I. Ogawa

K. Tanaka

M. Nishiura

S. Ikeuchi

S. Ito

S. Kobayashi

S. Kubo

S." Ogasawara

T. "Saito

T. Idehata

T. Kuwahara

T. Shimozuma

Y. Tatematsu

Y. Yamaguchi

Y. Yoshimura

National Institute for Fusion Science (NIFS-Repository)

The objective of this study is to stabilize vertical instabilities of elongated tokamak plasmas by the use ofsaddle coils which make helically perturbed fields. Although several studies have been made on the effects [1], they have been conducted in tokamaks with circular cross-sections. For the next step of this study, we have been constructing a small tokamak device (R = 0.33 m, a = 0.09 m, N = 1.8, Bt = 0.3 T) which has a highly elongated cross-section to demonstrate the stabilizing effects. Among the progress of the construction, we report the following topics: strength tests of a trial toroidal field coil and discharge tests of flywheel power supply for poloidal field coils.Firstly, we designed support structures for the toroidal field coils by stress analysis using the finite element method. After we confirmed soundness of the support structurenumerically, we performed strength tests on a trial TF coilas shown in Fig. 1. The maximum electromagnetic force which was estimated to be a centering force of 550 kgf was simulated by the load of a heavy weight of 1000 kg in thetests. It was found from the strain measurements that the deformation of the support structure can be elastic even under overload up to twice of the estimated maximum centering force. After we got the result, we made all sixteen toroidal field coils and assembled them (Fig. 2).We proposed a new simple flywheel generator system inspired by micro hydro power technology as the power supply for poloidal field coils (Fig. 3)[2]. This system uses a self-excited induction generator (SEIG) converted from a commercially available induction motor so that it can be cost effective. The SEIG can maintain nearly constant terminal voltage in combination with rectifier circuit much longer than commonly used capacitor banks. The post stage dc-chopper which was controlled by taking voltage fluctuations in SEIG into account converts the varyingvoltage to arbitrary voltage to control currents flowing in poloidal field coils. We conducted discharge test of the power supply with a sample coil comparable to poloidal field coils (Fig. 4). It was demonstrated that the proposed power supply can maintain command square-wave current flowing in the test coil for 1 s which is long enough to our small tokamak experiments.1) Ikezi, H., Schwarzenegger, K. F.: Phys. Fluids 22 (1979)2009.2) Hatakeyama, S., et al.: 1PoCL-07, MT-23, 2013.Fig. 1 Pictures of strength tests of a trial toroidal field coil.Fig. 4 Experimental waveforms of the sample coil current and the generator voltage.Fig. 2 Assembling of the toroidal field coils.Fig. 3 Circuit configuration of the flywheel generator system.i) IntroductionUse in collective Thomson scattering (CTS) diagnostics of fusion plasmas is mostly fit for gyrotrons on their distinctive ability of high power at high frequencies. At present, gyrotrons around 100 GHz developed for electron heating are used in CTS diagnostics [1], [2]. EM waves within this frequency band suffer from severe refraction and/or absorption in a plasma. Moreover, strong electron cyclotron emission is a large noise source. Use of a gyrotron in a sub-THz frequency range will resolve these problems.We have been developing high power sub-THz gyrotrons. Firstly, second harmonic (SH) oscillation was used. We have succeeded in high power single mode oscillation [3] and realized a maximum power approaching 100 kW at 389 GHz [4]. However, mode competition with fundamental harmonic (FH) modes has prevented achievement of much higher power [5].Then, we have started a task of development of an FH mode sub-THz gyrotron under the NIFS collaboration study scheme. We have reported over 200 kW oscillation of a prototype gyrotron [6]. In this report, we show new experimental results approaching 250 kW and the preliminary design work for an actual gyrotron.ii) Experimental resultsThe prototype gyrotron is mounted on a liquid He-free 12 T super conducting magnet, the diameter of the room temperature bore of which is 100 mm. The electron gun was very carefully designed for realization of a high quality electron beam at large beam currents [7]. This gyrotron is equipped with an internal mode convertor to deliver a Gaussian beam. It is of sealed-off type for high power oscillation. The vacuum window is made from a single crystal sapphire disk of c-axis cut. The design oscillation mode and the frequency are co-rotating TE14,2 mode and 295 GHz, respectively.Succeeding the previous result [8], operation conditions of the gyrotron was carefully optimized and the beam current Ib was increased up to 14 A. The result is indicated in Fig. 1. It plots the window power P measured with a water load and the efficiency as functions of Ib. Thevalue of P increases with Ib approaching 250 kW at Ib of 14 A and Vk of 65 kV. The efficiency gradually decreases with Ib but still higher than 25%.The TE14,2 mode is well isolated from possible competing modes and very stable single mode oscillation is obtained for wide rages of operation conditions.iii) Design of an actual gyrotronThe design concept to realize single mode high power oscillation of sub-THz gyrotron has been verified with the prototype gyrotron. An actual gyrotron will be designed upon this concept and, in the fiscal year 2014, we will manufacture a gyrotron that will be really used in the CTS diagnostics on LHD. The power target is 300 kW with the pulse width up to 1 ms. Figure 2 shows an initial design calculation for the case of TE18,2 mode. Over 300 kW power is expected. Now, works are under way for the final decision of the oscillation mode including the design of an electron gun congruent with the oscillation mode.[1] Bindslev, H. et al.: Phys. Rev. Lett. 97 (2006) 205005.[2] Nishiura, M. et al.: Nucl. Fusion 54 (2014) 023006.[3] Notake, T. et al.: Phys. Rev. Lett. 103 (2009) 225002.[4] Saito, T. et al.: Phys. Plasmas 19 (2012) 063106.[5] Saito, T. et al.: Phys. Rev. Lett. 109 (2012) 155001.[6] Saito, T. et al.: IRMMW and THz 2013, Mo5-3.[7] Yamaguchi, Y. et al.: Phys. Plasmas 19 (2012) 113113.[8] Yamaguchi, Y. et al.: Plasma Fusion Res. 8 (2013)1205165.0501001502002503000204060801000 2 4 6 8 10 12 14Power (Vk=55)Power (Vk=60)Power (Vk=65)Efficiency (Vk=55)Efficiency (Vk=60)Efficiency (Vk=65)Power (kW)Efficiency (%)Beam Current (A)IGUN = 80 AVR = 140/350IGUN = 25 AIGUN = 50 AFig. 1 Oscillation power and efficiency are plotted with closed circles and open circles respectively as functions of the beam current.010020030040050000.20.40.60.810 5 10 15 20D = 1.1D = 1.3Cavity Power (kW)EfficiencyBeam Current (A)TE18,2 (303.321 GHz)LC = 8.0 mmRC = 4.015 mmRB = 3.009 mmVK = 65 kVFig. 2 Preliminary design calculation of the actual gyrotron for the case of the TE18,2 mode..247§12. Study of High Power Sub Terahertz Pulse Gyrotron for Application to Collective Thomson Scattering Diagnostics in LHDSaito, T., Idehata, T., Ogawa, I., Tatematsu, Y., Yamaguchi, Y., Ikeuchi, S., Kuwahara, T. (Univ. Fukui, FIR FU),  Shimozuma, T., Kubo, S., Tanaka, K., Nishiura, M., Yoshimura, Y., Kobayashi, S., Ito, S., Ogasawara, S.

Study of High Power Sub Terahertz Pulse Gyrotron for Application to Collective Thomson Scattering Diagnostics in LHD

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Study of High Power Sub Terahertz Pulse Gyrotron for Application to Collective Thomson Scattering Diagnostics in LHD

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