RADIOFREQUENCY WAVES, HEATING AND CURRENT DRIVE IN MAGNETICALLY CONFINED PLASMAS
6.4. GYROTRONS FOR ECR HEATING AND CURRENT DRIVE
6.4.6. Prospects and future directions
Multi-MW gyrotrons are attractive for heating systems that require tens of MW of power, such as the ITER ECH heating system. A major first step in
this direction has been taken by the EU team as shown in Table 6.3. The EU 2 MW gyrotron is also shown in Fig. 6.85(c). An additional incentive for the development of a 2 MW gyrotron for ITER is the possibility of expanding the power level of the ECH system from 24 MW to 48 MW in a second stage of the project. A separate effort is under way at CPI to develop a 2 MW, 95 GHz gyrotron oscillator [6.277]. The gyrotron is designed to minimize weight and size, while maximizing efficiency. The gyrotron, which is shown in Fig. 6.86, is under test. The gyrotron has a collector made of dispersion-hardened copper to permit power dissipation in a minimized volume. The design of a 4 MW, 170 GHz gyrotron has been recently developed at Forschungszentrum Kalsruhe (FZK) [6.278].
FIG. 6.86. A 2 MW, 95 GHz gyrotron at CPI. Reprinted from Ref. [6.277]. Copyright (2011) by the World Scientific Publishing Company.
6.4.6.2. Frequency tuneable gyrotrons
High power gyrotrons are oscillators that are usually optimized for a single frequency of operation. Gyrotron amplifiers would have attractive features for plasma heating, such as control of power, phase and frequency, but gyrotron amplifiers have lower efficiency and lower average power capability. The ability to tune the frequency of a gyrotron over both small (tens of MHz) and large (several to tens of GHz) frequency intervals is of interest. Small frequency steps,
tens of MHz, may be used for a proposed fast switching application [6.279].
These small frequency steps, several tens of MHz, may be accomplished by voltage tuning of the gyrotron. Larger frequency steps are required when the magnetic field of the plasma is changed or the location of heating or current drive is changed. The possibility to step-tune in frequency by exciting the modes of the cavity sequentially was demonstrated in early experimental research [6.280]. A two-frequency gyrotron has been successfully developed by GYCOM for ASDEx-Upgrade with high power at the frequencies of 105 and 140 GHz.
A four-frequency gyrotron is currently under development [6.281].
6.4.6.3. Improvement of gyrotron efficiency
The required efficiency of modern gyrotrons operating with a depressed collector is in excess of 50%. The demonstration of a reliable 1 MW, 170 GHz CW gyrotron with 55% efficiency is a major advance in demonstrating high efficiency at high power [6.282]. The depressed collector is able to recover a large fraction of the axial energy of the gyrotron. The electron cyclotron maser interaction removes energy from the transverse portion of the electron beam;
the parallel energy is not affected. In the non-relativisitic limit, the fraction of transverse energy is simply v / v 2. For typical gyrotron beams with helicity
1.3–1.5, about two thirds of the energy is in the transverse component and one third in the axial component. If the gyrotron can extract 70% of the transverse energy components and recover 90% of the axial component, an efficiency of about 75% can be achieved. In fact, a 0.8 MW, 75 GHz gyrotron has achieved 70% efficiency in 0.1 s pulsed operation [6.276]. The possible use of multi-stage depressed collectors could facilitate achievement of very high gyrotron efficiency.
Since fusion requires a very high Q factor, defined as power out divided by power in, the achievement of 70% efficiency in gyrotron heating systems would provide an advantage in fusion energy development by significantly reducing the required wall plug input power for the auxiliary heating system.
6.4.6.4. Gyrotrons for DEMO
With the completion of the ITER design and the need for rapid development of fusion energy, attention is now turning to the requirements of a demonstration fusion reactor or DEMO. Since there are several designs for a DEMO, it is premature to specify the ECH system. However, DEMO may well require a higher frequency heating system than the 170 GHz gyrotrons for ITER, possibly near 220 GHz. Basic research has shown that a gyrotron can achieve MW power levels in the 200–300 GHz frequency range [6.283]. Future development of gyrotrons operating at these higher frequencies will provide the sources needed for heating plasmas in the worldwide fusion energy programme.
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