Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak

Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak

Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak 150 150 Mathew
CCFE-PR(17)56

Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak

The merging-compression method of plasma start-up in the Mega Amp Spherical Tokamak (MAST) involved the creation of two plasma tori with parallel currents, which merged at the vacuum vessel midplane due to their mutual attraction. Magnetic reconnection occurred in this process causing strong heating of both ions and electrons on millisecond timescales, and resulting in a plasma equilibrium with a single set of closed magnetic flux surfaces. The merging process also resulted in the prompt acceleration of substantial numbers of ions and electrons to highly suprathermal energies. Accelerated field-aligned ions (deuterons and protons) were detected using a neutral particle analyser at energies up to about 20 keV during merging in early MAST pulses, while nonthermal electrons have been detected indirectly in more recent pulses through microwave bursts. However no increase in soft X-ray emission was observed until later in the merging phase, by which time strong electron heating had been detected through Thomson scattering measurements. A test-particle code CUEBIT is used to model ion acceleration in the presence of an inductive toroidal electric field with a prescribed spatial profile and temporal evolution based on Hall-MHD simulations of the merging process. The magnetic field used in the modelling includes a poloidal component consistent with this electric field. The simulations yield particle distributions with properties similar to those observed experimentally, including the acceleration of protons to higher energies than deuterons, as expected from simple estimates, and the strong field alignment of the fast ion pitch angle distributions. Particle-in-cell modelling of a plasma containing a dilute field-aligned suprathermal electron component suggests that at least some of the microwave bursts can be attributed to the anomalous Doppler instability driven by anisotropic fast electrons, which do not produce measurable enhancements in soft X-ray emission either because they have energies below about 1 keV or because the nonthermal bremsstrahlung emissivity during this phase of the pulse is below the detection threshold. There is no evidence of runaway electron acceleration duing merging in MAST, possibly due to the presence of three-dimensional field perturbations.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP
Published date:
12/01/2017