Session DI3: Alternate Configurations and Startup

Formation of a Long-Lived Hot Field Reversed Configuration by Merging Two Colliding High-$\beta$ Compact Toroids

A new compact toroid (CT) device, C-2, has been built to form and sustain fusion-relevant field reversed configurations (FRC), one of the simplest magnetic confinement entities with average $\beta $ (ratio of average plasma to magnetic pressure inside the separatrix) $\sim $10. High temperature FRCs are produced in C-2 by dynamically merging two oppositely directed, highly supersonic high-$\beta $ deuterium plasmoids preformed by the conventional $\theta $-pinch technology, achieving record lifetimes of over 2 ms based on external diamagnetic measurements, with plasma diameter $\sim $ 1 m, poloidal flux \textit{$\phi $}$_{p} \quad \sim $ 15 mWb, electron density $n_{e} \quad \sim $ 10$^{20}$ m$^{-3}$, and $T_{i}+T_{e} \quad >$ 0.5 keV. Most of the kinetic energy is converted into thermal energy upon collision, predominantly going into the ion channel: $T_{i} \quad \sim \quad T_{e} \quad \sim $ 30 eV before merging, while $T_{i} \quad \sim $ 4.5$T_{e}$ with $T_{e} \quad \sim $ 100 eV after merging, as derived from radial pressure balance and multi-chord, muli-pulse Thomson scattering measurements. Such high ion temperatures are also consistent with Doppler spectroscopy and neutron measurements. Strong poloidal flux amplification occurs during the merging process with a flux amplification factor exceeding 10, the highest ever obtained in a magnetic confinement system. Both temperatures and poloidal fluxes of the merged FRCs depend strongly on the speed of the initial individual plasmoids, favoring fast translation. The dynamics of the merging/reconnection process of the translated CTs are reproduced, for the first time, by a newly developed 2-D resistive magnetohydrodynamic code, LamyRidge. What is even more remarkable is that the final merged FRC state exhibits a dramatic improvement in transport with flux confinement times approaching classical values. The formation of such a well-confined, long-lived, high-$\beta $ plasma state via collisional merging and magnetic reconnection should be of wide interest to fusion energy sciences and basic plasma physics research.   

Effect of pressure-driven MHD instabilities on confinement in reactor-relevant high-beta helical plasmas

Plasmas with 5{\%} volume averaged beta have been routinely produced in the Large Helical Device. The effect of the global modes and short-wave-length turbulence from MHD instabilities on the confinement is assessed. Even in globally stable plasmas, magnetic fluctuations due to MHD modes are observed localized in the plasma periphery, where there is a magnetic hill. They are enhanced as the beta increases and the magnetic Reynolds number decreases. These MHD instabilities are pressure driven because there is no net-current. Comparison of plasmas with and without the global MHD instabilities shows that the instability with a mode width of 5{\%} of the plasma minor radius reduces the energy confinement time by 10{\%}. In addition, the thermal transport in the magnetic hill region degrades gradually as the beta increases. The degradation has a clear correlation with the amplitude of density fluctuations with short wave length. These behaviors are consistent with a transport model based on a resistive interchange driven turbulence. Although the MHD modes do not produce a hard operational limit, this work reports quantitative analysis of their degradation of confinement giving a soft limit.   

Simulated and Measured Electron Thermal Transport with Varying Magnetic Stochasticity in the MST RFP

New high spectral resolution simulations with the 3D, nonlinear, resistive MHD code D\textsc{ebs} have been made at a Lundquist number of $S \approx $ 4 $\times $ 10$^{6}$, matching that of 400 kA standard discharges in the MST. At this Lundquist number: (1) D\textsc{ebs} reproduces many features of the MST plasma including a well defined sawtooth cycle during which the spectrum of tearing modes dynamically evolves, (2) simulations were run for many sawtooth cycles so that synthetic diagnostic data could be sawtooth-ensembled and compared directly to experimental data, (3) the experimentally measured electron thermal diffusion profile agrees reasonably well with Rechester-Rosenbluth thermal diffusion when the degree of magnetic stochasticity is high, and (4) when the stochasticity is relatively low, magnetic structures are seen in the D\textsc{ebs} data that correlate to electron temperature structures observed in the core region of MST with the recently upgraded Thomson scattering system. The TS system on MST is now capable of measuring the temporal evolution of the \textit{Te} profile at up to 25 kHz, representing an important advance in the diagnosis of high-$\beta $ plasmas that are inaccessible to ECE. In addition to resolving the \textit{Te} dynamics of the sawtooth cycle, this enhanced diagnostic capability has enabled confinement measurements for plasmas that spontaneously exhibit extended sawtooth-free periods with reduced stochasticity. During these periods, the energy confinement time triples, reaching 3 ms, while the central \textit{Te} increases to over 1 keV, and runaway electrons, generally absent from standard discharges, are observed. These reduced stochasticity periods can last for many energy confinement times with an approximately constant magnetic equilibrium, and thus may be a useful future development path for the RFP. This work supported by the US DOE.   

Demonstration of Inductive Flux Saving by Transient CHI on NSTX

Experiments in NSTX have now demonstrated the saving of central solenoid flux equivalent to 200kA of toroidal plasma current after coupling plasmas produced by Transient Coaxial Helicity Injection (CHI) to inductive sustainment and ramp-up of the toroidal plasma current [R. Raman, et al., PRL 104, 095003 (2010)]. This is a record for non-inductive plasma startup, and an important step for developing the spherical torus concept. With an injector current of only 4kA and total power supply energy of only 21 kJ, CHI initiated a toroidal current of 250 kA that when coupled to 0.11 Vs of induction ramped up to 525 kA without using any auxiliary heating, whereas an otherwise identical inductive-only discharge ramped to only 325 kA. This flux saving was realized by reducing the influx of low-Z impurities during the start-up phase through the use of electrode conditioning discharges, followed by lithium evaporative coating of the plasma-facing surfaces and reducing parasitic arcs in the upper divertor region through use of additional shaping-field coils. As a result of these improvements, and for the first time in NSTX, the electron temperature during the CHI phase continually increased with input energy, indicating that the additional injected energy was contributing to heating the plasma instead of being lost through impurity line radiation. Simulations with the Tokamak Simulation Code (TSC) show that the observed scaling of CHI start-up current with toroidal field in NSTX is consistent with theory, suggesting that use of CHI on larger machines is quite attractive. These exciting results from NSTX demonstrate that CHI is a viable solenoid-free plasma startup method for future STs and tokamaks. This work supported by U.S. DOE Contracts DE-AC02-09CH11466 and DE-FG02-99ER54519 AM08.