Journal of the Korean Physical Society, Vol. 49, December 2006, pp. S18∼S24 Plasma Breakdown using Second-Harmonic EC Waves in Helical Devices K. Nagasaki, * T. Mizuuchi, F. Sano, H. Okada, S. Kobayashi and Heliotron J Team Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan A. Cappa, F. Castej´ on, A. Fern´ andez, E. de la Cal, T. Estrada, V. Tribaldos, F. Tabar´ es, D. Tafalla and TJ-II Team Laboratorio Nacional de Fusi´on, CIEMAT, Madrid, Spain Y. Yoshimura and CHS Team National Institute for Fusion Science, Toki 509-5292, Japan K. Kondo and K. Takahashi Graduate School of Energy Science, Kyoto University, Uji 611-0011, Japan H. Shidara Centre de Recherches en Physique des Plasmas, Lausanne, CH-1015, Switzerland (Received 29 August 2005) Plasma breakdown using second-harmonic electron cyclotron (EC) waves has been studied ex- perimentally in three helical devices, Heliotron J, TJ-II and CHS. Recent technological progress on electron cyclotron heating (ECH) systems enables us to determine what the important factors for second-harmonic plasma breakdown are. Comparison of the experimental results among three devices shows a common feature that the plasma starts up from the good confinement region and strongly depends on the nonlinear interactions with the electric field of the local beam rather than the multi-reflected field. This suggests that the confinement of high energy electrons generated by ECH has a dominant role in second-harmonic plasma breakdown. PACS numbers: 52.50.Gj, 52.55.Hc Keywords: Plasma breakdown, Electron cyclotron waves, Helical devices I. INTRODUCTION Electron cyclotron waves of fundamental and second- harmonic resonance frequencies are routinely used for plasma start-up and heating in toroidal fusion plasmas. In helical systems, the plasma start-up is easily realized without any consideration of ECH launching conditions if the resonance is positioned within the confinement re- gion. Therefore no much attention has been paid to breakdown study so far. However, the physics of second- harmonic plasma breakdown is not as simple as that of fundamental plasma breakdown. Fundamental plasma breakdown can be explained simply by a linear theory, while second-harmonic plasma breakdown requires con- sideration of nonlinear wave-particle interactions, since the linear energy increment of seed electrons is propor- tional to the gyroradius squared, which is practically zero at the initial phase [1–3]. According to a nonlinear the- * E-mail: nagasaki@iae.kyoto-u.ac.jp ory, the accelerated electrons should be well confined for causing electron avalanche until they collide with neu- trals. It is pointed out that two conditions have to be satis- fied for effective breakdown. One is that the peak energy of trapped electrons in their periodic oscillations must exceed the ionization potential of neutrals. The other is that the electron-neutral collision frequency must be low enough not to prevent the nonlinear interaction but high enough to avoid the deeply trapped electrons es- caping from the confinement region before the ionization process. This limits the neutral gas pressure range for successful breakdown. ECH is also an attractive scheme for the start-up sce- nario in tokamaks. Low voltage start-up is very impor- tant for large tokamaks to reduce large voltage spike for plasma breakdown. In ITER, for example, toroidal electric field of less than 0.3 V/m is required, because of slow voltage diffusion through the thick vessel wall, which is strongly dependent on the filling pressure, error -S18-