TRANSP NBCD bug

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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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