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A Magnetic Recoil Spectrometer (MRS) has been built and successfully used at OMEGA for measurements of down-scattered neutrons (DS-n), from which areal density (ρR) in both warm-capsule and cryogenic-DT implosions have been inferred. Another MRS is currently being commissioned on the National Ignition Facility (NIF) for diagnosing low-yield tritium-hydrogen-deuterium (THD) implosions and high-yield DT implosions. As CR-39 detectors are used in the MRS, the principal sources of background are neutron-induced tracks and intrinsic tracks (defects in the CR-39). The Coincidence Counting Technique (CCT) was developed to reduce these types of background tracks to the required level for the DS-n measurements at OMEGA and the NIF. Using this technique, it has been demonstrated that the number background tracks is reduced by a couple of orders of magnitude, which exceeds the requirement for the DS-n measurements at both facilities. a) Also Visiting Senior Scientist at the Laboratory for Laser Energetics, University of Rochester.

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

A 0D circuit model for predicting in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface flux of finite- plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modified and expanded to treat highly shaped, ultralow- tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the Pegasus spherical tokamak, and are found to agree within 15% of experimental . High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the first half of the current ramp, or 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a significant drop in external plasma inductance, effectively using the plasma’s initially high inductance to drive the current ramp and provide  >70% of the current drive V-s. Projections using this model indicate the relative influences of higher helicity input rate and injector current on the attainable total plasma current.

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.