Relaxation of pressure anisotropy due to Alfvén-ion-cyclotron fluctuations observed in ion-cyclotron-range-of-frequency–heated mirror plasmas

Abstract

Alfven ion-cyclotron modes are generated over a wide range of anisotropy parameter P±(T±/T\\) 2 in ICRF-heated central cell plasmas of the GAMMA 10 tandem mirror. The fluctuations are observed at a small driving force in the convectively unstable region predicted theoretically in an infinite plasma. Before reching the ft limit of p± = 3.52x (T\\/T±) 2 , above which the absolute instability is predicted, relaxation of pressure anisotropy is observed for the first time by use of an array of diamagnetic loops together with a Faraday cup and secondary-electron emission detectors.PACS numbers: 52.35.Qz, 52.50.Gj, 52.55.Jd Generation of an Alfven ion-cyclotron (AIC) instability is expected in a magnetized plasma with strong pressure anisotropy. However, there have been limited experimental works, especially in laboratory plasmas. The first experimental evidence of the AIC mode was performed in the end cell of the TMX tandem mirror [1,2]. A high p± (the ratio of the perpendicular plasma pressure to the magnetic field pressure) plasma with strong pressure anisotropy is produced by intense neutral-beam injection (NBI) in the direction perpendicular to the magnetic field line at the midplane of the minimum-/? mirror field. The AIC mode was also observed in an ion-cyclotron-rangeof-frequency-(ICRF-) heated plasma in the central cell of the TAR A tandem mirror [3]. The interaction between alpha particles and the generated Alfven modes is going to be a critical subject for future fusion devices. Effects of the AIC mode on the plasma parameters were described precisely in computer simulations [4][5][6][7]. Also, rapid pitch angle scattering due to the AIC mode has been simulated in a space plasma [8]. Relaxation of the pressure anisotropy is predicted theoretically; however, a direct observation of the relaxation due to the generation of the AIC mode has not been reported in laboratory plasmas.In this Letter we report a parametric study of the AIC fluctuation for a wide range of the anisotropy parameter, in order to compare with an available theory. We report the first observations of the anisotropy relaxation associated with the AIC mode in the ICRF-heated central cell plasma of the GAMMA 10 tandem mirror.GAMMA 10 is a minimum-/? anchored tandem mirror with axisymmetric plug/thermal-barrier regions at both ends [9,10]. The central cell is 5.6 m long with a field strength of 0.4 T at midplane. The mirror ratio is 5. The diameter of the limiter which is set near the midplane of the central cell is 0.36 m. Double half-turn antennas are located near the end of the central cell for ion heating at the fundamental ion-cyclotron resonance layer near the midplane. So-called Nagoya type-Ill antennas [11] are installed on the outer side of the double half-turn antennas. They are used for producing and sustaining a plas-ma, together with hydrogen gas puffing into the central cell and short-pulse (1 ms) plasma guns at both ends [12]. In the present experiment, no other heating systems such as electron-cyclotron heating (ECH) and NBI a...

Measurement of Spatiotemporal Behavior of Alfvén-Ion-Cyclotron Waves in the GAMMA10 Tandem Mirror

“…In GAMMA10, AIC waves are spontaneously excited when the driving force β(T ⊥ /T ) 2 exceeds a threshold value, which is well below the theoretical prediction [9]. Figure 3 (a) shows the temporal evolution of the diamagnetism measured near (DM cc ) and 1.5 m away from (DM cw ) the midplane of the central cell.…”

Direct Observation of Electron Heating by Electron Landau Damping of Alfvén Ion Cyclotron Waves Using Thomson Scattering System in the Tandem Mirror GAMMA 10/PDX

“…Detailed description of GAMMA 10/PDX has been provided elsewhere [1][2][3][4][5][6][7][8][9][10][11][12]. The magnetic field strength at the midplane of the central cell is 0.405 T, and the mirror ratio of the central cell is 5.…”

“…A maximum ion temperature of 10 keV and a temperature anisotropy (defined as the temperature ratio between directions perpendicular, T ⊥ , and parallel, T // , to the magnetic field line) of more than 10 have been achieved. In such a strong temperature anisotropy condition, slow Alfvén ion cyclotron (AIC) waves were spontaneously excited in the central cell of GAMMA 10/PDX [3][4][5][6]. The AIC mode is normally observed using magnetic probes [7] and reflectometers [8,9].…”

Multi-point measurement using two-channel reflectometer with antenna switching for study of high-frequency fluctuations in GAMMA 10

“…AIC waves have several discrete peaks in the unstable frequency range that occur just below the ion cyclotron frequency near the midplane of the GAMMA 10 central cell. 7 The boundary conditions required for the formation of such eigenmodes in the unstable frequency range are not clear to date. The three-dimensional spatial structure of the AIC waves, which is crucial to the clarification of the boundary conditions, has not yet been measured.…”