Temporal behavior of the plasma current distribution in the ASDEX tokamak during lower-hybrid current drive

Abstract

Measurements of the time evolution of the current-density distribution in ASDEX show that lowerhybrid current drive leads to broader profiles, whereby q increases from q < 1 to q > 1 in the plasma central region. Simultaneously, the electron temperature is observed to peak, thus demonstrating that the lower-hybrid-driven current distribution is decoupled from the classical conductivity profile.

Safety factor measurements on the magnetic axis of the Texas experimental tokamak plasma in Ohmic and electron-cyclotron-resonance heated discharges

“…The present diagnostic agrees with most diagnostics on various tokamaks that the qo values for the Ohmically heated nonsawtoothing discharges are near or larger than unity [2,3,5,7,9,23,24], and disagrees with the result on TEXTOR [4]. A statistical analysis of these results must preclude both systematic errors (such as whether all qo diagnostics were biased by the theoretical predictions of either qo 1 or qo &&1 for the stable discharges) and accidental errors (such as technical problems in a diagnostic).…”

“…The results of experimental studies are inconclusive. For nonsawtoothing discharges, most of the diagnostics [2,3,5,7,9,23,24], except the result on TEXTOR [4], consistently demonstrated qo values near or larger than unity, as shown in Table II, in agreement with the prediction for qo in magnetohydrodynamically (MHD) stable plasmas [1].…”

“…It has also been observed that radio-frequency wave heating and neutral-beam heating could stabilize MHD instabilities [5,25,26]. Such auxiliary heating increases the local plasma temperature, which, in turn, changes the plasma current-density distribution and modifies the q profile.…”

10.1007/s11452-008-3002-4

“…Additional information required to narrow the range of possible solutions is extracted from measurements (if available) of the positions of the resonant magnetic surfaces, the pitch angle of the magnetic field (from Stark line polarization signals) [6], or the Faraday rotation of the plane of polarization of a diagnostic laser beam [7][8][9][10]. Along with the problem of adjusting the solutions to the GSh equation to the measurement data, there is always a question of how unique the reconstruction of the equilibrium is and of how broad the class of other equilibria that could be consistent with the same data within their error bars is.…”

The Theory of Variances in Equilibrium Reconstruction

“…Because of the extreme importance of the equilibrium information, different diagnostics are being developed to provide additional information for reconstruction of the current distribution inside the plasma. Examples include kinetic measurements of electron and ion temperatures, plasma density, contribution of the fast particles into the plasma pressure, polarimetry (measuring the line integrated Faraday rotation of polarized light from the laser beam [8,9], or spectral lines [10,11,12]), signals related to Motion Stark Effect (MSE) [13], position of resonant surfaces from internal magneto-hydrodynamic modes, etc.…”