Characterization of impurity confinement on Alcator C-Mod using a multi-pulse laser blow-off system.

A new laser blow-off system for use in impurity transport studies on Alcator C-Mod was developed and installed for the 2009 run campaign. Its design included capabilities for multiple impurity injections during a single plasma pulse and remote manipulation of the ablated spot size. The system uses a 0.68 J, Nd:YAG laser operating at up to 10 Hz coupled with the fast beam steering via a 2D piezoelectric mirror mount able to move spot locations in the 100 ms between laser pulses and a remote controllable optical train that allow ablated spot sizes to vary from ∼0.5 to 7 mm. The ability to ablate a wide range in target Z along with Alcator C-Mod's extensive diagnostic capabilities (soft x-ray, vacuum ultraviolet (VUV), charge exchange spectroscopy, etc.) allows for detailed studies of the impurity transport dependencies and mechanisms. This system has demonstrated the achievement of all its design goals including the ability for non-perturbative operation allowing for insight into underlying impurity transport processes. A detailed overview of the laser blow-off system and initial results of operation are presented. This includes an investigation into the characterization of impurity confinement in the I-mode confinement regime recently investigated on C-Mod.

[1]  J. L. Cecchi,et al.  System for rapid injection of metal atoms into plasmas , 1975 .

[2]  Naoto Tsujii,et al.  I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod , 2010 .

[3]  J. Rice,et al.  Spectroscopic measurement of impurity transport coefficients and penetration efficiencies in Alcator C-Mod plasmas , 1995 .

[4]  Robert S. Granetz,et al.  Edge x-ray imaging measurements of plasma edge in Alcator C-Mod , 1999 .

[5]  Impurity Transport in Alcator C-Mod Plasmas , 2007 .

[6]  P. Stek.  Reflectometry measurements on alcator C-mod , 1997 .

[7]  J. Rice,et al.  Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod plasmas , 2009 .

[8]  J. Rice,et al.  Five‐chord high‐resolution x‐ray spectrometer for Alcator C‐Mod , 1990 .

[9]  G. Magyar,et al.  Laser blow-off injected impurity particle confinement times in JET and Tore Supra , 1995 .

[10]  T. Fujita,et al.  Chapter 2: Plasma confinement and transport , 2007 .

[11]  R. Dux,et al.  Effects of plasma shape on laser blow-off injected impurity transport in TCV , 2003 .

[12]  P. T. Bonoli,et al.  Characterization of enhanced Dα high-confinement modes in Alcator C-Mod , 1999 .

[13]  J. Rice,et al.  Impurity injection experiments on the Alcator C tokamak , 1982 .

[14]  Y. Wang,et al.  H mode confinement in Alcator C-Mod , 1997 .

[15]  J. Stober,et al.  H-mode power threshold and transition in ASDEX Upgrade , 1998 .

[16]  F. Imbeaux,et al.  Experimental impurity transport and theoretical interpretation in a Tore Supra lower-hybrid heated plasma , 2008 .