RADIOFREQUENCY WAVES, HEATING AND CURRENT DRIVE IN MAGNETICALLY CONFINED PLASMAS
6.4. GYROTRONS FOR ECR HEATING AND CURRENT DRIVE
6.4.5. State of the art gyrotrons
The required power of ECH/ECCD systems will increase as future fusion devices will increase both in terms of size and in pulse length (CW). Table 6.2 presents the parameters of some of the major, multi-MW ECH systems that are currently in operation or planned in the near term. In order to meet the demands for high power plasma heating, present day gyrotrons are being designed, built and tested for operation at a power level of at least 1 MW and at frequencies in the 110–170 GHz range. The largest operating ECH system is the 6 MW, 110 GHz gyrotron system at General Atomics (GA) in San Diego.
TABLE 6.2. PARAMETERS OF MULTI-MW ECH SYSTEMS Plasma
device Institution ECH power
[MW] Frequency [GHz] Pulse
duration [s]
Gyrotron developer ITER
tokamak (under develop- ment)
ITER
organization 24 170 1000 JAEA/
Toshiba, GYCOM, EU ITER TEAM W7-x
stellarator (under develop- ment)
IPP
Greifswald 10 (10 × 1.0) 140 CW Thales (9), CPI (1)
DIII-D
tokamak GA 6 (6 × 1.0) 110 10 CPI
JT-60
tokamak JAEA 4 (4 × 1.0) 110 5 JAEA/
Toshiba LHD stellarator NIFS 2 (2 × 1.0)
0.9 (2 × 0.45) 0.8/0.2 1 (2 × 0.5)
7782.7 84168
52 3/CW1
GYCOM, Toshiba
ASDEx-U tokamak IPP
Garching 4 (4 × 1.0) 140/105 10 GYCOM TCV
tokamak CRPP
Lausanne 3 (6 × 0.5)
1.5 (3 × 0.5) 82.7
110 2
2 GYCOM,
Thales T-10
tokamak Kurchatov Institute, Moscow
1.5 (2 × 0.75) 1.5 (3 × 0.5) 0.5
129140 158.5
0.5 0.5 0.5
GYCOM
Figure 6.83 shows one of the six operating 1 MW, 110 GHz gyrotrons installed at GA capable of 10 s pulse operation. The reliability of the ECH system has been high (about 84%). Research is also under way to develop a higher power gyrotron for DIII-D: a short pulse prototype gyrotron has operated at the 1.5 MW power level at MIT with 50% efficiency [6.255] and an industrial prototype of a 1.3 MW, 110 GHz gyrotron has operated in long pulse mode at CPI [6.270]. A comparable system of four 1 MW, 110 GHz, 5 s pulse length gyrotrons is operating at JT-60, while a 1.5 MW, 110 GHz gyrotron has also been demonstrated at JAEA [6.271].
FIG. 6.83. The CPI 1 MW, 110 GHz gyrotron installed at General Atomics for heating the DIII-D tokamak (photo courtesy of John Lohr, General Atomics, USA). See also Ref. [6.272].
Gyrotron development received a strong incentive and challenge in order to meet the needs of the W7-x and ITER projects, each of which requires CW operation of 1 MW gyrotrons. The status of the development of gyrotrons to meet these needs is summarized in Table 6.3, which is a modified version of a table presented in Ref. [6.273].
TABLE 6.3. PARAMETERS OF MW AND MULTI-MW GYROTRONS AT 140–170 GHz
Organization Frequency
(GHz) Cavity
Mode Power
(MW) Efficiency
(%) Pulse Length
CPI 140 TE28,7 0.9 35 30 min
EU / TED 140 TE28,8 0.92 44 30 min
JAEA / Toshiba 170 TE31,8 1.0
0.8 55
57 13.3 min
60 min
IAP/GYCOM 170 TE25,10 0.95 53 1.7 min
EFDA / Fusion
for Enegy/TED 170 TE34,19
coaxial 2 Under
development Under development JAEA / Toshiba 170 TE31,12 1.5 27 0.1 s / under development GYCOM / IAP 170 TE28,12 1.44 41 0.1 s / under development
All of the long pulse gyrotrons in the Table 6.3 operate with a single stage depressed collector, and the gyrotrons use diode electron guns, except for the JAEA gyrotrons, which have triode electron guns. The first of the gyrotrons to achieve the 1 MW power level was the CPI 140 GHz gyrotron for W7-x. It produced a 30 min pulse at 0.9 MW in 2005. This achievement was followed closely in time by a 140 GHz gyrotron developed by the EU gyrotron team and built by Thales TED [6.274]; this produced 0.92 MW at 140 GHz with a 30 min pulse length and is shown in Fig. 6.84. Nine Thales gyrotrons are currently in production, which, together with the CPI tube, will make up the 10 MW system for heating W7-x.
FIG. 6.84. The 1 MW, 140 GHz Thales gyrotron for W7-X. Reprinted from Ref. [6.274].
Copyright (2011) by the World Scientific Publishing Company.
Gyrotron development at a frequency of 170 GHz has been very successful.
The feasibility of achieving 1–1.5 MW of gyrotron output power at 170 GHz was first demonstrated in short pulse experiments at MIT, where 1.5 MW at 170 GHz was achieved in the TE28,8 mode in 3s pulsed testing in 1997 [6.275]. Recently, JAEA has developed a 1 MW, 170 GHz gyrotron that has operated at a pulse length of 800 s (13.3 min) at over 50% efficiency [6.267]. This gyrotron meets the full ITER specification in terms of power, pulse length and efficiency — a remarkable achievement and a major breakthrough in gyrotron development. The ITER project will have 24 gyrotrons, with 8 gyrotrons supplied by each of three parties: Japan, the Russian Federation and the EU, assuming 1 MW per unit. The first prototype tube has been successfully developed in Japan and is now in use for component testing. The Institute of Applied Physics (IAP) and GYCOM have made excellent progress in developing a 170 GHz gyrotron, achieving a power
level of 1 MW with a 100 s pulse, despite being limited at present by the power supply [6.276]. The EU has chosen to supply 2 MW, 170 GHz gyrotrons utilizing a coaxial cavity. The 2 MW gyrotron will be tested at a new test stand that has been recently constructed at CRPP in Lausanne. A first gyrotron has been tested to confirm the basic features such as the correct operating mode. Successful demonstration of the 2 MW gyrotron will be another major breakthrough in gyrotron development. The gyrotrons under development for ITER are shown in Fig. 6.85. As shown in Table 6.3, both JAEA and IAP/GYCOM have initiated development of gyrotrons at the 1.5 MW power level for ITER. These gyrotrons would allow for higher levels of heating power or they could simply be operated at the 1 MW power level with an additional safety margin.
FIG. 6.85. Left: The 170 GHz gyrotrons for ITER: (a) Japan, (b) Russian Federation, and (c) EU coaxial gyrotron. The JAEA 1 MW, 170 GHz gyrotron has successfully met the ITER specifications by achieving 1 MW of output power for 800 s [6.267]. Right: A cross-sectional view of the JAEA 170 GHz gyrotron [6.271]. Reprinted from Ref. [6.267]. Copyright (2011), American Nuclear Society.