Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Abstract

The Pegasus Toroidal Experiment ͓R. Fonck et al., Bull. Am. Phys. Soc. 41, 1400 ͑1996͔͒ is a spherical torus designed to study the limits of plasma behavior as the aspect ratio A approaches unity. Access to near-unity A is achieved through the use of a novel high-stress reinforced solenoid magnet. High toroidal beta ␤ t is obtained in ohmically-heated plasmas by operation at low field with densities up to the Greenwald limit. Values of ␤ t up to 20% and normalized beta up to 5 have been obtained. The ratio of plasma current to toroidal field rod current, known as the toroidal field utilization, reaches values as large as 1 but appears to approach a ''soft'' boundary at that level related to both ohmic flux limitations and the onset of resistive magnetohydrodynamic ͑MHD͒ activity. The m/nϭ2/1 and 3/2 modes are most frequently observed, in agreement with the inferred safety factor profiles. Experiments are beginning to access the external kink stability boundary at edge safety factor q 95 ϭ5, which is significantly higher than that observed in conventional tokamaks. Calculations using the DCON code ͓A. H. Glasser and M. S. Chance, Bull. Am. Phys. Soc. 42, 1848 ͑1997͔͒ confirm instability to the ideal kink.

Recent progress on spherical torus research

“…137 The current seems to increase with $P rf rather than $P rf 0. 5 , as expected for the pressure-driven current approach. It is noted that LHCD start-up is being investigated in TST-2, where a plasma current of 15 kA was generated in low-density start-up plasmas.…”

“…Since the Alfv en velocity V A scales as b À0. 5 Cs, where Cs is the acoustic velocity, it is possible to access the super-Alfv en regime in NBI heated, high-b ST plasmas. As shown in Fig.…”

“…1 and 2. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Those ST facilities operated before 2000, including the START device, are covered in the previous review/overview papers. 34,35 It also should be noted that most of the ST facilities are located at universities with a strong student education mission.…”

“…5,49,50 The device has a highly engineered ohmic heating solenoid, with very high magnetic field (B sol 15) capability within the constraints of very tight space. As shown in Fig.…”

“…1 and 2), there are a large number of significant ST results which are unique to STs, while at the same time reaffirming many common physics features with conventional tokamaks. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] As a member of the tokamak family, 19 ST research contributes to advancing conventional tokamaks such as ITER 20,21 by providing data that extends into a unique plasma and device parameter space. Since ITER represents a significant extrapolation from present day tokamaks, ST research provides a different set of tokamak-related data for the improvement of predictive capabilities by providing leverage over a wide range of parameter space.…”