B.S. Victor

Imposed-dynamo current drive

A mechanism for steady inductive helicity injection (SIHI) current drive has been discovered where the current driving fluctuations are not generated by the plasma but rather are imposed by the injectors. Sheared flow of the electron fluid distorts the imposed fluctuations to drive current. The model accurately predicts the time dependent toroidal current, the injector impedance scaling, and the profile produced in the HIT-SI experiment. These results show that a stable equilibrium can be efficiently sustained with imposed fluctuations and the current profile can, in principle, be controlled. Both are large steps for controlled fusion. Some of the effects of the fluctuations on the confinement of tokamak and spheromak reactors are assessed and the degradation may be tolerable. The mechanism is also of interest to plasma self-organization, fast reconnection and plasma physics in general.

Recent results from the HIT-SI experiment

New understanding and improved parameters have been achieved on the Helicity Injected Torus with Steady Inductive helicity injection current drive (HIT-SI) experiment. The experiment has a bowtie-shaped spheromak confinement region with two helicity injectors. The inductive injectors are 180° segments of a small, oval cross section toroidal pinch. Spheromaks with currents up to 38 kA and current amplification of 2 have been achieved with only 6 MW of injector power. The Taylor-state model is shown to agree with HIT-SI surface and internal magnetic profile measurements. Helicity balance predicts the peak magnitude of toroidal spheromak current and the threshold for spheromak formation. The model also accurately predicts the division of the applied loop voltage between the injector and spheromak regions. Single injector operation shows that the two injectors have opposing, preferred spheromak current directions. An electron locking relaxation model is consistent with the preferred direction, with ion Doppler data and with bolometric data. Results from higher frequency operation are given. The impact of the new understanding on the future direction of the HIT programme is discussed.

New understandings and achievements from independent-injector drive experiments on HIT-SI

The Helicity Injected Torus-Steady Inductive (HIT-SI) experiment investigates steady inductive helicity injection with the aim of forming and sustaining a high-beta equilibrium in a spheromak geometry using two semi-toroidal injectors. Results of experiments with unequal helicity injection rates produced the highest spheromak current (38kA), current amplification (Itor/Iinj quad ≈ 2) and poloidal flux amplification (ψpol/ψinj quad > 6) to date. Single-injectoroperationsestablishapreferreddirectionofgeneratedspheromakcurrentforeachinjectordepending on the sign of the injected helicity and its orientation relative to the confinement volume. Yet, the HIT-SI injectors prefer to drive opposing spheromak currents because they are mounted on opposite sides of the confinement volume. Single-injector operations also eliminate the spontaneous spheromak current flipping observed during dual-injector operations. PACS numbers: 52.55.Wq, 52.55.Ip (Some figures in this article are in colour only in the electronic version)

Sustained spheromaks with ideal n = 1 kink stability and pressure confinement

Increasing the helicity injector drive frequency up to 68.5 kHz on the Helicity Injected Torus-Steady Inductive (HIT-SI) experiment has produced spheromaks with current amplifications of 3.8, ideal n = 1 kink stability, improved toroidal symmetry and pressure confinement. Current centroid calculations from surface magnetic probes show an outward shift in the magnetic field at frequencies above 50 kHz. Grad-Shafranov equilibria indicate pressure confinement at higher injector operating frequencies. The minimum characteristic frequency needed to achieve this confining effect on HIT-SI plasmas is found to be approximately 30 kHz by analysis of the density fluctuations.

Validation of single-fluid and two-fluid magnetohydrodynamic models of the helicity injected torus spheromak experiment with the NIMROD code

We present a comparison study of 3-D pressureless resistive MHD (rMHD) and 3-D presureless two-fluid MHD models of the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI). HIT-SI is a current drive experiment that uses two geometrically asymmetric helicity injectors to generate and sustain toroidal plasmas. The comparable size of the collisionless ion skin depth di to the resistive skin depth predicates the importance of the Hall term for HIT-SI. The simulations are run with NIMROD, an initial-value, 3-D extended MHD code. The modeled plasma density and temperature are assumed uniform and constant. The helicity injectors are modeled as oscillating normal magnetic and parallel electric field boundary conditions. The simulations use parameters that closely match those of the experiment. The simulation output is compared to the formation time, plasma current, and internal and surface magnetic fields. Results of the study indicate 2fl-MHD shows quantitative agreement with the experiment ...

Relaxation-time measurement via a time-dependent helicity balance model

A time-dependent helicity balance model applied to a spheromak helicity-injection experiment enables the measurement of the relaxation time during the sustainment phase of the spheromak. The experiment, the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI), studies spheromak formation and sustainment through inductive helicity injection. The model captures the dominant plasma behavior seen during helicity injection in HIT-SI by using an empirical helicity-decay rate, a time-dependent helicity-injection rate, and a composite Taylor state to model both the helicity content of the system and to calculate the resulting spheromak current. During single-injector operations, both the amplitude and the phase of the periodic rise and fall of the toroidal current are predicted by this model, with an exchange of helicity between the injector states and the spheromak state proposed as the causal mechanism. This phenomenon allows for the comparison of the delay between the current rises in the ...

Reduction of plasma density in the Helicity Injected Torus with Steady Inductance experiment by using a helicon pre-ionization source

A helicon based pre-ionization source has been developed and installed on the Helicity Injected Torus with Steady Inductance (HIT-SI) spheromak. The source initiates plasma breakdown by injecting impurity-free, unmagnetized plasma into the HIT-SI confinement volume. Typical helium spheromaks have electron density reduced from (2-3) × 10(19) m(-3) to 1 × 10(19) m(-3). Deuterium spheromak formation is possible with density as low as 2 × 10(18) m(-3). The source also enables HIT-SI to be operated with only one helicity injector at injector frequencies above 14.5 kHz. A theory explaining the physical mechanism driving the reduction of breakdown density is presented.

A Proof of Principle of Imposed Dynamo Current Drive: Demonstration of Sufficient Confinement

Abstract The conceptual design of an experiment for demonstrating and developing the efficient sustainment of a spheromak with sufficient confinement is presented. “Sufficient” means that the current drive power can heat the plasma to its stability β limit. Previous transient experiments showing sufficient confinement in the kilo-electron-volt temperature range with no external toroidal field coil, recent results on Helicity Injected Torus with Steady Inductive (HIT-SI) showing sustainment with sufficient confinement, the potential of imposed dynamo current drive (IDCD) of solving other fusion issues, and a very attractive reactor concept justify a proof-of-principle experiment for a high-β spheromak sustained by IDCD. A machine with 1-m minor radius with the required density control, wall loading, and neutral shielding for a 10-s pulse is described. Peak temperatures of 3 keV and toroidal currents of 3.2 MA and 16% wall-normalized plasma β are envisioned.

Numerical studies and metric development for validation of magnetohydrodynamic models on the HIT-SI experimenta)

We present application of three scalar metrics derived from the Biorthogonal Decomposition (BD) technique to evaluate the level of agreement between macroscopic plasma dynamics in different data sets. BD decomposes large data sets, as produced by distributed diagnostic arrays, into principal mode structures without assumptions on spatial or temporal structure. These metrics have been applied to validation of the Hall-MHD model using experimental data from the Helicity Injected Torus with Steady Inductive helicity injection experiment. Each metric provides a measure of correlation between mode structures extracted from experimental data and simulations for an array of 192 surface-mounted magnetic probes. Numerical validation studies have been performed using the NIMROD code, where the injectors are modeled as boundary conditions on the flux conserver, and the PSI-TET code, where the entire plasma volume is treated. Initial results from a comprehensive validation study of high performance operation with different injector frequencies are presented, illustrating application of the BD method. Using a simplified (constant, uniform density and temperature) Hall-MHD model, simulation results agree with experimental observation for two of the three defined metrics when the injectors are driven with a frequency of 14.5 kHz.

Simulation of injector dynamics during steady inductive helicity injection current drive in the HIT-SI experiment

We present simulations of inductive helicity injection in the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI) device that treats the entire plasma volume in a single dynamic MHD model. A new fully 3D numerical tool, the PSI-center TETrahedral mesh code, was developed that provides the geometric flexibility required for this investigation. Implementation of a zero-β Hall MHD model using PSI-TET will be presented including formulation of a new self-consistent magnetic boundary condition for the wall of the HIT-SI device. Results from simulations of HIT-SI are presented focusing on injector dynamics that are investigated numerically for the first time. Asymmetries in the plasma loading between the two helicity injectors and progression of field reversal in each injector are observed. Analysis indicates cross-coupling between injectors through confinement volume structures. Injector impedance is found to scale with toroidal current at fixed density, consistent with experimental observation. Comparison to experimental data with an injector drive frequency of 14.5 kHz shows good agreement with magnetic diagnostics. Global mode structures from Bi-Orthogonal decomposition agree well with experimental data for the first four modes.