2024 – Radial magnetic compression of DIII-D plasmas

Radial magnetic compression of DIII-D plasmas

2024 Research Campaign, Frontier Science

Purpose of Experiment

The present experiment aims to revive the magnetic plasma compression technique applied to tokamaks and, in combination with external heating systems such as neutral beam injection (NBI), its potential for improving fusion performance using, for example, the fusion triple product as a metric. Specifically, we expect to learn how the energy confinement tE, the plasma density ne, the electron (Te) and ion (Ti) temperatures, the toroidal plasma current Ip, the toroidal field BT, and toroidal vT, etc., evolve during the radial compression and how some of these interdependent quantities vary with compression time tc and specially with the compression factor CR. Here, CR = Ro(i)/Ro(f), and here i and f refer to the initial and final positions of the plasma column major radius Ro inside of the toroidal vacuum metal chamber of the DIII-D tokamak. We expected to observe some level of adiabatic heating (increase in Te and Ti), alongside simultaneous ne and vT increments, using a relatively fast radial compression time tfc to respect tE, i.e., tfc < tE. This semi-adiabatic compression will lead higher triple product if tE is maintained.

Experimental Approach

The radial plasma compression will be conducted mainly via fast (10kA-turn/s) increase in the currents through the out-most poloidal field coils (a symmetrical pair, i.e., in a Helmholtz configuration) which create the major vertical field to maintain the plasma column in equilibrium (a balance between the plasma pressure gradient and the Lorentz’s forces). We will compress plasmas from a reference campaign with high (H) confinement time (H-mode) (tE =0.13-0.16s) using tfc =0.02-0.1s which leads to modest values of CR =1.08-1.22. Their performance will be compared the with those of the same reference campaign where a very slow radial compression (tsc ~1s, CR ~1.03) was conducted which, nevertheless, lead to an increase in the triple product despite of no benefit of any adiabatic compression was envisaged since tsc > tE. With our fast compression this reference performance is expected to increase further because of some level of adiabatic properties. The values of CR are increased in a shot-to-shot basis and, overall, are relatively small. This allows the use of the unique multi-chord plasma diagnosis capability in DIII-D, while also help modeling the NBI power core deposition because of the minor changes of Ro (this will later facilitate tE analysis) and to maintain the plasma control pos-compression. The small value of the relative tfc (intrinsically related CR) allows the safety-factor q to be minimally unaltered during this compression thus maintaining the plasma stability. This q behavior is the result of another semi-adiabatic compression property due to the almost conservation of toroidal FT and poloidal magnetic fluxes (q = FT/Fp). Simulations of this radial plasma compression have currently been conducted by a new Next Step Fusion (NSF) Simulator, which is a free-boundary equilibrium and transport solver based on the renowned simulation approach used in DINA code.