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This public data set contains openly-documented, machine readable digital research data corresponding to figures published in A.C. Sontag et al., 'The New Pegasus-III Experiment,' IEEE Transactions on Plasma Science 50, 4009 (2022).

Magnetic measurements during dc helicity injection tokamak startup indicate Alfvénic turbulence in the injected current streams mediates magnetic relaxation and results in macroscopic plasma current drive. Localization of such activity to the injected current streams, a bias voltage dependence to its onset, and higher-order spectral analysis indicate super-Alfvénic electrons excite instabilities that drive the observed turbulence. Measured fluctuation helicity is consistent with an α-dynamo electromotive force driving net current comparable to the macroscopic equilibrium current density. These results imply new constraints for scaling local helicity injection to larger devices.

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in N.J. Richner et al., ‘Magnetic Turbulence and Current Drive During Local Helicity Injection,’ Phys. Rev. Lett. 128, 105001 (2022).

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in J.A. Reusch et al., ‘A Coaxial Helicity Injection System for Non-Solenoidal Startup Studies on the Pegasus-III Experiment,’ accepted for publication in IEEE Transactions on Plasma Science.

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in M.W. Bongard et al., ‘Digital Control and Power Systems for the Pegasus–III Experiment,’ accepted for publication in IEEE Transactions on Plasma Science.

This grant supported a long-term research and development program to explore the physics and technology of spherical tokamak (ST) plasma behavior at near-unity aspect ratio (A). The PEGASUS Toroidal Experiment is a university-scale (R) ~ 0.40 m, α ~ .35 m, I_p ≤ 0.3 MA, B_T ≤ 0.15 T) experiment that accesses A → 1, to provide large values of the normalized plasma current, I_N = I_p/αB_T, or equivalently the toroidal field utilization factor I_p/I_TF. The research activities focused on: 1) developing the technique of Local Helicity Injection (LHI) as a viable means of initiating and growing an ST plasmamore » without the use of a central solenoid; 2) exploring the equilibrium and stability properties of spherical tokamak plasmas at extremely low aspect ratio; and 3) obtaining new insights in the properties of the enhanced confinement H-mode regime at A ~ 1. The initial (Phase I) facility employed a unique high-stress central solenoid Ohmic for current drive and heating with simple magnet current waveforms. A subsequent major upgrade (Phase II) offered more flexible control of the plasma evolution along with a powerful LHI system for plasma initiation, heating and sustainment. The LHI technique consists of injection of high-density current streams into vacuum toroidal and vertical magnetic fields. These streams become unstable and support magnetic reconnection to allow the system to relax into a tokamak-like magnetic configuration. This program focused on: 1) development of electron current sources that provide sufficiently high injection currents in the challenging edge plasma region; 2) validation of theoretical limits to achievable I_p arising from magnetic helicity and energy conservation plus absolute limits arising from relaxation to a minimal energy states; 3) tests of a power balance model to describe the evolution of the plasma current I_p(t); and 4) study of magnetic fluctuations and their role in the I_p evolution. This culminated in the production of toroidal currents of I_p ≥ 0.2 MA with only ~8 kA of injected current. Operation with Ohmic induction alone in Phase I created plasmas with soft limits on the achievable I_p and field utilization (I_p/I_TF ~ 1) due to the onset of large low-order (2/1 or 3/2) internal resonant tearing modes. With the expanded capabilities in Phase II operation, methods employed to mitigate those modes and achieve I_p/I_TF > 1 were: 1) I_TF ramp-downs; 2) non-solenoidal startup via LHI; and 3) extremely low I_TF Ohmic operations assisted by LHI current injector pre-ionization. Extensive use of LHI-driven startup and drive for non-solenoidal operation produced favorable current profiles to stably operate tokamak plasmas with unprecedentedly high I_N = I_p/aB_T ~ 15 and I_p/I_TF > 2. With strong reconnection-driven anomalous ion heating, this in turn provided access to tokamak plasmas with B_t up to 100%, a world record 2 times higher than that previously achieved in advanced or spherical tokamaks. This high B_t regime had a large fraction (~50%) of the plasma volume in a region of an absolute minimum-B configuration. The MHD stability limit in this regime was the ideal external kink mode without a stabilizing wall. These results provided the first demonstration of access to tokamak plasmas with very high B_t, high i_N, high elongation κ, and low internal inductance ℓ_i, and as such served as a proof-ofprinciple for access to this long-sought ST regime. The naturally low B_T needed for stability at A ≤ 1.2 reduces the L-H mode power threshold, P_LH, so that H-mode plasmas were achieved with Ohmic heating alone. The power threshold for the transition to the H-mode regime was found to be 10× the accepted values from the ITER international scaling, confirming initial trends indicated by experiments on MAST and NSTX, and suggesting that the physics behind this transition is not yet fully understood. The density dependence of the transition and the observed insensitivity to whether the plasma is bounded by a limiter or magnetic divertor are consistent with the recent FM³ model. Multiple unstable toroidal magnetic modes were measured during ELM (Edge Localized Mode) crashes, with generally lower toroidal mode numbers than seen at higher A, which was attributed to the larger peeling mode instability drive at low A. First-ever edge current profile measurements through an ELM crash showed current-carrying filaments were generated, as seen in ELM simulations and in electromagnetic blob transport theory.« less

This article corrects an error in M.W. Bongard et al., 'On Virial Analysis at Low Aspect Ratio,' Phys. Plasmas 23, 072508 (2016) pertaining to a typographical error in its equation 21. Furthermore, all numerical calculations reported in this paper utilized the correct formulation described here. Accordingly, no changes to the conclusions of the original article are required.

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in G.M. Bodner et al., ‘Initial Characterization of Electron Temperature and Density Profiles in PEGASUS Spherical Tokamak Discharges Driven Solely by Local Helicity Injection,’ Physics of Plasmas 28, 102504 (2021).

Local helicity injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate tokamak discharges. Viable non-solenoidal startup techniques require high central T_e to combat resistive losses and enhance coupling to auxiliary methods of current drive/heating. Thomson scattering measurements of LHI discharges in PEGASUS showed peaked T_e profiles at I_p ~ 0.15 MA and B_t ~ 0.15 T with T_(e,0) ~ 100–150 eV. These results are similar to T_e profiles observed with Ohmic induction. At lower levels of B_t, LHI T_e profiles were hollow with T_(e,0) ~ 40 eV and T_(e,max) ≤ 120more » eV depending upon the helicity input. Regardless of the B_t level and helicity input, the electron pressure profiles were flat/peaked with hollow J(R) profiles. Equilibrium reconstructions and measurements of core absolute extreme ultraviolet radiation suggest the hollow T_e profiles are the result of very low resistive heating power in the core due to the edge-localized nature of LHI and low-Z line radiation losses. Estimates of Z_eff from the plasma conductivity indicate averaged values of ~1 or ~3 assuming neoclassical or Spitzer conductivity, respectively. When auxiliary heating power from magnetic reconnection is considered, this observed LHI performance is comparable to expectations from a linear Ohmic confinement scaling estimate and a collisional stochastic confinement scaling estimate of the core plasma region.« less

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in M.W. Bongard et al., 'Advancing Local Helicity Injection for Non-Solenoidal Tokamak Startup,' Nucl. Fusion 59, 076003 (2019).