1 FIP/2-2Ra Prototype Development of the ITER EC system with 170GHz - PDF document

1 FIP/2-2Ra Prototype Development of the ITER EC system with 170GHz gyrotron Y. Oda 1 , K. Kajiwara 1 , R. Ikeda 1 , K. Ohshima 1 , K. Hayashi 1 , K. Takahashi 1 , K. Sakamoto 1 , D. Purohit 2 , F. Gandini 2 , T. Omori 2 , C. Darbos 2 , M. Henderson 2 1 Japan Atomic Energy Agency (JAEA), Naka, Japan 2 ITER Organization, St Paul Lez Durance, France E-mail contact of main author: oda.yasuhisa@jaea.go.jp Abstract . To study the operational performance of ITER EC heating and current drive system (H&CD), a mock- up of the ITER mm wave system has been assembled using the high power long gyrotron test stand in JAEA. The prototype system is composed of the primary parts of the EC H&CD system, including: 170GHz gyrotron, power supply, transmission line (TL) and mock-up of equatorial launcher (EL) and control system. The JAEA test stand is a flexible system with its center piece a frequency-step-tunable gyrotron at 170GHz/137GHz/104GHz. The output beam is radiated to the identical direction from the output window for each frequency, consequently the power was transmitted to the end of the TL at these three frequencies. The system has achieved CW 5 kHz power switching, which demonstrates the compatibility for MHD control of ITER plasma. The modulation was achieved using a novel configuration of the electron beam acceleration power supply. In the experiment, stable 5 kHz of power modulation was demonstrated with minimized spurious frequency excitation at the ramp-up phase of each pulse which satisfied the ITER criteria. 1. Introduction On ITER, EC H&D is one of key components, which is used from the first plasma, and is expected as an actuator for plasma control in addition to the heating and current drive tool[1- 4]. In Japan Atomic Energy Agency (JAEA), EC system prototype is fabricated to investigate the system characteristics prior the construction to ensure the maximum performance on ITER by modifying the JAEA gyrotron test stand. As for the gyrotron development, 1MW/800 s shot was achieved in 2006 with TE 31,8 cavity mode[2]. Followed this, we proceed to the development of the higher power gyrotron capable of >1.2 MW power output. The gyrotron was designed to be frequency-step-tunable at 170 GHz/137 GHz/104 GHz. This gyrotron is used as a power source of the prototype system. The objective of the prototype EC system is (1) to demonstrate the performance of each component and fix the specification prior to the construction, (2) to develop the control system and operational software package to be used on ITER. Here, novel PS configuration with anode switch is introduced to realize CW 5 kHz power modulation. The high-power TL operation with three frequencies were also demonstrated and its transmission efficiency was measured. 2. Multi-Frequency Gyrotron and its RF Power Transmission The gyrotron is designed to be a frequency-step-tunable at 170GHz/137GHz/104GHz with the oscillation mode TE 31,11 /TE 25,9 /TE 19,7 , respectively. The selection of the operation modes is

2 FIP/2-2Ra the key point of the multi frequency gyrotron design which has to adapt the restrictions of 1) window thickness, 2) cavity size, and 3) built-in mode converter. Firstly the operation frequency was selected according to thickness of single disk diamond window. The selected frequencies were transparent through the diamond window of 1.853mm in thickness. Then the cavity size and mode was determined to be operated with similar electron beam radius at the cavity. The triode electron gun has advantage in its wide operation region compared to diode gun since its pitch factor can be optimized for the selected mode under different magnetic field. To adapt the restriction of the built-in mode converter, we selected these modes which have similar rotational angle in the beam tunnel. In our gyrotron, the rotational angles of the selected three modes have less than 0.1 degree difference to each other. When the rotational angle is similar, the built-in mode converter works similarly for all the operation frequencies. [5] Figure 1 shows the beam profile at the output window of the gyrotron measured using a thermal imaging camera. The output beams locate at the center of the window and radiated to the identical direction for each frequency. The output power at 2s operation was obtained up to 1.2 MW, 1.0 MW, and 1.0 MW at 170 GHz, 137 GHz, and 104 GHz respectively. The pulse duration was extended up to 1000 s at 510 kW, 250 s at 330 kW and 20 s at 260 kW for 170 GHz, 137 GHz, and 104 GHz, respectively. The waveform of long pulse operation were shown in Fig.2. 170 GHz 137 GHz 110 GHz Fig.1 Output beam profile of multi-frequency gyrotron 170 GHz / 1000 s / 510 kW 137 GHz / 250 s / 330 kW 104 GHz / 20 s / 260 kW Fig. 2 Waveforms of long pulse operation of multi-frequency gyrotron

3 FIP/2-2Ra Consequently the power was transmitted to the end of the TL for these three frequencies. Figure 3 shows the TL test stand which have short line (7 m) and long line (40 m) and both lines are connected using a waveguide switch. The dummy loads for 1MW-class CW operation are connected at both end of short and long lines. The gyrotron produced 20 s high power RF pulses with all the three frequencies and transmitted RF to the dummy loads and loss in TL components and matching optical unit (MOU) were measured. The identical MOU mirror and the same mirror angle adjustment setting was utilized for the different frequency operation. The transmission efficiencies for each line were deduced from the measured power and loss and the result is listed in table 1. The transmission efficiencies in long line composed of waveguides were the same values among different frequencies. On the other hand, the transmission efficiency in MOU was varied on frequency and the efficiency decreased with increase of wavelength since the MOU mirror was designed for 170 GHz and the deflection loss of quasi-optical transmission in MOU was increased. As total transmission efficiency Fig.3 Schematic of TL test stand in JAEA Table 1 Efficiency of TL at multi frequency operation 170 GHz 137 GHz 104 GHz Transmission power efficiency Transmission efficiency at MOU 96.7 % 94.2 % 90.2 % Transmission efficiency at Long TL 93.8 % 95.1 % 93.7 % Total transmission efficiency (MOU+LongTL) 90.7 % 89.6 % 84.5 % Mode purity LP 01 mode purity at MOU output 94.4 % 92.6 % 92.7 % LP 01 mode purity at Long TL end 93.1 % 92.5 % 94.0 %

4 FIP/2-2Ra The transmission mode purity is the key issue of RF transmission design in ITER launchers since the ITER launcher is designed based on quasi-optical transmission system and the mode contents affect the characteristics of the radiated beam profile. The mode content at the inlet of TL and outlet of long line were measured for all the frequencies and the result is listed in table 1. The input mode purity of the fundamental transmission mode, LP 01 , was 94% at 170 GHz, the designed frequency, and >90% were obtained for both 137 GHz and 104 GHz. Since the gyrotron produced the similar beam profile for three frequencies as shown in Fig.1, the identical MOU mirror could couple the RF power to TL at high efficiency and TL caused small mode conversion loss.[6] These results indicate that the TL system and components are adaptable for the high-power multi-frequency operation. In addition, the multi-frequency gyrotron can operate high-power long-pulse with the single set of MOU mirror to couple RF power without any adjustment while different frequency operation. 3. 5 kHz modulation and ITER relevant PS configuration The 5 kHz power modulation is carried out with the electron beam switching using the anode voltage control. The electron beam is completely suppressed when anode is short-circuited to cathode voltage and this is a great advantage of triode gyrotrons. The power supply configuration with shortcut-circuit switch device, namely anode switch, is promising system for high frequency power modulation and the modulation operation was demonstrated. The issue of the modulation is the excitation of spurious frequency (167GHz; TE 30,11 mode) at the ramp-up phase of the anode voltage. The reduction of spurious mode is mandatory for high efficient gyrotron operation and protection of plasma measurement devices.[7] Fig.4 Configuration plan of HVPSs for gyrotorn operation in ITER.

5 FIP/2-2Ra To suppress the spurious frequency, novel configuration of the power supply is developed as shown in Fig.4. The double anode switch is adopted and the anode PS is applied to feed the anode voltage without interference from body and main PSs. These are effective to minimize the capacity of the circuit which is a main cause to prevent the quick ramp-up of the voltage. As shown in the Fig.5, the short rising time of anode voltage was achieved. As a result, the spurious mode (167GHz) start to growth but that is immediately suppressed after 2.4  s by the excitation of the main mode (170GHz) as shown in Fig.5. Fig.5 Waveform of 5 kHz modulation with the ITER EC prototype system and IF signal of RF detector (right ) 4. ITER relevant Control System Also, new control system was developed according to ITER standard of control system, namely Plant Control Design Handbook (PCDH), as a prototype of the ITER CODAC compatible gyrotron local controller. The conceptual view and the picture of the EC prototype system fabricated in JAEA is shown in Fig.6 and 7. Fig.6 High voltage PS configuration and circuit of Fig.7 Configuration of the ITER EC prototype system ITER triode gyrotron operation system and the gyrotron local controller.