Clive Challis With thanks to: Doug McCune and Yuriy Baranov TRANSP NBCD bug Clive Challis With thanks to: Doug McCune and Yuriy Baranov Clive Challis - TRANSP User Group Meeting - 21 May 2004

Clive Challis - TRANSP User Group Meeting - 21 May 2004 Fast Ion Current ionisation Beam current driven by tangentially injected fast ions Fast ions that can circulate toroidally provide a current which depends on: Beam energy Number of fast ions Toroidal velocity component Fast ion slowing-down time Fast ions can be trapped in B field gradient if v||/v too small Trapped if |v||/v|<(2)0.5 (approx – Wesson, =r/R) Trapped ions do not drive current beam neutrals plasma fast ions Clive Challis - TRANSP User Group Meeting - 21 May 2004

Back Electron or Shielding Current Thermal electrons dragged by collisions with fast ions Bulk electron rotation given by momentum balance with fast and thermal ions For: Zbeam=Zplasma; Mbeam=Mplasma ve>>vi R>>r (no particle trapping) Momentum balance gives: nfast(v||fast-<ve>)-nion<ve>=0 Jnet=nfastev||fast-nee<ve> ne=nion+nfast  Jnet=0 thermal ions (nion) thermal electrons (ne, <ve>) fast ions (nfast, v||fast) Clive Challis - TRANSP User Group Meeting - 21 May 2004

Clive Challis - TRANSP User Group Meeting - 21 May 2004 Net Current Thermal electrons can be trapped in B field gradient if v||/v too small Trapped electrons do not carry current Trapped electrons drag on rotating thermal electron Shielding current reduced by factor 1.0-1.460.5 (approx – Wesson) electron-ion coulomb cross-section scales like Z2ion, so higher Z ions preferentially drag on electrons to prevent complete cancellation of fast ion current Resulting net current scales roughly as: Jnet~Jfast{1.0-(Zfast/Zeff)(1.0-1.460.5)} For Zbeam=1, high Zeff and/or large  advantageous Clive Challis - TRANSP User Group Meeting - 21 May 2004

Clive Challis - TRANSP User Group Meeting - 21 May 2004 TRANSP NBCD Bug TRANSP Zfast used to calculate the shielding current was accidentally reset by mixed H & He beams, or non-NBI fast ion calculations (e.g. fusion products, including He) When alpha-simulations were switched on (typical for JET, but not for MAST) the beam driven current was miscalculated using Zfast=2 rather than 1 to give: Jnet~Jfast{1.0-(2/Zeff)(1.0-1.460.5)} This reduces the current significantly in the core of low Zeff plasmas, but the effect diminishes at large  On-axis current drive vanishes for Zeff2, and the current profile becomes hollow (normally peaked for JET) Clive Challis - TRANSP User Group Meeting - 21 May 2004

Clive Challis - TRANSP User Group Meeting - 21 May 2004 Typical Case In this typical JET ITB plasma the TRANSP neutral beam driven current profiles are compared with and without the bug at different times Better agreement with PENCIL is achieved without the bug The corrected TRANSP calculation is better than PENCIL due to superior physics model In this case the bug resulted in a 40% reduction in total beam driven current Clive Challis - TRANSP User Group Meeting - 21 May 2004

Clive Challis - TRANSP User Group Meeting - 21 May 2004 History and Checks Doug McCune, who promptly located the bug from my symptoms, has determined the history of the bug: Introduced 18 March 2002 approx (30 May 2002 at JET - Jim) Removed 19 April 2004 approx For 2004 runs the absence of the bug can be confirmed by the presence of the following signals in the multiplot JBFACS: JBFAC,JBFACZ1,JBFACZ2 (as opposed to JBFAC,JBFAC0,JBFACNC with the bug) Alternatively, the shielding current calculation can be checked directly: JBFAC(=0)=1.0-Zbeam/Zeff (Zbeam=2 for H, D or T beams being the fault condition) Clive Challis - TRANSP User Group Meeting - 21 May 2004