Improved confinement events triggered by emissive electrode biasing on the tokamak ISTTOK

Edge polarization experiments were carried out on ISTTOK using an emissive electrode. The dependence of the bias current on the radial electric field, measured by a radial array of Langmuir probes, is in qualitative agreement with the expression derived by Stringer for the transitional region between plateau and collisional transport regimes (Stringer T.E. 1993 Nucl. Fusion 33 1249). Improved particle confinement is clearly observed for negative bias associated with a large radial electric field. In some cases, a double-peaked profile of the radial electric field is observed just after biasing, evolving afterwards to a single-peaked profile as confinement improves. These types of profiles may be related to the multiple solutions of the non-linear equation for the radial electric field, recently put forward by Kasuya et al (2003 Nucl. Fusion 43 244). Above a certain threshold of the bias current stronger improved confinement events are observed, during short periods, for both bias polarities.

[1]  G. Oost,et al.  Influence of electrode and limiter biasing on the ISTTOK plasma , 2004 .

[2]  Á. Cappa,et al.  Improved confinement regimes induced by limiter biasing in the TJ-II stellarator , 2004 .

[3]  I. Nedzelskiy,et al.  Limiter Biasing Experiments on the Tokamak ISTTOK , 2003 .

[4]  Y. Takase,et al.  Accessibility to a Double-Peaked Er Shear Layer Structure by Double Electrode Biasing in Tokamak Plasmas , 2003 .

[5]  G. Oost,et al.  Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results , 2003 .

[6]  Y. Takase,et al.  Effect of electrode biasing on the radial electric field structure bifurcation in tokamak plasmas , 2003 .

[7]  Guido Van Oost,et al.  Measurements with an emissive probe in the CASTOR tokamak , 2002 .

[8]  S. Jachmich,et al.  Bifurcation of the radial electric field in the presence of edge polarization in tokamaks , 2001 .

[9]  Shuichi Takamura,et al.  Effect of space-charge limited emission on measurements of plasma potential using emissive probes , 2000 .

[10]  I. Caldas,et al.  Magnetic and electrostatic fluctuations in the CASTOR tokamak , 1999 .

[11]  P. Belo,et al.  Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing , 1998 .

[12]  A. Fukuyama,et al.  Solitary radial electric field structure in tokamak plasmas , 1997 .

[13]  J. Anderson,et al.  Enhanced Confinement with Plasma Biasing in the MST Reversed Field Pinch , 1997 .

[14]  H. Sanuki,et al.  Physical Mechanism Determining the Radial Electric Field and its Radial Structure in a Toroidal Plasma , 1994 .

[15]  G. Tynan,et al.  Electrostatic biasing of the ALT-II pump limiter , 1994 .

[16]  S. Luckhardt,et al.  Scrape-off layer current in diverted Tokamak , 1994 .

[17]  André Rogister,et al.  Revisited neoclassical transport theory for steep, collisional plasma edge profiles , 1994 .

[18]  G. Oost,et al.  Predicting the radial electric field imposed by externally driven radial currents in tokamaks , 1994 .

[19]  N. Sato,et al.  Plasma structures in front of a floated emissive electrode , 1993 .

[20]  T. Stringer.  Explanation of the L-H mode transition induced by applied voltage , 1993 .

[21]  W. Stacey.  Neoclassical theory of the plasma edge , 1992 .

[22]  G. Oost,et al.  Confinement and profile changes induced by the presence of positive or negative radial electric fields in the edge of the TEXTOR tokamak , 1992 .

[23]  S. Lebedev,et al.  LETTER: Radial current in a tokamak caused by a biased electrode , 1992 .

[24]  T. Petrie,et al.  Limiter bias experiments in DIII-D , 1990 .

[25]  R. N. Dexter,et al.  First results from the Madison Symmetric Torus reversed field pinch , 1990 .

[26]  R. Taylor,et al.  Dynamics of H-mode-like edge transitions brought about by external polarization , 1990 .

[27]  Brown,et al.  H-mode behavior induced by cross-field currents in a tokamak. , 1989, Physical review letters.

[28]  E. C. Crume,et al.  Bifurcation theory of poloidal rotation in tokamaks: A model for L-H transition. , 1989, Physical review letters.

[29]  R. S. Dallaqua,et al.  Particle diffusion in TBR , 1984 .

[30]  Roger H. Taylor,et al.  Suppression of Metallic Impurities by Electron Injection in a Tokamak , 1979 .