18 Search Results

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.L. Barr et. al, 'A Power-Balance Model for Local Helicity Injection Startup in a Spherical Tokamak,' Nuclear Fusion 58, 076011 (2018).

A 0D circuit model for predicting I_p(t) in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface flux of finite-A plasmas developed are modified and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A ~ 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental I_p(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the first half of the current ramp, or I_p ≲ 75 kA. The second half ofmore » the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. Here, 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.« 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).

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in D.J. Schlossberg et al., 'Non-Inductively Driven Tokamak Plasmas at Near-Unity Toroidal Beta,' Phys. Rev. Lett. 119, 035001 (2017).

This public data set contains openly-documented, machine readable digital research data corresponding to figures published in M.G. Burke et. al., 'Continuous, Edge Localized Ion Heating During Non-Solenoidal Plasma Startup and Sustainment in a Low Aspect Ratio Tokamak,' Nucl. Fusion 57, 076010 (2017).

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