Tokamak Startup Using Point-Source dc Helicity Injection

Abstract

Startup of a 0.1 MA tokamak plasma is demonstrated on the ultralow aspect ratio Pegasus Toroidal Experiment using three localized, high-current density sources mounted near the outboard midplane. The injected open field current relaxes via helicity-conserving magnetic turbulence into a tokamaklike magnetic topology where the maximum sustained plasma current is determined by helicity balance and the requirements for magnetic relaxation.

Full-wave modeling of the O–X mode conversion in the Pegasus toroidal experiment

“…The plasma core can be heated-up and EBW heating can provide a tool to control the current profile. Furthermore, as discussed in the introduction, synergies are possible between EBW heating and helicity injection 8,9 and between EBW heating and fast wave heating 10 .…”

Full-wave modeling of the O-X mode conversion in the Pegasus Toroidal Experiment

“…EBW heating is also useful during the ramp-up phase, as it increases the electron temperature and decreases the collisionality, thus resulting in more effective induction. Helicity injection 8,9 and fast wave heating and current drive 10 are also expected to benefit from electron heating by EBWs in Pegasus. Synergistic combinations of these start-up techniques have the potential to create a suitable plasma target to hand over to Ohmic heating and neutral beam injection in larger spherical tokamaks including a possible future component test facility 11 .…”

Exact axisymmetric Taylor states for shaped plasmas

“…For equilibria with a magnetic X-point, the location of the X-point must also be specified. The flexibility and simplicity of these solutions make them useful for verifying the accuracy of numerical solvers and for theoretical studies of Taylor states in laboratory experiments.Plasmas in both astrophysical and laboratory settings have a strong tendency to relax to minimum energy states known as Taylor states or Woltjer-Taylor states [1][2][3][4][5][6][7][8][9][10][11][12] in which the magnetic fields are force-free fields given by the equationwhere λ is a global constant. A well-known analytic solution to equation (1) is often used for theoretical studies and to interpret experiments [5][6][7]13 .…”

“…Plasmas in both astrophysical and laboratory settings have a strong tendency to relax to minimum energy states known as Taylor states or Woltjer-Taylor states [1][2][3][4][5][6][7][8][9][10][11][12] in which the magnetic fields are force-free fields given by the equation…”

“…The plasma boundary can have a magnetic separatrix, if desired, and the location of the separatrix can be specified. The equilibria we describe in this article can thus be useful for a variety of applications, including the study of non-solenoidal current start-up in low aspect ratio toroidal devices 10 and plasma dynamics in spheromaks. They can also be used to verify the accuracy of numerical schemes developed to solve equation (1) in fusionrelevant geometries 14,15 .…”