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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).

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).

Experiments on the A ~ 1 PEGASUS ST are advancing the physics and technology basis of Local Helicity Injection (LHI). LHI injects helicity with relatively intense electron current sources in the plasma edge. It creates high toroidal current, toroidally-averaged tokamak-like plasmas that have been efficiently transitioned to Ohmically driven tokamak plasmas. Tradeoffs between physics and engineering goals are tested with LHI systems on the low-field-side and the high-field-side of PEGASUS, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive, respectively. An extensive LHI source development campaign comparing active arc sources, passive and gas-effused electrode sources lead to themore » selection of active arc sources for present and next-step LHI deployments. LHI plasmas with net toroidal current I_p=0.225 MA, T_e>100 eV, and n_e ~ 10¹⁹ m^-3 are attained to date. A predictive 0D power-balance model describes experimental I_p (t) and partitions the active current drive sources. High-frequency MHD activity is found to be present during LHI current drive, in addition to n=1 modes previously found in NIMROD simulation and experiment. A new system of reduced MHD activity was detected where n=1 activity is suppressed, LHI CD efficiency improves, and long-pulse plasmas are sustained with V_IND ~ 0.« less

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in J.M. Perry et al., 'Initiation and Sustainment of Tokamak Plasmas with Local Helicity Injection as the Majority Current Drive,' Nuclear Fusion 58, 096002 (2018).

Local helicity injection (LHI) is a non-solenoidal current drive capable of achieving high-I_p 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, ismore » 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 n = 1 fluctuations at 20–50 kHz are abruptly reduced on the outboard side results in improved current drive. In conclusion, 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.« less

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in J.A. Reusch et al., 'Non-inductively Driven Tokamak Plasmas at Near-Unity βt in the Pegasus Toroidal Experiment,' Phys. Plasmas 25, 056101 (2018).

Amore » major goal of the spherical tokamak (ST) research program is accessing a state of low internal inductance ${\ell }_i$, high elongation $\kappa$ , and high toroidal and normalized beta ( ${\beta }_t$ and ${\beta }_N$ ) 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 ${\ell }_i$. The low aspect ratio ( $R_0/a~1.2$) of Pegasus allows access to high κ and high normalized plasma currents $I_N=I_p/a{B}_T>14$). Magnetic reconnection during LHI provides auxiliary ion heating. Together, these features provide access to very high ${\beta }_t$ plasmas. Equilibrium analyses indicate that ${\beta }_t$ up to ~100% is achieved. Finally, these high ${\beta }_t$ discharges disrupt at the ideal no-wall β limit at ${\beta }_N~7.$« less

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in D.J. Schlossberg et. al., 'A Novel, Cost-Effective, Multi-Point Thomson Scattering System on the Pegasus Toroidal Experiment,' Rev. Sci. Instrum. 87, 11E403 (2016).