Progress on the Development of High Power Long Pulse Gyrotron and Related Technologies

1 FTP 1-3 Rb

K. Kajiwara1, K. Sakamoto1, Y. Oda1, K. Hayashi1, K. Takahashi1 and A. Kasugai1

1Japan Atomic Energy Agency, Naka, Japan

Abstract: Recent progress on the development of a high power gyrotron and related technologies is presented. In the development of a higher power multi-frequency gyrotron, a high order mode gyrotron, which permits to select the oscillation at 170 GHz-TE31,11 and 137 GHz- TE25,9 (104 GHz-TE19,7 as an option), has been fabricated and tested. In long pulse experiments, 905 kW/45 %/75 s at 170GHz and 540 kW/42 %/20 s at 137 GHz are achieved. Since the RF beam direction from the output window is designed to be almost the same for frequencies, good power couplings to the transmission line are obtained by using a pair of identical phase correcting mirrors. Pulse extension is underway aiming for >1 MW at CW operation. In parallel, a 5kHz full power modulation experiment is performed using the 170 GHz gyrotron of TE31,8 mode oscillation. The 5 kHz full power modulation was achieved with full beam modulation by employing a fast voltage switching between the anode and the cathode of the triode type electron gun. In addition, another fast switch is inserted for the faster ramp up of the anode-cathode voltage for the suppression of an unwanted mode oscillation. The other is the fast frequency change (∼3 GHz) by changing the magnetic ﬁeld. The fast sweeping coil is located inside the bore of super conducting magnet which allows to change the magnetic ﬁeld of 0.2 T within 20 s. 170 GHz→167 GHz→170 GHz every 3.5 s is successfully demonstrated.

1 Introduction

A 170GHz high power and long pulse gyrotron is essential for the EC H&CD system in ITER[1]. In JAEA, 1MW/800 s was achieved by a gyrotron with TE31,8 cavity mode[2]. Using this gyrotron, reliability test was carried out and demonstrated a repetitive operation of 800 kW/600 s pulses in every 30 min as the ITER relevant operation[3]. In the next stage, we proceed to develop a higher order cavity mode gyrotron, which is capable of more than 1 MW power output at 170 GHz. To expand the operation region of the EC H&CD system, a multi-frequency gyrotron is designed and fabricated. The progress of the multi-frequency gyrotron can be found in ref.[4, 5, 6]. The output frequency is 170GHz or 137GHz, which is selected by changing the oscillation mode TE31,11 or TE25,9, respectively[7]. The control of the oscillation mode is realized by changing the magnetic FTP 1-3 Rb 2

filed configuration with the solenoid coil, and optimizing the pitch factor of the electron beam using the triode type electron gun. Short pulse experiments (0.5ms) were successfully performed with 1.3MW power output at more than 30% of the oscillation efficiency without depressed collector for both frequencies[7]. Long pulse experiments are underway. Other activities are a power modulation and a fast frequency change (∼3 GHz). The gyrotron output power is required to on/off modulation for the suppression of the neo- classical tearing mode (NTM) on ITER up to 5 kHz. The fast frequency change is aiming for the step-tunable gyrotron operation in future fusion device so as to control the position of the resonance layer by changing the frequency.

2 Multi-frequency gyrotron

Collector

magnetic field line

electron beam Output window (CVD Diamond)

Internal RF mirrors

DC Break Mode convertor

Cavity

Triode Electron gun (MIG)

FIG 1: Picture and schematic view of the multi-frequency gyrotron.

The multi-frequency gyrotron (170GHz/137GHz, 104GHz as an option) has been de- veloped in JAEA. A picture and a schematic view of the gyrotron are shown in Fig. 1. One of the features of the gyrotron is the triode electron gun. Most of the present multi- frequency gyrotron employs a diode electron gun. As the diode gun is difficult to control the beam position and the pitch factor in the cavity independently, that restricts the selection of the oscillation mode, which causes a degradation of the oscillation efficiency. On the other hand, the triode electron gun can optimize the beam position and the pitch factor separately, therefore, high efficiency oscillation is expected for both frequencies. Cavity modes are selected as TE31,11 for 170 GHz and TE25,9 for 137 GHz from considera- tion of the RF beam optics. Because these modes have very close caustic radii in a mode converter, the RF radiation angles from the mode converter are almost the same for both 3 FTP 1-3 Rb frequencies. As a result, very similar RF beam direction and beam profile are realized at the gyrotron output window. These modes have a large margin in the heat load on the cavity wall for the 1MW operation. In the experiment, high power (>1.3MW) and high oscillation efficiency (>30%, without depressed collector) capability for both frequencies were shown with the short pulse operation (<1 ms)[7]. The output power is coupled with HE11 mode of the transmission waveguide using a pair of two phase correcting mirrors. In the experiment, high mode purities of HE11 contents were confirmed, i.e., 96% for 170GHz and 94% for 137GHz. The mode purity of 96% satisfies the ITER requirement of 95% at 170 GHz. The reason of good coupling at two frequencies with the identical mirrors is attributed to the similarity of the RF beam direction, and RF power and phase profiles.

Body voltage 30.0kV Body voltage 28.1kV

Anode voltage -5.1kV Anode voltage -8.8kV

Cathode voltage -45.6kV Cathode voltage -43.3kV

Beam current 44.1A Beam current 29.5A

RF signal RF signal

Ion pump 3micro A Ion pump 5micro A

FIG 2: Waveforms of the long pulse operation of the multi-frequency gyrotron. Left hand side is 170 GHz and right hand side is 137 GHz.

In Fig. 2, the waveforms of long pulse operations for both frequencies are shown. It is the first time long pulse experiments with the multi-frequency gyrotron/triode electron gun. The 905 kW/45%/75 s for 170 GHz and 540 kW/42%/20 s for 137 GHz are achieved. A higher power operation is also progress. The 1080 kW/45 %/5 s is achieved with 170 GHz oscillation and higher power operation is going to attempt for both frequencies. The pulse length and the efficiency will be increased by accessing the hard excitation region with adjustments of the anode voltage and the magnetic field during the pulse[2]. For the detailed adjustments, pulse length of more than 100 s is preferable. The conditioning and the adjustments in order to access the hard excitation region are underway. The goal of the long pulse operation is >1MW/CW for both frequencies with 50% electrical efficiency. As same as the TE31,11 and the TE25,9, a TE19,7 has also close caustic radii. It cor- responds to 104 GHz oscillation which also penetrate the 1.853 mm thickness output window. A first attempt to generate 104 GHz is performed with short pulse (<1 ms). The output pattern and the frequency are successfully detected. FTP 1-3 Rb 4

3 5kHz beam on/oﬀ modulation with the short-circuited switch

3.1 Single anode switch conﬁguration

-45kV Cathode -45kV Cathode Anode Anode switch switch

- >300mA Main >300mA Main e ~10mA switch ~10mA switch Anode Anode

-5kV 40A -45kV -45kV -45kV

Anode Body -45kV Anode Body -45kV Gyrotron voltage PS Gyrotron voltage PS divider 30kV divider 30kV +30kV Main +30kV Main Body PS Body PS 0kV 0kV

~10mA ~10mA Collector Collector

0kV 0kV RF on (Electron beam on) RF off (Electron beam off) FIG 3: Circuit of the gyrotron power supply for the high speed modulation. The current and the voltage are shown in the case for the RF on phase (left hand side) and the RF oﬀ phase (right hand side) with the anode switch.

A novel high speed modulation method is proposed with the triode electron gun. As a nature of the triode electron gun, the electron beam inside the gyrotron is controlled by the voltage difference between the anode and the cathode (Vac). Especially, it is cut off when the Vac goes to zero, because the electric field around the electron emission belt becomes close to zero. Figure 3 shows the circuit for the modulation. As shown in the figure, the current (=40 A) for the main circuit in the RF off phase is zero when the electron beam is cut off by short-circuited switch (“anode switch”) between the anode and the cathode. Remarkably, the output voltages of two power supply (PS), i.e., the main PS and the body PS, do not need to change. It means the Main PS and the Body PS can be a DC PS. Moreover the electron beam is zero during RF off phase, which reduces heat load on the collector significantly. It allows the increase of the beam current in the RF on phase for higher RF power operation. One issue is the control of the anode voltage divider. The anode voltage divider is consisted with 100 series of Zener diodes parallel to a photo diode. Each Zener diode can sustain 1 kV. The number of the active Zener diode is remotely controlled by the photo diodes. The 40 Zener diodes should be turn on/off with high speed in order to change the anode voltage from -5 kV to -45 kV. 5 FTP 1-3 Rb

RF on RF off phase phase *note: duty 45% case

20 (a) 0 Vc Va -20 Vb

Voltage [kV] -40

60 (b) 40

Ib[A] 20

0.3 (c) 0.2

0.1 RF[a.u.]

170 (d) 169

168

Freq. [GHz] 167 500 500.1 500.2 500.3 500.4 500.5 Time[ms]

FIG 4: Typical waveforms of the 5 kHz modulation with the anode switch[8]. (a) Vc (Cathode), Va (Anode), Vb (Body) voltage, (b) electron beam current, (c) RF signal and (d) frequency.

The 5kHz modulation by using the anode switch is successfully demonstrated as shown in Fig. 4. As expected, the beam current goes to zero during the RF off phase and the cathode and the body voltage kept constant. By this method, the 1.16 MW/60 s is achieved with the electrical efficiency of 48%. As shown in the Fig. 4(c) and (d), the 170 GHz oscillation period is shorter than the RF on phase. It is because the anode voltage is under the excitation voltage of target mode TE31,8 in the early period of the RF on phase. In this early period, the adjacent mode TE30,8 167 GHz is generated instead of 170 GHz (Fig. 4(d)). The period of 170 GHz oscillation becomes shorter with higher efficiency operation. The duty cycle of 170 GHz oscillation is 24% in the 1.16MW shot with 50% duty cycle of the RF on phase.

3.2 Double anode switch configuration The single anode switch operation, which is introduced in the previous section, needs anode voltage divider control. It causes the slow anode voltage increase at the beginning of the RF on phase. In order to improve it, a double anode switch configuration is proposed. So as to keep the constant output voltage at the anode voltage divider, another switch is added between the divider and the anode electrode. The circuit of the double switch configuration is shown in Fig. 5. As shown in the figure, the output voltage of the anode voltage divider is same -5 kV for both the RF on phase and the RF off phase. The typical waveforms with double anode switch configuration are shown in Fig. 6(a). As shown in the figure, the oscillation period is increased compared to Fig. 4(c). The reason FTP 1-3 Rb 6

is the fast rising of Vac. Figure 6(b) shows the comparison of the Vac between the single switch configuration and the double switch configuration with 5kHz-50% duty operation. As shown in the Fig. 6(b), the rising time of the double switch operation is faster than the single switch, which realizes the longer period of the 170 GHz RF generation by suppressing the unwanted 167 GHz RF generation at the beginning of the RF on phase. The test of the double switch configuration toward the high power and high efficiency is underway.

-45kV Cathode -45kV Cathode Anode Anode switches switches - >300mA Main >300mA Main e ~10mA switch switch Anode Anode

-5kV -5kV 40A -45kV -5kV -45kV -45kV

Anode Body -45kV Anode Body -45kV Gyrotron voltage PS Gyrotron voltage PS divider 30kV divider 30kV +30kV Main +30kV Main Body PS Body PS 0kV 0kV

~10mA Collector Collector 0kV 0kV RF on (Electron beam on) RF off (Electron beam off) FIG 5: Circuit of the gyrotron power supply for the high speed modulation with the double anode switch. The current and the voltage are shown in the case for the RF on phase (left hand side) and the RF oﬀ phase (right hand side).

(a) Vc Va Vb (b) Single switch 20 Double switch 1

0.8 -20 Voltage [kV]

-40 0.6 1.5

0.4 1

0.5 Anode-cathode voltage[a.u.] 0.2 RF[a.u.]

0 0 141.5 141.6 141.7 141.8 141.9 142 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Time[ms] Time[ms] FIG 6: Typical waveforms of the 5 kHz modulation with the double anode switch configuration. (a) Vc (Cathode), Va (Anode), Vb (Body) voltage and RF signal. (b) Comparison of the anode-cathode voltage (Vac) between the single anode switch configuration and the double anode switch configuration. The vertical axis is normalized. 7 FTP 1-3 Rb

4 High speed frequency change

A super conducting magnet (SCM) for the gyrotron is designed for high speed frequency change. A fast sweeping coil, located inside the bore, generates 0.2 T at the cavity. This fast sweeping coil can quickly change the magnetic ﬁeld from -0.2 T to 0.2 T within 20 s. The 170 GHz gyrotron[9] is used to demonstrate the frequency change. Figure 7 shows the waveforms of the demonstration and magniﬁed waveforms at the pulses. Here, the pulse width is 1 ms and the operation is performed without depressed collector. As shown in the Fig. 7, the frequency is changed from 170 GHz to 167 GHz within 3.5 s and is changed back to 170 GHz. The power of 170 GHz and 167 GHz are 610 kW and 540 kW, respectively.

Cathode voltage[kV] Anode voltage [kV] Beam current [A] RF signal [a.u.] Fast sweeping coil current[A]

0 0

-50 -50

170 170 Frequncy

Frequncy 165 165 -0.001 -0.0005 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 3.49 3.4905 3.491 3.4915 3.492 3.4925 3.493 3.4935 3.494 Time [s] Time [s] FIG 7: Waveforms of the fast frequency change demonstration.

5 Summary

The multi-frequency 170 GHz/137 GHz (104 GHz as an option) gyrotron with triode electron gun is successfully designed and tested. In the short pulse experiments (<1 ms), more than 1.3 MW is generated for both frequencies. The first attempt to generate 104 GHz FTP 1-3 Rb 8 are also succeeded in the short pulse. The long pulse experiments show 905 kW/45%/75 s for 170 GHz and 540 kW/42%/20 s for 137 GHz. The 5 kHz modulation achieves the 1.16 MW with 48% electrical efficiency by using the short-circuited switch between the anode and the cathode. In order to improve the rising time of the cathode-anode voltage, the double switch configuration is introduced. As a result, the suppression of the unwanted mode at the beginning of the RF on phase is achieved. The fast frequency change by using the SCM with the fast sweeping coil inside the bore for quick magnetic field change is successfully demonstrated. A 3 GHz frequency change is succeeded within 3.5 s. Larger frequency change with longer pulse will be attempted.

References

[1] HOW, J., et al. (Eds.), ITER 2009 Baseline: Plant Description (PD), ITER Orga- nization, Route de Vinon-sur-Verdon, CS 90 046 13067, St. Paul-lez-Durance Cedex, France, 2009, Ch. 7.3 Electron Cyclotron System, http://www.iter.org.

[2] SAKAMOTO, K., et al., “Achievement of robust high-eﬃciency 1 MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron”, Nat. Phys. 3 (2007) 411–414.

[3] KAJIWARA, K., et al., “Repetitive gyrotron operation for ITER”, Fusion Eng. Des. 86 (2011) 955–958.

[4] DENISOV, G., et al., “Development in russia of high-power gyrotrons for fusion”, Nucl. Fusion 48 (2008) 054007.

[5] THUMM, M., et al., “Frequency step-tunable (114-170 GHz) megawatt gyrotrons for plasma physics applications.”, Fusion Eng. and Des. 53 (2001) 407–421.

[6] ZAPEVALOV, V. E., et al., “Development of a prototype of a 1-MW 105-156-GHz multifrequency gyrotron”, Radiophysics and Quantum Electronics 47 (2004) 396–404.

[7] KAJIWARA, K., et al., “Development of dual-frequency gyrotron with triode mag- netron injection gun”, Applied Physics Express 4 (2011) 126001.

[8] KAJIWARA, K., et al., “5kHz modulation of 170 GHz gyrotron with anode-cathode short-circuited switch”, in: Fusion Engineering (SOFE), 2011 IEEE/NPSS 24th Sym- posium on, 2011.

[9] KASUGAI, A., et al., “Steady-state operation of 170GHz-1MW gyrotron for ITER”, Nucl. Fusion 48 (2008) 054009.