Transport and confinement studies in the RFX-mod reversed-field pinch experiment

Abstract

In the modified RFX experiment (RFX-mod) external magnetic field coils and a close fitting thin conductive shell control radial magnetic fields. In the so-called virtual shell (VS) operation, radial field zeroing at the thin shell radius is stationary provided by the feedback-controlled coils. First experiments on RFX-mod proved the capability of the active scheme to steadily reduce the radial magnetic field. Furthermore it has been found that such edge magnetic field control extends its beneficial effects to the whole plasma. With respect to the old RFX, where magnetohydrodynamic modes amplitude was controlled by the use of a passive thick conductive shell, a stationary 2- to 3-fold reduction of the B r field amplitude in the core is obtained. The reduction of field fluctuations positively reflects on confinement. In fact, a strong reduction of the loop voltage is observed and correspondingly a 3-fold increase in pulse length is achieved by using the same poloidal flux swing. Temperature and particle measurements confirm the improved confinement properties of the VS operation. With a lower ohmic input power, higher electron temperature and lower particle influx are measured. Particle and heat transport have been studied by means of a 1D code. Local power balance was used to compute the heat conductivity profile: for the VS discharges a lower conductivity over a significant region of the plasma is found. The improved properties of RFX-mod VS operation provide a better confinement scaling in terms of plasma current. The results show that compared with the thick shell configuration, a significant confinement improvement can be obtained under stationary conditions by actively controlling the plasma magnetic boundary.

A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

“…While the issue of the LCFS will be considered in the next section, here we would like to underline the key role of the safety factor profile that is an input constraint to VMEC and has to be determined independently. This is done either from the SHEq equilibrium in a semi-analytical way or numerically estimated with the FLiT [INN07] or ORBIT [ORBIT] field line tracing codes following field lines as they wind around the helical axis. Both solutions provide the same result though the second one requires much more computational time.…”

Flow Measurements in the Edge Region of the RFX‐Mod Experiment

“…Specifically, a step in the reversal parameter F has been induced during the discharge to compare different magnetic equilibria: a phase with a shallow reversal (F > -0.07) and a phase with deeper reversal (F < −0.15) [11]. An example is shown in fig.4, together with the time evolution of the parallel and perpendicular flow M par , M perp and a reconstruction of the local magnetic topology (Poincaré plots) obtained with the FLiT code [14] at three different time instants. Also, a preliminary work has been performed in order to describe the time evolution of the flow components during a pellet injection.…”

Single-Helical-Axis States in Reversed-Field-Pinch Plasmas

“…In order to interpret the new condition in terms of magnetic topology, the field-line tracing code FLIT [16] has been used. The peculiarity of this code is that it uses mode eigenfunctions computed, in toroidal geometry, according to Newcomb equation supplemented with boundary conditions derived from Mirnov coil measurements [17].…”