1 FIP/2-2Rb Development of dual frequency gyrotron and launcher - - PDF document

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FIP/2-2Rb Development of dual frequency gyrotron and launcher for the JT-60SA ECH/ECCD system

• T. Kobayashi1, S. Moriyama1, K. Yokokura1, M. Sawahata1, M. Terakado1, S. Hiranai1,
• K. Wada1, Y. Sato1, J. Hinata1, K. Hoshino1, A. Isayama1, M. Saigusa2, K. Kajiwara1,
• Y. Oda1, R. Ikeda1, K. Takahashi1, K. Sakamoto1

1Japan Atomic Energy Agency (JAEA), Naka, Ibaraki 311-0193 Japan 2Ibaraki University, Hitachi, Ibaraki 316-8511 Japan

E-mail contact of main author: kobayashi.takayuki@jaea.go.jp

• Abstract. The development of a gyrotron and a launcher operated at two frequencies, 110 GHz and 138 GHz,

has made a significant progress toward electron cyclotron heating (ECH) and current drive (ECCD) in JT-60SA. Oscillations of 1 MW for 100 s were achieved at both frequencies, for the first time in the world as a dual- frequency gyrotron, by optimizing electron pitch factor using a triode electron gun resulting in the fulfillment of the target output power and pulse length for JT-60SA. In high-power experiment, low diffraction loss and cavity Ohmic loss enabling 1.5 - 2 MW for several seconds were confirmed. Oscillations at 82 GHz were also demonstrated providing an additional frequency for the use of fundamental harmonic waves at the toroidal magnetic field of ~ 2.3 T. In addition to the above results of the gyrotron development, the ECH/ECCD launcher and polarizer developments toward dual-frequency operations were carried out. Quasi-optical design of the launcher showed not much difference in the poloidal beam width for these frequencies. Prototype test of a wide- band polarizer for operations at the above frequencies at low power (< 1 mW) showed that almost arbitrary elliptical polarization can be generated. The Ohmic loss of the prototype polarizer obtained in a high-power (~0.25 MW, 3s) test qualitatively agreed with the design.

• 1. Introduction

High-power, long-pulse gyrotrons are required for the JT-60SA ECH/ECCD system which has the total injection power of 7 MW and the pulse duration of 100 s using 9 gyrotrons [1]. The millimeter wave frequency in the original specification is 110 GHz, which is effective for

• ff-axis ECH/ECCD to sustain a high-beta plasma at the toroidal field, Bt, of ~ 1.7 T. On the
• ther hand, the higher frequency waves at 130 ~ 140 GHz enable ECH/ECCD in the core

plasma region at the maximum Bt of 2.3 T in JT-60SA. An application of a dual-frequency gyrotron has an advantage as demonstrated in ASDEX-U [2, 3] since it enables to realize a dual-frequency ECH/ECCD system, in which the number of gyrotron (including transmission lines and power supplies etc.) and the total injection power are unchanged from the original single-frequency system. However, a dual-frequency gyrotron that can be operated at the target output power and pulse length of JT-60SA (1 MW for 100 s) at two frequencies did not exist [4]. Simultaneous realization of a high oscillation efficiency to obtain high output power and low diffraction loss to achieve long pulse at both frequencies is a key to develop a dual- frequency gyrotron. In order to extend the operation regime of the ECH/ECCD system in JT-60SA, we have developed a dual-frequency gyrotron (110 GHz, 138 GHz) equipped with a triode type electron gun, which enabled to obtain high oscillation efficiency at both frequencies. In parallel to the gyrotron development, a quasi-optical characteristic of the ECH/ECCD launcher to be installed into the JT-60SA tokamak was numerically studied in order to clarify

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the poloidal beam focusing property operating at two

• frequencies. Moreover, the

injected wave needed to have an elliptical polarization suitable for the efficient coupling to the pure X-mode in target plasma. In order to produce arbitrary elliptical polarizations at two frequencies, a development of a wide band polarizer was started in collaboration between Japan Atomic Energy Agency (JAEA) and Ibaraki University. In this paper, we describe the results of the dual-frequency gyrotron development, design and mock-up tests of the ECH/ECCD launcher and the wide band polarizer.

• 2. The dual-frequency gyrotron development

An application of a dual-frequency gyrotron for the JT-60SA ECH/ECCD system was discussed in 2010 [5]. Then, the detail design of the dual-frequency gyrotron was carried out in JAEA in 2011 [6], and the gyrotron was fabricated in Toshiba Electron Tube and Devices Co., Ltd. in 2011-2012. An initial short pulse commissioning result in early 2012 was reported in the last IAEA FEC [7]. Since the last conference, optimizations of gyrotron operation parameters and conditioning operations toward high-power, long-pulse oscillations have been carried out as shown in Fig .1. As a result of this work, the target values for the dual- frequency gyrotron (110 GHz and 138 GHz, 1 MW, 100 s) of the JT-60SA ECH/ECCD system have been fully satisfied. 2.1. Design of the dual-frequency gyrotron In the dual-frequency gyrotron design, the selection of TEmn modes and frequencies are one of the important issues, because design parameters of the electron gun, cavity, mode convertor, and output window are closely related to each other through eigenvalue of the mode ’m,n and m-1,1 and the frequency, f, or the wavelength, c/f. Here mn is an n-th root of derivative of the m-th order Bessel function, and c is the speed of light. The key parameters on the dual- frequency gyrotron design to be satisfied at both frequencies simultaneously are the cutoff frequency of the cavity, fcutoff = c mn / 2a, the thickness of the output window, dw = nw w / 2, the electron beam radius in the cavity to get maximum coupling efficiency, rb = m-1,1  / 2, the transverse radiation angle in the mode convertor,  = 2 cos-1(m / mn), where a, nw, and w are the radius of the cavity, an integer and the wavelength in the window material, respectively. We found that the parameters shown in Table 1 were the best combination for operation at the frequencies of 110 GHz and 138 GHz for this gyrotron by careful consideration of each parameter and numerical calculations [6]. The shape of cavity resonator was optimized for chosen operating modes. The output power of higher than 1 MW

• Fig. 1. Progress in high-power, long-pulse operations of the

newly developed dual-frequency gyrotron for JT-60SA. The target value for JT-60SA (1MW, 100 s) was achieved at both frequencies (110 Hz, 138 GHz).

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was expected with the beam current > 40 A at the acceleration voltage of 85 kV (the pitch factor of 1.1 was assumed). The calculated heat loads of the cavity at 40 A are ~ 0.8 kW/cm2 at 110 GHz and ~1.35 kW/cm2 at 138 GHz. These values are much smaller than the limitation

• f the material (< 2 kW/cm2). The quasi-optical mode convertor was optimized for two

frequencies, and the calculated diffraction loss was ~ 3% at 110 GHz and ~ 2% at 138 GHz. The other parts (electron gun, beam tunnel, collector, etc.) are the same as those of the previous 110 GHz JT-60 gyrotrons. These calculation results showed that oscillations of 1 MW for 100 s are possible at both frequencies, and the higher power will be also possible with a limitation of the pulse length. 2.2. 1 MW, 100 s oscillations Since the last FEC, a matching optics unit (MOU) of the gyrotron and a dummy load were connected by an evacuated corrugated waveguide transmission line (TL) with the diameter of 60.3 mm for conditioning operation and for evaluating the gyrotron characteristics in long-

• pulse. The total length of the TL was approximately 7 m. In the TL, three miter-bends (a

power monitor miter-bend, plane miter-bend and arc-detector miter-bend) and a vacuum pump-out tee were included. The transmission efficiency of the TL (including loss in MOU ~ 4%) was approximately 92% for 110 GHz and 93% for 138 GHz by changing the angles of mirrors in the MOU at each frequency using ultra-sonic motors. Oscillations up to 1 MW/10 s were confirmed at both frequencies in 2013 [8] while the maximum pulse length or the maximum energy was limited mainly by the increase in the waveguide temperature and the vacuum pressure in the TL and the dummy load. After upgrading the cooling capability of the TL and many conditioning pulses to reduce the outgas from the dummy load surface, which was a main source of the increase in the vacuum pressure in the TL, we successfully achieved

• scillations of 1 MW for 100 s at both frequencies in 2014 as shown in Fig. 1, for the first

time as a dual-frequency gyrotron. The pulse length of 100 s at 1 MW or the maximum energy

• f 100 MJ was again limited by the temperature increase of the TL (~ 100 °C). The relatively

large increase in the TL temperature might be caused by a reflection from the dummy load. It is noted that the longest pulse length of 198 s, which is the maximum pulse length available with the present pulse timing control system of the ECH/ECCD system developed for JT-60U, were also successful with the output power of approximately 0.5 MW (100 MJ) [8]. Although the pulse length longer than 100 s is not explicitly required for the purpose of JT-60SA, it will be possible to expand pulse length at 1 MW by improving the TL and the control system since no evidence of the limitation of the pulse length at 1 MW was found in the gyrotron itself. Figure 2 shows the time traces of the voltages applied to the cathode, Vk, anode, Va, and body, Vb, electrodes, beam current, Ibeam, rf signal detected by a diode detector installed at a directional coupler of the power monitor miter-bend in the TL, and the power absorbed at the dummy load, Pload, for the 1 MW oscillations. The response time of the calorimetrical power measurement by the dummy load (cooled by water with the flow rate of ~ 950 L/min) is about 15 s due to the large heat capacity of the load. The actual rise time of the rf power is about

Table 1. Key parameters of the dual-frequency gyrotron design for JT-60SA. Nominal frequency [GHz] TEmn mn a [mm] fcutoff [GHz] dw [mm] nw rb [mm]  [degree] 110 TE22,8 52.66 22.9 110.0 2.29 4 10.1 130.63 138 TE27,10 65.85 137.6 5 9.9 131.59

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100 ms as seen in the rf signal since the rise time of the power supplies used in this experiment is approximately 100 ms. In the above operations, a pre-programed anode voltage control was applied to keep the oscillations even with the slightly decreased beam current due to cathode cooling [9]. In addition, a pre-programmed heater control was applied and the stable beam current was obtained for pulse length longer than 30 s. Thus, the oscillations will be able to be kept even for the pulse length longer than 100 s. In the above

• scillations

(1 MW/100 s), the

• scillation

efficiencies without collector potential depression (CPD), which is defined by

• Pgyro. (kW)

/ Ibeam (A) / Vbk (kV) × 100, at 110 GHz and 138 GHz are 34% and 32%, respectively. Here, the acceleration voltage, Vbk = Vb - Vk, is 85 kV and the output power, Pgyro., and Ibeam are the time averaged values. The obtained oscillation efficiencies are sufficiently high for long-pulse operations at 1 MW. The total efficiencies with CPD, which is defined by Pgyro. (kW) / Ibeam (A) / |Vk| (kV) × 100, are 47% and 44%, respectively. In this

• peration, the CPD voltage was ~ 23 kV. Since the increase in the anode current, which is a

measure of the amount of trapped electrons, was not observed, it will be possible to increase the CPD voltage further to increase the total efficiency. However, the main power supply used in this experiment was not able to change the cathode voltage (~ 62 kV). In JT-60SA, a new power supply, which can change the output voltage, will be installed, and the total efficiency

• f 50% will be obtained by increasing the CPD voltage while decreasing the cathode voltage.

An important result demonstrated in this development is that the optimization of the voltage between anode and cathode, Vak, is required for multi-frequency gyrotrons. As shown in

• Fig. 2, the Vak at 138 GHz is larger than that at 110 GHz by 5 kV. The magnetic field applied

by a superconducting magnet (SCM) almost linearly depends on the operating frequency of the gyrotron. Since the electron pitch factor depends on both the anode voltage and the magnetic field strength, the anode voltage needed to be increased to operate at the higher frequency (the higher magnetic field). Moreover, the optimum pitch factor is also different at each frequency depending on the cavity design. Thus, it is quite important to optimize the pitch factor or the anode voltage at each operation condition. From a viewpoint of high- efficiency oscillations at two/multi-frequencies, a triode electron gun has a significant advantage compared with a diode electron gun [3].

• Fig. 2. Time traces of the beam current, voltages applied to

the cathode, Vk, anode, Va, and body, Vb, electrodes, rf signal detected by a diode detector installed at the directional coupler of the power monitor miter-bend, calorimetrically measured power at the dummy load, Pload, for oscillations of the gyrotron output power, Pgyro., of 1 MW for 100 s at 110 GHz and 138 GHz.

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2.3. Calorimetric measurement of the losses in the gyrotron In high-power, long-pulse experiments, it was clarified that the diffraction loss, which was evaluated by calorimetric measurements of the cooling water, was sufficiently low at both frequencies (3.8% at 110 GHz and 3.2% at 138 GHz) for the long-pulse operation at 1 MW. The increase in the gyrotron internal pressure due to temperature increase in the internal components heated by the diffracted waves was the order of 10-6 Pa, which was lower than the

• perational limit of 1×10-5 Pa for long pulse operations. It is noted that the amount of the

diffraction loss at each frequency is comparable to that of the previous 110 GHz gyrotron with improved mode convertor (The measured diffraction loss was also ~ 3% [10]). Moreover, the cavity Ohmic loss, which generally limits oscillation power even at a short pulse of 1 s, was successfully reduced by 20% (~29 kW at 1 MW oscillation) by increasing the oscillation mode number from TE22,6 to TE22,8 at 110 GHz compared to the previous gyrotron (~36 kW at 1 MW oscillation). The increase in the surface area of the cavity also enabled to reduce peak heat load of the cavity wall. From the above results, it is expected that oscillations of 1.5 ~ 2 MW (at least several seconds), which exceeds previous records (1.5 MW/4 s, 1.4 MW/9 s) of the improved 110 GHz gyrotron of JT-60U [7], will be possible at 110 GHz as expected in the design. 2.4. Oscillation at an additional frequency of 82 GHz Since a frequency of 82 GHz satisfies the condition dw = 3 × w, it is possible to transmit the 82 GHz wave with small reflection sufficient for 1 MW oscillations through the same output window with the thickness of 2.29 mm. In the gyrotron design, we evaluated an oscillation characteristic at this additional frequency of 82 GHz at the operating mode of TE17,6 (’mn = 39.46, fcutoff = 82.4 GHz, rb = 10.5 mm,  = 128.97°). In calculation, it was shown that the beam profile at the output window is slightly elongated as shown in Fig. 3 while it has the diffraction loss of lower than 10%. The diffraction loss is comparable with the original 110 GHz gyrotron for JT-60U, by which

• scillations of 1 MW/5s, 1.5 MW/4 s were

demonstrated [10]. A cavity output power of ~ 1 MW was obtained with slightly higher beam current of ~ 50A, in calculation. From these investigations, we expect that this gyrotron can be operated at 82 GHz with an output power

• f < 1 MW, and a pulse length of < 1s.

So far, an oscillation of 0.3 MW for 20 ms at 82 GHz was obtained by operating the gyrotron with the cavity magnetic field of ~ 3.3T [8]. The measured frequency was 82.5 GHz as

• expected. The pulse length in this preliminary experiment was limited by the capability of the

short pulse dummy load directly connected to the output window of the gyrotron. Since the burned pattern at the MOU inlet (Fig. 3) agreed with the calculation, the longer pulse length and the higher power experiment will be possible at this frequency.

• Fig. 3. Cold resonance surface of 82 GHz

(left), calculated gyrotron output profile (right top) and the burned pattern (right bottom).

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The above result

• f the oscillation at

82 GHz showed a possibility of the use of fundamental harmonic resonance waves at Bt ~ 2.3 T for start- up assist [11] and ECH wall-cleaning [12] where relatively low power and short pulse (<1 MW, < 1 s) are sufficient in JT-60SA. Since the main frequencies of 110 GHz and 138 GHz are injected as second harmonic resonance waves (Fig. 3), the use of the 82 GHz wave as the fundamental harmonic resonance wave will extend the operation regime of the ECH/ECCD system significantly.

• 3. Development of ECH/ECCD launcher for dual-frequency operation

3.1. Launcher development Development of an ECH launcher with high reliability is in progress based on a linear- motion antenna concept [13] as shown in

• Fig. 4 and Fig. 5. Though the quasi-optical

antenna mirrors were designed for only 110 GHz in the previous works [14, 15], the quasi-optical characteristics of the antenna at 138 GHz was investigated by a numerical code, which calculates a Kirchhoff integral based on Huygens principle. It was newly clarified that the antenna concept is suitable for dual frequency operation since the calculated poloidal beam width was insensitive to the frequency as shown in

• Fig. 6. In this design, there are no

curvatures in toroidal direction for both first and second mirrors. Thus the beam divergence in toroidal direction is different for each frequency due to its difference in the diffraction effect. However, in the poloidal direction, the second mirror has the small curvature radius of 700 mm. This curvature results in the difference in the beam waist position and its radius at each frequency, and then the similar divergence after the waist position for both frequencies are obtained. From a viewpoint of the ECCD current profile width, the poloidal beam width is more important than

• Fig. 4. CAD image of the ECH/ECCD launcher and photographs of the full

scale mock-up of the steering structure using two bellows and the quasi-

• ptical antenna mirrors.
• Fig. 5. Beam steering concept of the launcher.

Linear motion and rotation of the 1st mirror enable poloidal steering of 60° and toroidal steering of 30°.

• Fig. 6. Calculated toroidal (top) and poloidal

(bottom) beam width for 110 GHz and 138 GHz at each major radius, R, of the tokamak.

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the toroidal beam width. Thus, the characteristic of poloidal beam width insensitive to the

• perating frequency is suitable for dual-frequency operation.

The mechanical design and test of the launcher components are also in progress. In this antenna concept, a bellows structure is a key to obtain wide steering range in two directions [15]. A full size mock-up of the steering structure with improved design for easy maintenance of the bellows has been fabricated (Fig. 4) to confirm its reliability by a cyclic steering test. Since the offset of the axis of the bellows for the toroidal steering is relatively large, the life time of this bellows will be shorter than the long straight bellows for linear

• motion. In order to realize quick maintenance, the design of this bellows was improved so that

the bellows can be replaced easily. The cyclic tests of 105 cycles for poloidal steering and 104 cycles for toroidal steering are under preparation, and it is expected to be finished in 2014. Moreover, the design of the large curved second mirror, which has an internal cooling line, is in progress by evaluating the thermal stress and the electro-magnetic force during a disruption. In order to establish the fabrication process by a hot isostatic pressing, a full scale mock-up of this mirror is under fabrication. Prototypes of a long steering shaft with a bearing structure enabling both rotational and linear motions are also designed and under fabrication to confirm its reliability and to confirm its fabrication process. 3.2. Polarizer development In order to inject the millimeter waves into the plasma with a sufficiently high coupling efficiency to the desired mode (extra-ordinary (X-) mode in the case of low field side injection

• f the second harmonic resonance waves), it is required to make an optimum elliptical

polarization of the injected waves. However, a miter-bend type polarizer used with a corrugated waveguide transmission line was usually optimized only for single frequency. Recently, a polarizer with a groove depth, width, and period optimized for dual frequency

• peration has been designed in a collaboration research between JAEA and Ibaraki University.

In calculation, it was found that the wider ratio between the groove with and the period is suitable for the wide band operation [16]. Since the higher heat load is anticipated for the twister polarizer with the deeper grooves (~ /4) compared with that of the circular polarizer (~ /8), a prototype twister with grooves

• ptimized in the above calculation was fabricated. In a low power (~1 mW) experiment of the

prototype twister, it was confirmed that it is possible to obtain any target angle of polarization at both frequencies by rotating the grooved mirror of the twister polarizer as expected. A high-power test was also carried out using the dual-frequency gyrotron at the gyrotron output power of ~ 0.25 MW for 3 s at 110 GHz. The high-power test result showed that the heat load at the angle of polarization,  = 180° is approximately 2 times higher than that at  = 0° as expected in the design. A circular polarizer was also designed and tested at low

• power. By combining the results of low power

tests, it was shown that the Poincare sphere can be covered by combinations of the rotating angles of the designed twister and circular polarizers at both frequencies as shown in

• Fig. 7. Almost covered Poincare sphere
• btained by a low power measurement of the

newly designed twister and circular polarizers.

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• Fig. 7. Thus it is possible to obtain almost arbitrary elliptical polarizations required in the

experiments, at two frequencies, independently of the TL configuration.

• 4. Summary

In JAEA, a dual-frequency gyrotron (110 GHz/138 GHz) has been developed successfully. Oscillations of 1 MW for 100 s, which is the world record of a dual-frequency gyrotron and the target values for the ECH/ECCD system in JT-60SA, were achieved at both frequencies. The high oscillation efficiencies without CPD of 34% (110 GHz) and 32% (138 GHz) were

• btained by optimizing the pitch factor with the triode electron gun. The measured diffraction

and the cavity Ohmic loss were acceptable for the higher power oscillations 1.5 ~ 2 MW (at least several seconds) at 110 GHz. An oscillation of an additional frequency of the dual- frequency gyrotron at 82 GHz, which is effective for EC-wall cleaning and start-up assist, was

• demonstrated. The operation regime of the ECH/ECCD system in JT-60SA will be extended

significantly by enabling to inject the 82 GHz waves as fundamental harmonic resonance

• waves. Development of the ECH/ECCD launcher based on the linear motion antenna concept

and a wide band polarizer are in progress. It was clarified that the antenna concept is suitable for the dual-frequency operation with almost the same poloidal beam width at both

• frequencies. Moreover, a newly designed polarizer has been tested at low and high power. It

was shown that almost arbitrary elliptical polarization can be generated by this polarizer, and the initial high power test result showed that the observed heat load on the grooved mirror of the twister polarizer is qualitatively consistent with the design calculation. In the above aspects, the operation regime of the ECH/ECCD system in JT-60SA is expected to be extended.

References

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• M. Thumm, State-of-the-Art of High Power. Gyro-Devices and Free Electron Masers.

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[6]

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[7]

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[8]

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