Noninductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta

Access to and characterization of sustained, toroidally confined plasmas with a very high plasma-to-magnetic pressure ratio (${\beta }_t$), low internal inductance, high elongation, and nonsolenoidal current drive is a central goal of present tokamak plasma research. Stable access to this desirable parameter space is demonstrated in plasmas with ultralow aspect ratio and high elongation. Local helicity injection provides nonsolenoidal sustainment, low internal inductance, and ion heating. Equilibrium analyses indicate ${\beta }_t$ up to $\sim 100\%$ with a minimum $|B|$ well spanning up to $\sim 50\%$ of the plasma volume.

Figure 1

High-${\beta }_t$ discharge parameters. (a) $I_p$ and $I_{TF}$, (b) ${\overline{n}}_e$, (c) core $T_i$ (diamonds) and $T_e$ (squares), and (d) MHD fluctuations (vertically offset for clarity). Panel (c) and solid black waveforms from the ramped-$B_t$ scenario.

Figure 2

Kinetically constrained reconstruction of a ${\beta }_t=95\%$, ${\beta }_N=6.7$ discharge. (a) Equilibrium flux surfaces and vessel geometry, (b) safety factor, (c) current profile, and (d) plasma parameters.

Figure 3

Kinetic measurements from Thomson scattering and impurity spectroscopy at 24.5 ms in the ramped-$B_t$ scenario of Fig. 2. (a) Electron temperature profile and core ion temperature, (b) electron density profile, and (c) electron pressure profile.

Figure 4

Troyon plot of Pegasus LHI discharges (markers) and other high- and low-$A$ tokamaks (hatched, shaded regions). Circles (triangles) denote constant (decreasing) $B_t(t)$. Solid (open) markers denote kinetic (scaled magnetic) ${\beta }_t$ equilibrium reconstructions.

Figure 5

MHD analysis of disruptive ramped-$B_t$ discharge. (a) Measured $n=1$ $d{B}_z/dt$ fluctuations near the time of disruption; (b) dcon calculated poloidal harmonic content of marginally unstable configuration at no-wall $\beta$ limit.

Figure 6

High-${\beta }_t$ $|B|$ well formation. (a) Equilibrium flux surfaces (dotted blue), $|B|$ isocontours (solid black), and $|B|$ well (shaded red); (b) profiles of $|B|$ and its components along the plasma midplane.