R. G. O’Neill

Flux amplification in Helicity Injected Torus (HIT-II) coaxial helicity injection discharges

Recent coaxial helicity injection (CHI) studies using the Helicity Injected Torus device [Redd et al., Phys. Plasmas 9, 2006 (2002)] have produced discharges with measured toroidal plasma currents up to 350kA and direct evidence of both poloidal flux amplification and toroidal current buildup, resulting from a steady process on millisecond time scales. Internal magnetic probes directly measure the poloidal flux amplification, and also measure a strong paramagnetism. Equilibrium reconstructions of these flux amplification discharges, using only surface magnetics, match the internal probes and multipoint Thomson scattering, and show current-profile relaxation during toroidal current ramp up. The criteria for producing flux amplification include both a sufficiently thin electrode-driven edge region and a large magnetic shear in the CHI injector region, which allows injector reconnection activity to overcome resistive decay and build up a closed plasma core. If the interelectrode distance d is small, then bot...

Experimental demonstration of plasma startup by coaxial helicity injection

Experimental results on the transfer of a coaxial-helicity-injection (CHI) produced discharge to inductive operation are reported. CHI assisted plasma startup is more robust than inductive only operation and reduces volt-seconds consumption. After handoff to inductive operation, the initial 100 kA of CHI produced current drops to 50 kA, then ramps up to 180 kA, using only 30 mVs, about 40% higher than that produced by induction alone. Results show that initiation of CHI discharges at lower densities produce higher levels of coupling current. Coupling a CHI produced discharge to induction from a precharged central solenoid has produced record currents of 290 kA using only 52 mWb of central solenoid flux. CHI discharges can also be generated while the central transformer is in the process of being precharged, during which period it induces a negative loop voltage on the CHI discharge. These significant results were obtained on the Helicity Injected Torus-II (HIT-II) [T.R. Jarboe, Fusion Technol. 15, 7 (1989...

Coaxial helicity injection in open-flux low-aspect-ratio toroidal discharges

Open-flux low-aspect-ratio toroidal discharges generated and sustained by coaxial helicity injection (CHI) in the Helicity Injected Torus device (HIT-II) are described. The discharges in this study are flux tubes directly connected to the CHI electrodes, with poloidal flux less than or equal to the CHI injector flux, and no possibility of a significant closed-flux plasma core. Theoretically derived scalings for the dependence of CHI injector current on the toroidal field current and magnitude of the injector flux are experimentally confirmed, and empirical models are developed for the poloidal magnetic field and toroidal plasma current in open-flux discharges. In particular, the toroidal plasma current is independent of the toroidal magnetic field, both theoretically and empirically. Variations in injector flux geometry demonstrate that the CHI injector current leaves the electrode surfaces at the flux strike points, and that the relative width of the CHI injector determines whether the dominant observed ...

Transient coaxial helicity injection for solenoid-free plasma startup in HIT-II

The favorable properties of the spherical torus (ST) arise from its very small aspect ratio. Methods for initiating the plasma current without relying on induction from a central solenoid are essential for the viability of the ST concept. In steady state tokamaks, the central solenoid can be dispensed with if suitable methods for initiating the plasma current are on hand. Coaxial helicity injection (CHI) is a promising candidate for solenoid-free plasma current startup in STs and tokamaks. Experiments on the Helicity Injected Torus (HIT-II) machine at the University of Washington [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] have demonstrated the capability of a new method, referred to as transient CHI, to produce a high quality closed-flux equilibrium that has been successfully coupled to induction demonstrating that this new plasma current startup method is compatible with the conventional inductive method. This paper presents physics requirements for implementing this method in STs and tokamaks and supporting experimental results from the HIT-II device.

A fully relaxed helicity balance model for an inductively driven spheromak

Magnetic helicity balance and a fully relaxed Taylor-state model are shown to predict the magnitude of sustained equilibrium current in an inductively driven spheromak. The Helicity Injected Torus with Steady Inductive drive (HIT-SI) [T. R. Jarboe et al., Phys. Rev. Lett. 97, 115003 (2006)] forms and sustains spheromaks using two inductively driven helicity injectors. By assuming helicity is injected at a rate 2VΨ, and only decays through Spitzer resistivity using Te measured with a Langmuir probe, the magnitude of the sustained equilibrium current is predicted with no fitting parameters. The model correctly predicts a threshold helicity injection rate for spheromak formation. Although the experimental results suggest a higher effective helicity dissipation rate by a factor of ∼1.37 compared to the Spitzer value, the prediction is still within the uncertainties of the measured parameters.

Ion heating during magnetic relaxation in the helicity injected torus-II experiment

Ion doppler spectroscopy (IDS) is applied to the helicity injected torus (HIT-II) spherical torus to measure impurity ion temperature and flows. [A. J. Redd et al., Phys. Plasmas 9, 2006 (2002)] The IDS instrument employs a 16-channel photomultiplier and can track temperature and velocity continuously through a discharge. Data for the coaxial helicity injection (CHI), transformer, and combined current drive configurations are presented. Ion temperatures for transformer-driven discharges are typically equal to or somewhat lower than electron temperatures measured by Thomson scattering. Internal reconnection events in transformer-driven discharges cause rapid ion heating. The CHI discharges exhibit anomalously high ion temperatures >250eV, which are an order of magnitude higher than Thomson measurements, indicating ion heating through magnetic relaxation. The CHI discharges that exhibit current and poloidal flux buildup after bubble burst show sustained ion heating during current drive.

Temperature and density characteristics of the Helicity Injected Torus-II spherical tokamak indicating closed flux sustainment using coaxial helicity injection

The electron temperature and density profiles of plasmas in the Helicity Injected Torus [HIT-II: T. R. Jarboe et al., Phys. Plasmas 5, 1807 (1998)] experiment are measured by multipoint Thomson scattering (MPTS). The HIT-II device is a small low-aspect-ratio tokamak (major radius 0.3m, minor radius 0.2m, toroidal field of up to 0.5T), capable of inductive ohmic (OH) current drive, Coaxial Helicity Injection (CHI) current drive, or combinations of both. The temperature and density characteristics have been characterized by a ruby laser MPTS diagnostic at up to six locations within the plasma for a single diagnostic time per discharge. Observed hollow temperature profiles of CHI discharges are inconsistent with open flux only predictions for CHI and indicate a closed flux region during CHI current drive.

SPHEROMAK FORMATION BY STEADY INDUCTIVE HELICITY INJECTION

A spheromak is formed for the first time using a new steady state inductive helicity injection method. Using two inductive injectors with odd symmetry and oscillating at 5.8 kHz, a steady state spheromak with even symmetry is formed and sustained through nonlinear relaxation. A spheromak with about 13 kA of toroidal current is formed and sustained using about 3 MW of power. This is a much lower power threshold for spheromak production than required for electrode-based helicity injection. Internal magnetic probe data, including oscillations driven by the injectors, agree with the plasma being in the Taylor state. The agreement is remarkable considering the only fitting parameter is the amplitude of the spheromak component of the state.