106 GHz electron cyclotron heating experiment on Heliotron-E

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m Fusion Engineeringand Design 26 (1995) 153-158

ELSEVIER

Fusion Engineering and Design

106 GHz electron cyclotron heating experiment on Heliotron-E K . N a g a s a k i a, H . Z u s h i a, M . S a t o b, F. S a n o ~, K . K o n d o a, S. S u d o b, T. M i z u u c h i a, S. B e s s h o u ", H . O k a d a a, M . I i m a a, S. K o b a y a s h i a, K . S a k a m o t o a, A . I s a y a m a c, T. O b i k i a " Plasma Physics Laboratory, Kyoto University, Uji, Kyoto, Japan b National Institute for Fusion Science, Nagoya, Japan c Faculty of Electrical Engineering, Department of Engineering, Kyoto University, Kyoto, Japan

Abstract

A new electron cyclotron heating (ECH) system has been installed on the Heliotron-E helical device. The gyrotron output 106 GHz TEj2,2 mode is converted into a gaussian beam by the quasi-optical Vlasov convertor. The gaussian beam is coupled to the HE~ waveguide mode, then transmitted by the corrugated waveguides. The transmitted beam is well focused and launched to the Heliotron-E device. Plasma has been produced and heated by this 106 GHz ECH. The second harmonic breakdown was investigated and compared with the fundamental harmonic breakdown. The improvement of the particle confinement was observed during 106 GHz second harmonic ECH. Compared with the 53 GHz fundamental ECH phase, the averaged electron density was increased by 20% without gas puffing. The outflux to the divertor plate was reduced.

I. Introduction

With stronger magnetic field strength and larger plasma volume in fusion devices, higher power (500kW and above) and higher frequency (100 GHz and above) millimetre wave system is required for electron cyclotron heating (ECH). Such systems have been designed and developed in current fusion experimental devices such as Heliotron-E [1], W7-AS [2], D I I I - D [3] and Tore Supra [4]. Gyrotrons in these systems have output modes of the whispering gallery mode (WGM) or the gaussian beam converted by the built-in convertor. Since the W G M is not suitable for long transmission, it must be converted into the gaus-

sian beam. The Vlasov convertor is a convenient tool for obtaining the gaussian beam. However, the conventional Vlasov convertor has some side lobes, leading to the degradation of the conversion efficiency. In Kyoto University, an improved Vlasov convertor was developed, which was designed on the basis of geometrical optics. It showed a high conversion efficiency (96% in the cold test) and the radiation pattern was found to be close to gaussian [5]. Two kinds of transmission lines are considered for the high power and high frequency systems. One is an overmoded circumferentially corrugated or dielectrically lined circular HE~ mode waveguide, and the other is a quasi-optical beam waveguide using focusing reflectors for transmission of the TEM00 mode [6].

0920-3796/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0920-3796(94) 00180-4

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For Heliotron-E, the tubular HE~I waveguides are used for transmission. A similar system is used in D I I I - D tokamak. We have two main advantages for the 106 G H z E C H in Heliotron-E. The first is that we can extend the cut-off density toward 7 x 1013 c m - 3 which is twice as much as in the 53 G H z ECH. The second is that the power is launched with the extraordinary (X) mode which has a good absorption efficiency (nearly 100%) in a single pass. This would lead to the well-localized power deposition. In this paper, we present the 106 G H z E C H system and plasma experimental results. Some components of the system are explained and the radiation measurement results are shown. The plasma is produced and heated by the 106 G H z ECH. The second harmonic breakdown is investigated and compared with the fundamental harmonic breakdown. The improvement of the particle confinement observed during the 106 G H z E C H is also described.

2. 106 GHz electron cyclotron heating system Fig. 1 shows a schematic view of the 106 G H z E C H system on Heliotron-E. The system consists of a pulsed 106 G H z gyrotron source with the TE12,2 W G M , conversion system, HEII transmission line and launching system. The attainable m a x i m u m gyrotron output power is 440 kW, and

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the pulsed width is 100 ms. For the experimental results reported here, however, the power and the pulse width were limited up to 250 kW and 20 ms respectively. We used the quasi-optical Vlasov convertor which was developed by Iima et al. [5]. The conventional Vlasov convertor has a conversion efficiency of only up to 80% in the low power test owing to the occurrence of large side lobes. In our system, the launcher wall is deformed so as to obtain the gaussian beam by only one reflector mirror. The cold test showed that the radiated power contained in the main lobe was 96% of the total power. The power distribution was found to be close to gaussian in the azimuthal direction. The convertor assembly was designed and constructed by taking into account the cold test results. The improved Vlasov launcher is shown in Fig. 2. The diameter of the waveguide is 38.1 mm, the helical pitch angle is 25.8 ° , and the length of the straight edge is 213 mm, which are determined by geometrical optics. Although the radiation pattern at the reflector plate was a little expanded in the axial direction, the main lobe was found to dominate 85% of the total radiated power. The converted beam is focused and coupled to the HE~, waveguide mode. The transmission line consists of 18 m corrugated waveguides (diameter D = 63.5 mm) and four mitre bends. The total transmission loss is theoretically estimated as

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Fig. 2. Modified quasi-optical Vlasov launcher. The diameter of the waveguides is 38.1 mm, The power distribution in the azimuthal direction can be changed by the deformed launcher wall, the so-called visor.

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K. Nagasaki et al./ Fusion Engineering and Design 26 (1995) 153-158

opposite side of the launching port. It was confirmed that the single-pass a b s o r p t i o n was quite good if the resonance layer (B = 1.9 T) was located in the p r o p e r position (high field side).

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3. I. P l a s m a b r e a k d o w n

Fig. 3. Radiation pattern from launcher mouth for injection to the Heliotron-E device• The measured position is 410 mm, which corresponds to the plasma centre.

5.2%. The radiation m e a s u r e m e n t at the waveguide m o u t h showed that the radiation pattern after long transmission was almost circular and there were no large side lobes. Since the conversion process gives rise to side lobes, and the main lobe is a little d e f o r m e d before transmission, it is concluded that this transmission line acts as a m o d e filter• The gaussian b e a m is launched f r o m the top of the torus with the X mode. Before launching, the b e a m is focused by the quasi-optical type launcher. The radiation pattern in the hot test is shown in Fig. 3. This launching system effectively t r a n s f o r m s the gaussian b e a m waist in the free space to a 2 cm (poloidal) x 3 cm (toroidal) 1/e p o w e r spot size in the plasma centre region. This b e a m size is small c o m p a r e d with the plasma m i n o r radius in the horizontal direction ( a b o u t 15 cm). Since the second h a r m o n i c X m o d e has a high single-pass a b s o r p t i o n ( a b o u t 100% at T~ = 1 keV, n e = 1 x 1 0 1 3 cm-3), we can expect that the p o w e r deposition is well localized near the p l a s m a central region. In the plasma experiment, we m e a s u r e d the transmission of the b e a m with a diode detector which was located on the

A currentless plasma has been initiated by the 106 G H z second h a r m o n i c E C H [7]. The breakdown greatly depends on the position of electron cyclotron resonance ( E C R ) layer (B = 1.90T). Fig. 4 shows positions of the resonance layer B = 1.90 T at several central field strengths. When the E C R layer was located at the high field side, that is, the focused gaussian b e a m did not cross the E C R layer, then the b r e a k d o w n never occurred within the pulse width ( a b o u t 15ms) in

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K. Nagasaki et al./ Fusion Engineering and Design 26 (1995) 153-158

this experiment. This is quite different from the 53 G H z ECH breakdown. In the 53 G H z ECH, the breakdown occurs even if the ECR layer is located in the high field side. The time evolution is also different between them. At the 106 G H z second harmonic ECH, the breakdown occurred near the magnetic axis, and then expanded to the peripheral region. The 53 G H z second harmonic breakdown at B h = 0.94 T also has such a behaviour. On the contrary, at the 53 G H z fundamental ECH, the edge density rose as soon as the breakdown started. Therefore the phenomenon that the breakdown starts in the central region seems characteristic of the second harmonic ECH. As shown in Fig. 5, it takes more than 10ms to reach the stationary state, which was about three times as much as the 53 G H z fundamental ECH. The particle confinement at the breakdown seems to be better in the 106 G H z ECH. The electron density profile was not hollow but close to parabolic, and the average density was reached twice at the same gas puffing conditions. Since the ECH pulse was short, about 15 ms, the plasma did not suffer from density clamping. A longer pulse length is needed for further analysis of the density profile. Breakdown

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The different characteristics of the breakdown may be explained by the wave-particle interaction. The non-linear interaction of a cold electron with the wave plays an important role on the second harmonic electron cyclotron breakdown. A theory [8] indicates that the breakdown would occur if the accelerated electron can be trapped in the well-confined region, e.g. the magnetic axis. On the contrary, the fundamental electron cyclotron breakdown would occur along the ECR layer, because the electron can be easily accelerated in the linear interaction with the wave. There is also a little discrepancy between the ray tracing calculation and the experiment. The ray tracing calculation showed that almost all power was absorbed in the single pass at B h ~> 1.91 T. However, the breakdown delayed about 4 ms even at Bh(0) = 1.92 T, and Te(0) was rather low, about 900 eV at ~e = 0.8 X 1013 cm -3. The best condition was found to be B h ( 0 ) = 1.96T which is the maximum field strength in Heliotron-E. This upshift may be due to the relativistic effect on the resonance frequency. The upshift to Bh(0) = 1.96 T corresponds to an electron energy of about 26 keV.

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The particle confinement was improved during the 106 G H z ECH. After a target plasma was produced by the 5 3 G H z ECH, the 1 0 6 G H z power was injected. Gas puffing was performed only before the discharge. There was no gas puffing during the discharge so that the plasma was sustained by wall recycling. A carbon limiter was slightly inserted into the plasma (r/a = 0.9). In the 53 G H z plasmas, once the density clamping happens, the electron density starts to decrease and the outflux to the wall is enhanced as shown in Fig. 6(a). On the contrary, Fig. 6(b) shows that the plasma heated by the 106 G H z ECH is different from that by the 53 G H z ECH. In the afterglow phase, the current flowing into the limiter and the floating potential at the wall went down to zero, and the ion saturation current and the D~ emission levelled down by half. These low levels were kept even when the 106 G H z power was injected. The central density increased to 20%

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observed, which were measured with some far infrared interferometer (FIR) channels [9]. These indicate that the improvement in the core plasma affects both the outflux to the divertor and the parallel particle transport.

4. Conclusion

A new 106 GHz ECH system (0.4 MW, 100 ms) was developed and installed on the Heliotron-E helical device. The TEI2,2 gyrotron output was effectively converted into the gaussian beam by modified quasi-optical Vlasov convertor developed in Kyoto Universtiy. The transmitted HE]~ mode was confirmed to be circular and radiated as predicted in the theory. It was experimentally

158

K. Nagasaki et al. / Fusion Engineering and Design 26 (1995) 153-158

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p l a s m a . T h e average electron density increased b y 20% w i t h o u t gas puffing. T h e Do~ emission at the wall a n d the ion s a t u r a t i o n c u r r e n t were k e p t as low as in the a f t e r g l o w phase. T h e edge Te at r/a >~ 0.8 was low, a b o u t 1 0 e V even in the h e a t i n g phase, while the central T e was 1.3 keV at tie ~ 1.0 x 10 ]3 cm -3. This m a y be because a l m o s t all p o w e r is a b s o r b e d n e a r the central region in the first pass, leading to the p e a k e d d e p o s i t i o n profile.

Acknowledgments T h e a u t h o r s w o u l d like to t h a n k the H e l i o t r o n - E G r o u p for o p e r a t i n g the H e l i o t r o n - E device. Calc u l a t i o n o f the r a d i a t i o n f r o m the m o d i f i e d Vlasov c o n v e r t o r b y Dr. M. N a k a j i m a a n d Dr. O. W a d a is a p p r e c i a t e d . T h e y also a c k n o w l e d g e Dr. K. O h k u b o a n d Dr. S. K u b o for the help o f designing the c o r r u g a t e d waveguide.

References

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100

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Fig. 7. Time evolution of pellet-injected ECH plasma. The reduction in the Ha emission and the outflux to the wall were maintained during the 106 GHz ECH. c o n f i r m e d t h a t the c o r r u g a t e d w a v e g u i d e a c t e d as a m o d e filter. T h e focused b e a m was l a u n c h e d f r o m the t o p o f the torus, a n d its radii were 2 c m ( p o l o i d a l ) x 3 c m ( t o r o i d a l ) in e-folding p o w e r spot size in the high p o w e r test. These are small e n o u g h c o m p a r e d with the p l a s m a m i n o r r a d i u s ( a b o u t 15 cm). T h e p l a s m a b r e a k d o w n by the 106 G H z E C H was investigated in H e l i o t r o n - E . T h e b r e a k d o w n occurs in the p l a s m a central region, a n d then e x p a n d s to the p e r i p h e r a l region, which is quite different in the f u n d a m e n t a l E C H . This phen o m e n o n m a y be e x p l a i n e d b y the n o n - l i n e a r i n t e r a c t i o n o f the electron with the wave in the g o o d electron c o n f i n e m e n t region. The i m p r o v e d state related to the particle c o n f i n e m e n t was a t t a i n e d in the 106 G H z E C H

[1] K. Nagasaki et al., 106 GHz ECH system for Heliotron-E, Fusion Technol., 25 (1994) 419. [2] W. Kasparek, Millimeter wave systems for high-power ECRH, status and future trend, Proc. 8th Joint Workshop on ECE and ECRH, Gut Ising, Germany, 1992, Vol. 2, p. 423. [3] C. Moeller et al., 110 GHz ECH System for D i l l - D , Proc. 18th EPS Conf. on Controlled Fusion and Plasma Physics, Berlin, 1991, Vol. III, p. 369. [4] M. Pain et al., The 110 GHz electron cyclotron heating and current drive system of Tore Supra, Proc. 8th Joint Workshop on ECE and ECRH, Gut Ising, 1992, Vol. 2, p. 523. [5] M. lima et al., Measurement of radiation field from an improved efficiency quasi-optical convertor for whisperinggalley mode, Proc. 13th Int. Conf. on Infrared and Millimeter Waves, Wiirzburg, 1989. [6] W. Henle et al., Study on ECW transmission lines for NET/ITER, IPP Rep. EUR-FU/80-99, 1990. [7] K. Nagasaki et al., Confinement improvement by 106 GHz 2nd harmonic electron cyclotron heating in Heliotron-E, Proc. 20th EPS Conf. on Controlled Fusion and Plasma Physics, Lisboa, 1993, Vol. 17C, Part I, p. 397. [8] M.D. Carter et al., Second harmonic electron cyclotron breakdown in stellarators, Nucl. Fusion 27 (1987) 985. [9] H. Zushi et al., Analysis of magnetic turbulence during pellet ablation and response of fueled particles by pellet injection to the SOL divertor, Proc. 20th EPS Conf. on Controlled Fusion and Plasma Physics, Lisboa, 1993, Vol. 17C, Part II, p. 715.