Non-inductively driven tokamak plasmas at near-unity βt in the Pegasus toroidal experiment

Abstract

A major goal of the spherical tokamak (ST) research program is accessing a state of low internal inductance ℓi, high elongation κ, and high toroidal and normalized beta (βt and βN) without solenoidal current drive. Local helicity injection (LHI) in the Pegasus ST [Garstka et al., Nucl. Fusion 46, S603 (2006)] provides non-solenoidally driven plasmas that exhibit these characteristics. LHI utilizes compact, edge-localized current sources for plasma startup and sustainment. It results in hollow current density profiles with low ℓi. The low aspect ratio (R0/a∼1.2) of Pegasus allows access to high κ and high normalized plasma currents (IN=Ip/aBT>14). Magnetic reconnection during LHI provides auxiliary ion heating. Together, these features provide access to very high βt plasmas. Equilibrium analyses indicate that βt up to ∼100% is achieved. These high βt discharges disrupt at the ideal no-wall β limit at βN∼7.

Advancing local helicity injection for non-solenoidal tokamak startup

“…They suggest a physical mechanism for the = 1 activity: macroscopic reconnection of the helical injected current streams into axisymmetric "current rings" which then contribute to growth. Such = 1 activity is ubiquitous in LHI discharges [3,4,11,13]. Internal and external magnetic measurements during LFS helicity injection experiments confirmed the = 1 mode structure is consistent with that of a line-tied kink to the sources [13].…”

“…The improved model reproduces ( ) trajectories to ~10% accuracy when using an approximate volume-averaged 〈 〉 for neoclassical plasma resistivity values in high plasmas driven by both LFS and HFS injector systems (Fig. 6) [13].…”

“…The physical mechanisms leading to LHI current drive generate strong MHD activity. Nonlinear, 3D resistive MHD NIMROD simulations of LHI startup have provided insight into dynamical processes observed in experiment and simulation [19], and spurred experimental investigations into MHD activity [13] and reconnection [20] during LHI.…”

A power-balance model for local helicity injection startup in a spherical tokamak

“…At constant I INJ and B T , an increase in V LHI is predicted to further increase achieved plasma currents. This leads to optimizing injector location (inboard injection) or large area injectors to increase the available helicity injection rate [10]. Additionally, an alternate and possibly more efficient route to higher performance is projected by increasing the Taylor limit in the beginning phases via I INJ and/or B T .…”

“…Magnetic reconnection of the injected current streams and relaxation processes give rise to the formation of a tokamaklike plasma, in which a net toroidal current can be driven to many times the injected current. To date, LHI has provided high current-multiplication ST startup with toroidal plasma currents ( I p ) up to 0.2 MA on Pegasus using less than 7.5 kA of injected current ( I INJ ) [10], giving a multiplication factor of M = I p /I INJ > 25. Efficient handoff to OH drive for further sustainment and transition to H-mode has also been demonstrated [11].…”