H-mode is obtained at A ∼ 1.2 in the Pegasus Toroidal Experiment via Ohmic heating, high-field-side fueling, and low edge recycling in both limited and diverted magnetic topologies. These H-mode plasmas show the formation of edge current and pressure pedestals and a doubling of the energy confinement time to H 98 y , 2 ∼ 1 . The L–H power threshold P LH increases with density, and there is no P LH minimum observed in the attainable density space. The power threshold is equivalent in limited and diverted plasmas, consistent with the FM3 model. However, the measured P LH is ∼ 15     × higher than that predicted by conventional International Tokamak Physics Activity (ITPA) scalings, and P LH / P ITPA 08 increases as A → 1 . Small ELMs are present at low input power P IN ∼ P LH , with toroidal mode number n ⩽ 4 . At P IN ≫ P LH , they transition to large ELMs with intermediate

Using a recently installed impurity powder dropper (IPD), boron powder (< 150 μm) was injected into lower single null (LSN) L-mode discharges in WEST. IPDs possibly enable real-time wall conditioning of the plasma-facing components and may help to facilitate H-mode access in the full-tungsten environment of WEST. The discharges in this experiment featured Ip = 0.5 MA, BT = 3.7 T, q95 = 4.3, tpulse = 12–30 s, ne,0 ~ 4×1019 m-2, and PLHCD ~ 4.5 MW. Estimates of the deuterium and impurity particle fluxes, derived from a combination of visible spectroscopy measurements and their corresponding S/XB coefficients, showed decreases of ~ 50% in O+, N+, and C+ populations during powder injection and a moderate reduction of these low-Z impurities (~ 50%) and W (~ 10%) in the discharges that followed powder injection. Along with the improved wall conditions, WEST discharges with B powder injection observed improved confinement, as the stored energy WMHD, neutron rate, and electron temperature Te increased significantly (10–25% for WMHD and 60–200% for the neutron rate) at constant input power. These increases in confinement scale up with the powder drop rate and are likely due to the suppression of ion temperature gradient (ITG) turbulence from changes in Zeff and/or modifications to the electron density profile.

A major goal of the spherical tokamak (ST) research program is accessing a state of low internal inductance ℓi, high elongation κ, and high toroidal and normalized beta (βt and βN) without solenoidal current drive. Local helicity injection (LHI) in the Pegasus ST [Garstka et al., Nucl. Fusion 46, S603 (2006)] provides non-solenoidally driven plasmas that exhibit these characteristics. LHI utilizes compact, edge-localized current sources for plasma startup and sustainment. It results in hollow current density profiles with low ℓi. The low aspect ratio (R0/a∼1.2) of Pegasus allows access to high κ and high normalized plasma currents (IN=Ip/aBT&gt;14). Magnetic reconnection during LHI provides auxiliary ion heating. Together, these features provide access to very high βt plasmas. Equilibrium analyses indicate that βt up to ∼100% is achieved. These high βt discharges disrupt at the ideal no-wall β limit at βN∼7.

Local helicity injection (LHI) is a non-solenoidal current drive capable of achieving high- tokamak startup with non-invasive current injectors in the plasma scrape-off layer. The choice of injector location within the edge region is flexible but has a profound influence on the nature of the current drive in LHI discharges. New experiments on the Pegasus ST with injection in the high-field-side, lower divertor region produce plasmas dominated by helicity injection current drive, static plasma geometry, and negligible inductive drive. Peak plasma current up to 200 kA, and a sustained plasma current of 100 kA for up to 18 ms, is demonstrated. Maximum achievable plasma current is found to scale approximately linearly with the effective loop voltage from LHI. A newly-observed MHD regime for LHI-driven plasmas in which large-amplitude fluctuations at 20–50 kHz are abruptly reduced on the outboard side results in improved current drive. A simultaneous increase in high frequency fluctuations (>400 kHz) inside the plasma edge suggests short wavelength turbulence as an important current drive mechanism during LHI.

Robust non-solenoidal startup methods may simplify the cost and complexity of next-step burning plasma devices, and especially STs. Experiments on the ~ 1 Pegasus ST are advancing the physics and technology basis of Local Helicity Injection (LHI). LHI creates high tokamak plasmas by injecting helicity with small current sources in the plasma edge. Its hardware can be withdrawn before a fusion plasma enters a nuclear burn phase. Flexible injector placement offers tradeoffs between physics and engineering goals. They are tested with LHI systems on the low-field-side (LFS) and the high-field-side (HFS) of Pegasus, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive (~), respectively. Record LHI plasmas with = 0.225 MA, > 100 eV, and ~ 10 19 m-3 are attained. A predictive 0D power-balance model describes experimental () and partitions the active current drive sources. Analysis of experimental discharges with the model confirms the dominance of non-solenoidal induction in LFS LHI and DC helicity drive in HFS LHI. Studies of HFS scenarios find favorable, positive scalings of with and with. High-frequency MHD activity is found to be present during LHI current drive, in addition to = 1 modes previously found in NIMROD simulation and experiment. A new regime of reduced MHD activity was discovered where = 1 activity is suppressed, LHI CD efficiency improves, and long-pulse plasmas are sustained with ~ 0. LHI facilitates access to favorable ST regimes with non-solenoidal sustainment, high , low ℓ , and high. Low LHI operation has led to record = 100%, high , and a minimum-| | well that may positively affect turbulence, transport, and fast particle confinement. Major facility upgrades are planned to extend LHI to higher , , and pulse length.