expected to start
in spring 2004 will allow investigation of RF generation and mode competition
as well as of the efficiency of the RF output system and measurement of the
amount of the internal stray radiation under relevant conditions.
In the case of step frequency tunable
gyrotrons the possibility of multi-frequency operation of a gyrotron designed
to oscillate in the TE22,8 mode at 140 GHz, the TE17,6
mode at 105 GHz and six other modes at frequencies in between is currently
under investigation. Key issues are here the
development of a broadband quasi-optical (QO) output system and the CVD-diamond
Brewster window. The QO mode converter of the
gyrotron consists of a dimpled-wall antenna (Denisov-type launcher) and a beam
forming mirror system. The first mirror is a large quasi-elliptical one, the
second and third are phase correcting mirrors with a non-quadratic shape of the
surface. The QO system has been optimized for broadband operation in the
various modes. A 140 mm diameter disk is being developed at Element Six (formerly
DeBeers Industrial Diamonds for a full-size diamond Brewster angle window).