The technique of fiber optic pulsed polarimetry, which provides a distributed (local) measurement of the magnetic field along an optical fiber, has been improved to the point where, for the first time, photocathode based optical detection of backscatter is possible with sub-mm spatial resolutions. This has been realized through the writing of an array of deterministic fiber Bragg gratings along the fiber, a so-called backscatter-tailored optical fiber, producing a 34 000-fold increase in backscatter levels over Rayleigh. With such high backscatter levels, high repetition rate lasers are now sufficiently bright to allow near continuous field sensing in both space and time with field resolutions as low as 0.005 T and as high as 170 T over a ∼mm interval given available fiber materials.

The goals were to collaborate with the MSX project and make the MSX platform reliable with a performance where pulsed polarimetry would be capable of adding a useful measurement and then to achieve a first measurement using pulsed polarimetry. The MSX platform (outside of laser blow off plasmas adjacent to magnetic fields which are low beta) is the only device that can generate high-beta magnetized collisionless supercritical shocks, and with a large spatial size of ~10 cm. Creating shocks at high Mach numbers and investigating the dynamics of the shocks was the main goal of the project. The MSX shocks scale to astrophysical magnetized shocks and potentially throw light on the generation of highly energetic particles via a mechanism like the Fermi process.

Injection of plasma via an annular array of coaxial plasma guns during the pre-ionization phase of field-reversed configuration (FRC) formation is shown to catalyze the bulk ionization of a neutral gas prefill in the presence of a strong axial magnetic field and change the character of outward flux flow during field-reversal from a convective process to a much slower resistive diffusion process. This approach has been found to significantly improve FRC formation in a conical θ-pinch, resulting in a ∼350% increase in trapped flux at typical operating conditions, an expansion of accessible formation parameter space to lower densities and higher temperatures, and a reduction or elimination of several deleterious effects associated with the pre-ionization phase.

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 experiment and the numerical model, enabling a measurement of the relaxation time. The measured relaxation time of 4.8 mu s +/- 2.8 mu s is shorter than the toroidal Alfven timescale. These results validate Hall MHD calculations of the Geospace Environmental Modeling challenge. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773401]

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.

The first sustainment of toroidal plasma current of 50 kA at up to 3 times the injected currents, added in quadrature, using steady inductive helicity injection is described. Separatrix currents-currents not linking the helicity injectors-are sustained up to 40 kA. Decreases in the n=1 toroidal mode of the poloidal magnetic field at higher current amplifications indicate more quiescent, direct toroidal current drive. Results are achieved in HIT-SI (with a spheromak of major radius 0.3 m) during deuterium operations immediately after helium operation. These results represent a breakthrough in the development of this new current drive method for magnetic confinement fusion.

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.

Pulsed polarimetry techniques are described for the determination of the local magnetic field distribution, B(r), in magnetically confined plasmas. Pulsed polarimetry is a non-perturbative Lidar-like technique that exploits both the Thomson scattering and magneto-optic Faraday effects to measure B-II at the position and along the sightline of a polarized light pulse propagating in the plasma. The implementation of pulsed polarimetry on high performance magnetized plasmas of relevance to magnetic fusion falls naturally into three categories based on the size and optical activity of the plasma and present-day laser and detector options: 1) large tokamak plasmas typified by ITER and DEMO, 2) meter-sized high energy density plasmas typified by the Magnetized Target Fusion (MTF) program and 3) cm-sized plasmas typified by wire Z-pinches. Plausible pulsed polarimetry systems are presented for each category along with the current interest in obtaining the local field measurement for the respective programs.

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 (38 kA), current amplification (I tor/I inj_quad ≈ 2) and poloidal flux amplification (ψpol/ψinj_quad > 6) to date. Single-injector operations establish a preferred direction of generated spheromak current for each injector depending 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.

Pulsed polarimetry in combination with a high speed photographic technique based on the optical Kerr effect is described. The backscatter in a pulsed polarimeter is directed through a scattering cell and photographed using an ∼1 ps shutter, essentially freezing the intensity pattern. The image provides both the local electron density and magnetic field distributions along and transverse to the laser sightline. Submillimeter spatial resolution is possible for probing wavelengths in the visible due to the high densities and strong optical activity. Pulsed polarimetry is thereby extended to centimeter-sized plasmas with ne>1019−1020 cm−3 and B>20–100 T (MG) produced by multiterawatt, multimega-ampere electrical drivers, wire Z pinches, and liner imploded magnetized plasmas.

direction based upon helicity sign and injector orientation. • Helicity balance dominates the preferred injector current drive direction. • This preference is robust against a vertical field that would otherwise lead to current drive in the opposite direction. • The opposing preferred directions of each injector lead to reversals in the direction of the toroidal current during the shot. Internal magnetic probe data indicate that this reversal is the result of a twisting of the current. • Recent operations with unbalanced injector demands have produced our best ratios of j/n Highlights Preferred current direction depends on the sign of the helicity and the orientation of the injector with respect to the confinement volume