Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

Abstract

Modifications to the plasma control capabilities and poloidal field coils of the National Spherical Torus Experiment (NSTX) have enabled a significant enhancement in shaping capability which has led to the transient achievement of a record shape factor (S ≡ q 95 (I p /aB t )) of ∼41 (MA m −1 T −1 ) simultaneous with a record plasma elongation of κ ≡ b/a ∼ 3. This result was obtained using isoflux control and real-time equilibrium reconstruction. Achieving high shape factor together with tolerable divertor loading is an important result for future ST burning plasma experiments as exemplified by studies for future ST reactor concepts, as well as neutron producing devices, which rely on achieving high shape factors in order to achieve steady state operation while maintaining MHD stability. Statistical evidence is presented which demonstrates the expected correlation between increased shaping and improved plasma performance. Plasmas with high shape factor have been sustained for pulse lengths which correspond to τ pulse = 1.6s ∼ 50τ E ∼ 5τ CR , where τ CR is the current relaxation time and τ E is the energy confinement time. Plasmas with higher β t ∼ 20% have been sustained for τ pulse = 1.2s ∼ 25τ E ∼ 3τ CR with noninductive current fractions f NI ∼ 50%, with ∼40% pressure driven current and ∼10% neutral beam driven current. An interesting feature of these discharges is the observation that the central value of the safety factor q(0) remains elevated and constant for several current diffusion times without sawteeth, similar to the 'hybrid mode'. Calculations of the profiles of inductive and non-inductive current are compared with measurements of the total current profile and are shown to be in quantitative agreement. Results are shown from experiments which investigate the applicability of high harmonic fast waves (HHFWs) and electron Bernstein waves (EBWs) as current drive and heating sources, and the possibility of LHCD for future ST devices is raised. A calculated scenario which provides 100% non-inductive current drive is described. NSTX operates with peak divertor heat fluxes which are in the same range as those expected for the ITER device, i.e. with P heat max ∼ 10 MW m −2 . High triangularity, high elongation plasmas on NSTX have been demonstrated to have reduced peak heat flux to the divertor plates to <3 MW m −2 .

Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas

“…A similar calculation was reported in Ref. [69], where good agreement was found between the current profile reconstructed from Grad--Shafranov solutions and that summed from calculations of the current profile constituents; only a small discrepancy in the on--axis current density was reported. Calculations of the non--inductive fraction in NSTX using TRANSP and NUBEAM have also been reported in…”

Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

“…High values of β T were achieved [13,147], but for only very short duration. In later years of NSTX, more emphasis has been placed on achieving sustained high values of the non-inductive fraction [103,125,148,149] or β N [78,109,111]. As a consequence, there were few or no attempts to access this very low q* regime during the run campaigns used for disruptivity analysis in section 4.…”

Exploration of the equilibrium operating space for NSTX-Upgrade

“…The plasma shape is a key parameter in determining the ability of a tokamak to achieve large bootstrap currents and sustain high-β [35,37,42,[101][102][103][104]; NSTX-Upgrade is no exception to this rule. In general, it is desirable to keep the inner-plasma-wall gap as small as reasonably possible in order to maintain low aspect ratio; this results in the best utilization of the toroidal field.…”

“…Among the most critical of these issues are the scaling of the electron transport with field and current [21][22][23][24][25], the physics of fast-particles in the lower field of the ST [26][27][28][29][30][31][32][33] and the ability to non-inductively sustain the high-beta ST configuration (see Refs. [34][35][36][37][38][39][40][41][42] for progress towards this goal in NSTX, and Refs. [43 & 44] for progress in MAST).…”