The upgraded Pegasus Toroidal Experiment

Abstract

The Pegasus Toroidal Experiment was developed to explore the physics limits of plasma operation as the aspect ratio (A) approaches unity. Initial experiments on the device found that access to high normalized current and toroidal beta was limited by the presence of large-scale tearing modes. Major upgrades have been conducted of the facility to provide the control tools necessary to mitigate these resistive modes. The upgrades include new programmable power supplies, new poloidal field coils and increased, time-variable toroidal field. First ohmic operations with the upgraded system demonstrated position and current ramp-rate control, as well as improvement in ohmic flux consumption from 2.9 MA Wb −1 to 4.2 MA Wb −1 . The upgraded experiment will be used to address three areas of physics interest. First, the kink and ballooning stability boundaries at low A and high normalized current will be investigated. Second, clean, high-current plasma sources will be studied as a helicity injection tool. Experiments with two such sources have produced toroidal currents three times greater than predicted by geometric field line following. Finally, the use of electron Bernstein waves to heat and drive current locally will be studied at the 1 MW level; initial modelling indicates that these experiments are feasible at a frequency of 2.45 GHz.

Tokamak Startup Using Point-Source dc Helicity Injection

“…1:15) spherical tokamak with a maximum plasma minor radius a of 0.39 m at a major radius (R 0 ) of 0.45 m [7]. The toroidal field rod current (I TF ) is as large as 288 kA, corresponding to a vacuum toroidal field of 0.13 T at R 0 ¼ 0:45 m. The focus of the nonsolenoidal startup experiments on Pegasus is to create a tokamak plasma that can be subsequently sustained by other current drive techniques.…”

Super X divertors for solving heat and neutron flux problems of fusion devices

“…These are being investigated further using theory, code simulations, and experiments. The NSTX [21], MAST [22], PEGASUS [23], and SST [24] groups are planning to implement and test SXD in their devices. SXD by now seems to have become an integral part of ST-CTF design evaluation [6].…”

Pedestal Structure without and with 3D Fields

“…Thus, because of their low aspect ratio, pedestal plasmas are typically not in the low collisionality (ν * e 1) "banana" regime; rather, they are usually in the "plateau" or even Pfirsch-Schlüter (ν e > ω te ) regimes [24,28]. This is especially true for very low aspect ratio tokamaks such as NSTX [11], Pegasus [30] and MAST [7]. The normalized electron viscosity coefficient μ e /ν e increases significantly in Hmode pedestals from the separatrix inward; e.g., for the DIII-D pedestal in Fig.…”