Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons(REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices.Experimental study on the suppression of runaway current by electrode biasing(EB) and limiter biasing(LB) has been performed on the J-TEXT tokamak,which could be an alternative way to suppress the runaway current.The experimental results show that the higher the voltage value,the smaller the runaway current in both EB and LB experiments.The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V.The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max(E_(RE)) and radiation temperature T_(HXR)hard x-rays(HXRs)are significantly reduced after the application of +200 V limiter biased voltage.The electric field generated by the biased voltage may be the key factor to suppress the runaway current,and the measured radial electric field increases obviously after the voltage is applied.This may result in an increase in the loss of REs to realize the suppression of runaway current.

Toward modeling of the scrape-off layer currents induced by electrode biasing using 2D particle-in-cell simulations

The property of scrape-off layer(SOL) currents induced by a biased electrode is investigated by fully kinetic collisionless two-dimensional particle-in-cell(PIC) simulations. A reduced Vlasov–Darwin model is employed, which is capable of describing the low-frequency kinetic behavior without electromagnetic vacuum modes(w^2=w_(pe)~2+ c^2k^2). A linear decay distribution of electron currents parallel to the background magnetic field is exhibited. Simulation analyses indicate that the cross field ion current is a key factor in sheath formation and global current balance. The influences of electrode area, biasing voltage and plasma source on the SOL current profile are studied, respectively.Characteristic plasma parameters in the far SOL region of the EAST tokamak are used in simulations to assess the current driving ability of the electrode biasing method. Due to the limitations of computational power, the geometrical size of the simulation domain is significantly smaller than the realistic SOL, which may lead to an absence of the quasi-neutral region in the upstream plasma.At last, a heuristic method is proposed to calculate the upper bound of the total current strength.

Response of the low-pressure hot-filament discharge plasma to a positively biased auxiliary disk electrode

In a steady-state plasma,the loss rate of plasma particles to the chamber wall and surfaces in contact with plasma is balanced by the ionization rate of background neutrals in the hot-filament discharges.The balance between the loss rate and ionization rate of plasma particles(electrons and ions)maintains quasi-neutrality of the bulk plasma.In the presence of an external perturbation,it tries to retain its quasi-neutrality condition.In this work,we studied how the properties of bulk plasma are affected by an external DC potential perturbation.An auxiliary biased metal disk electrode was used to introduce a potential perturbation to the plasma medium.A single Langmuir probe and an emissive probe,placed in the line of the discharge axis,were used for the characterization of the bulk plasma.It is observed that only positive bias to the auxiliary metal disk increases the plasma potential,electron temperature,and plasma density but these plasma parameters remain unaltered when the disk is biased with a negative potential with respect to plasma potential.The observed plasma parameters for two different-sized,positively as well as negatively biased,metal disks are compared and found inconsistent with the existing theoretical model at large positive bias voltages.The role of the primary energetic electrons population in determining the plasma parameters is discussed.The experimentally observed results are qualitatively explained on the basis of electrostatic confinement arising due to the loss of electrons to a biased metal disk electrode.