Modeling far-field radio-frequency sheaths in Alcator C-Mod

This paper is motivated by the recent measurement of large (>100 V) plasma potentials in Alcator C-Mod during ion cyclotron range of frequency (ICRF) heating. The plasma potential is measured on field lines that intersect a limiter but do not pass near a powered ICRF antenna. The measured potential correlates with the local ICRF fast wave electric field and is a prime candidate to cause increased Mo sputtering from the limiter surface. In this paper, it is shown that a theory of 'far-field' radio-frequency (rf) sheaths can qualitatively explain this experimental observation. The theory describes rf-sheath formation when unabsorbed fast ICRF waves are incident on a conducting boundary far from the antenna. It is shown that the rf-sheath drive is sensitive to the angle between the surface normal and the equilibrium magnetic field. The main conclusion of this work is that the rapid tangential variation in the B field-limiter geometry near the tip of the limiter promotes the formation of large sheath potentials of the same order as the measured ones.

[1]  D. D'Ippolito,et al.  ICRF-edge and surface interactions , 2011 .

[2]  Emilio Molina,et al.  Summary and Discussion , 2014 .

[3]  Erwin Frederick Jaeger,et al.  Power deposition in high-density inductively coupled plasma tools for semiconductor processing , 1995 .

[4]  James R. Wilson,et al.  Analysis of RF sheath interactions in TFTR , 1998 .

[5]  L. Giannone,et al.  ICRF antenna-plasma interactions and its influence on W sputtering in ASDEX upgrade , 2011 .

[6]  D. D'Ippolito,et al.  Resonance cone interaction with a self-consistent radio-frequency sheath. , 2008, Physical review letters.

[7]  R. Stenzel High‐frequency instability of the sheath–plasma resonance , 1989 .

[8]  Helmi Malova,et al.  Quasiadiabatic dynamics of charged particles in a space plasma , 2013 .

[9]  R. Mitteau,et al.  Key results of long pulse ICRH operation in Tore Supra , 2006 .

[10]  D. D'Ippolito,et al.  Analytic model of near-field radio-frequency sheaths. II. Full plasma dielectric , 2010 .

[11]  Irving Langmuir,et al.  The Effect of Space Charge and Initial Velocities on the Potential Distribution and Thermionic Current between Parallel Plane Electrodes , 1923 .

[12]  T. H. Stix Waves in plasmas , 1992 .

[13]  B. Lipschultz,et al.  RF-Plasma Edge Interactions and their Impact on ICRF Antenna Performance in Alcator C-Mod , 2007 .

[14]  D. D'Ippolito,et al.  Faraday screen sheaths and impurity production during ion cyclotron heating , 1990 .

[15]  Stenzel Instability of the sheath-plasma resonance. , 1988, Physical review letters.

[16]  James Myra,et al.  Numerical analysis of radio-frequency sheath-plasma interactions in the ion cyclotron range of frequencies , 2011 .

[17]  Myra,et al.  Sheath-plasma waves and anomalous loading in ion-Bernstein-wave experiments. , 1991, Physical review letters.

[18]  G. Oost,et al.  The interaction between waves in the ion cyclotron range of frequencies and the plasma boundary , 1993 .

[19]  L. Berry,et al.  Far-field sheaths due to fast waves incident on material boundaries , 2008 .

[20]  D. A. D’Ippolito,et al.  A radio-frequency sheath boundary condition and its effect on slow wave propagation , 2006 .

[21]  D. A. D’Ippolito,et al.  Far field sheaths from waves in the ion cyclotron range of frequencies , 1994 .

[22]  D. Russell,et al.  Nonlinear ICRF-plasma interactions , 2005 .

[23]  C. D. Child,et al.  Discharge From Hot Cao , 1911 .