Session GI2: Alternates

Extension of High-Ion-Temperature Regime in the Large Helical Device (LHD)

The production of high-ion-temperature hydrogen plasmas was successfully demonstrated in the LHD. The ion temperature (Ti) exceeded 5 keV (the record value in helical plasmas) at an average plasma density (n$_{e})$ of 1.2x10$^{19}$m$^{-3}$ and also achieved 3 keV at n$_{e}\sim $4x10$^{19}$m$^{-3}$. This achievement demonstrated the capability of high-ion-temperature plasma confinement in helical devices. The total injection power of neutral beams of 20 MW (3 parallel and 1 perpendicular injections) and ion cyclotron heating power of about 2 MW was applied in reduced helical-ripple magnetic configurations with Rax in the range of 3.575-3.65 m and B of 2.7-2.85 T. Here Rax is the vacuum magnetic axis position. High-Ti plasmas typically have large toroidal velocity, Vt, of order 50 km/s in the core region, accompanied by an increase of the Ti-gradient. \newline The measured core density of carbon-impurity ions strongly drops as the core-Ti increases. This implies that carbon-impurity ions are expelled from the core region. This unique feature may provide an efficient knob to avoid the impurity accumulation in reactor-relevant helical plasmas. Transport analysis has been performed, including comparison with relevant theories.~In these high-Ti plasmas, the ions are in 1/$\nu $ regime and the neoclassical ambipolar Er is expected to be negative (ion-root). This prediction indicates that the hollow impurity profile (usually anticipated from the positive Er (electron-root)) must be due to effects beyond neoclassical transport theory. The role of the large Vt in the improved ion heat confinement, with the viewpoint of plasma viscosity structure in three-dimensional magnetic configurations, will be discussed.   

Theoretical and experimental studies of high-beta plasmas formed by odd-parity rotating magnetic fields

Hamiltonian simulations of ion and electron heating by odd-parity rotating magnetic fields applied to FRC plasmas have predicted rapid heating of both electrons and ions to multi-keV temperatures, even at low relative RMF strengths. Both the onset of heating and saturation of energy have been explained by perturbation analysis in stochastic theory. These simulations assumed full RMF penetration to the major axis and collisionless particle trajectories, the latter expected in fusion reactor. However, most present RMF/FRC experiments do not achieve full RMF penetration and operate in a low-temperature collisional regime, far from fusion-reactor conditions. Recent experiments at Princeton, which employ commercial off-the-shelf hardware and non-invasive diagnostics and which use, for the first time in FRC research, remote divertor chambers, have achieved a thousand-fold reduction in collisionality to below 0.001, volume-averaged beta above 0.5, electron temperatures above 200 eV, and full penetration of the RMF while avoiding the radiation barrier encountered by other RMF/FRC experiments. Comparisons between theory and experiment show the important role of infrequent collisions, particularly with neutrals. Motivations for a superconducting next-step FRC and design considerations for a car-sized practical FRC reactor will be described.   

Higher Temperature Steady-State FRCs Formed and Sustained by Rotating Magnetic Fields in the New TCS-Upgrade Device

Previous work in the Translation, Confinement, and Sustainment (TCS) device has demonstrated the formation and steady-state sustainment of FRCs by rotating magnetic fields (RMF). The RMF was shown to provide complete FRC stability and good particle confinement. Simple theory shows that the particle density is set primarily by the applied RMF torque, but that the plasma temperature is separately determined by power balance. In TCS this temperature was limited to several 10s of eV due to high impurity content. A new upgraded device, TCSU, with a bakable, ultra-high vacuum chamber was built to reduce these impurities and overall recycling. Spectacular improvements were obtained in the first month of TCSU operation, with temperatures increasing to well over 100 eV. The higher temperatures resulted in higher magnetic fields and toroidal currents without higher RMF power inputs, indicating improvements in current drive efficiency and energy confinement time with temperature. These results were obtained with simple, even-parity RMF antennas, which cause field line opening. Results with both odd-parity antennas, which can achieve complete field line closure and reduce fundamental non-radiative energy loss rates, as well as with advanced wall conditioning methods will also be reported on.   

Gyrokinetic simulations of plasma turbulence, transport and zonal flows in a closed field line geometry

We present nonlinear gyrokinetic simulations of small-scale plasma turbulence and transport in closed field-line geometries relevant to the Levitated Dipole Experiment (LDX) and planetary magnetospheres: the Z-pinch and the ring-dipole. As in toroidal geometries, the instabilities present in the system depend on the steepness of the plasma pressure gradient: for sufficiently steep gradients, the system is unstable to ideal interchange modes, while for weaker gradients, short wavelength non-MHD modes at the ion gyro-radius scale (entropy modes) typically dominate. Considering the latter, ideally-stable case at low plasma beta, we find an enormous variation in the nonlinear dynamics and particle transport as a function of the density and temperature gradients and the plasma collisionality. This variation is explained in part by the damping and stability properties of spontaneously formed zonal flows in the system. As in toroidal systems, the zonal flows can lead to a strong nonlinear suppression of transport below a critical gradient that is determined by the stability of the zonal flows.   

Magnetic Field Generation and Energy Confinement with T$_{e} >$ 500 eV in the SSPX Spheromak

The understanding of confinement and energy transport in spheromaks is key the understanding the physics of spheromak formation and self-organization as well as addressing the feasibility of the concept as a reactor scenario. In the Sustained Spheromak Physics eXperiment (SSPX), increased understanding of the physics in building and sustaining self-organized magnetic equilibria has resulted in record electron temperatures T$_{e} \quad >$ 500 eV and plasma currents of $\sim $ 1 MA on the magnetic axis. We find that the highest edge magnetic field magnitudes (and correspondingly high T$_{e})$ is achieved when $\lambda =\frac{\mu _0 I_{gun} }{\Psi _{gun} }$ is near (but slightly below) the Kruskal-Shafranov instability limit $\lambda _{KS} \cong \frac{2\pi }{L}\cong 12.6\,m^{-1}$ where L is the length of the flux-conserver (0.5 m). Building on previously reported results, power-balance analysis has shown levels of electron thermal transport $\chi _e <$ 1 m$^{2}$/s, indicating good confinement and closed flux surfaces. With the addition of a modular capacitor bank we are able to highly tailor the gun current to take advantage of the sensitive dependence of spheromak performance on the plasma $\lambda $. When in this optimum operating range we also find that the efficiency of field build-up (defined as the ratio of edge poloidal magnetic field to gun current) is increased 20{\%} over prior results, to $\sim $1.0 T/MA. Additionally this brings the efficiency of spheromak formation into numerical agreement with results from the NIMROD 3-D MHD code. Plasma energy evolution has been studied by taking time-resolved measurements of T$_{e}$(r) and n$_{e}$(r) indicating a distinct and robust feature of spheromak formation; a hollow-to-peaked temperature transition with an inverse relationship to the electron density. This feature, as well as sub-microsecond transport, is being studied with the upgrade of the Thomson scattering diagnostic to double-pulse operation. We also present recent results of the impact of charge-exchange losses on overall power balance and estimates of the plasma ion temperature as measured with a neutral particle analyzer.   

High confinement in fusion oriented plasmas with kV-order potential, ion, and electron temperatures with controlled radial turbulent transport in GAMMA 10

The tandem mirror system has achieved improved energy confinement times ($>$ 60-90 ms) with radial transport dominating the Pastukhov axial energy confinement time ($>$ 100 ms). This high confinement regime establishes a proof of principle that the combination of electrostatic and magnetic mirror confinement can successfully insulate electrons from thermal ions. ECH controlled hot-layer formation facilitates plasma-rotation profile formation with a radially localized high-vorticity layer. In the vicinity of the layer, a radial transport barrier is formed [1], showing similar properties to ITB in toroidal plasmas. Coaxially nested intense E(r)$\times $B sheared flow [2] in the GAMMA 10 core plasma realizes an upgraded stable regime having (i) $>$ 0.75 keV bulk central electron temperature with (ii) an achievement of larger stored energy for axially potential-confined ions exceeding that (i.e., diamagnetism) for central magnetically confined ions ($\approx $ 7 keV). The radially sheared flow having peak-on-axis high vorticity guards and improves whole core plasma confinement, and is controlled by (iii) improved $\approx $ 3 kV ion-confining potential due to simultaneous central and plug ECH. X-ray imaging of the suppression of turbulent structures [1-3] will be shown [1,2]. \newline [1] T. Cho et al., Phys. Rev. Lett. \textbf{97}, 055001 (2006). \newline [2] T. Cho et al., Phys. Rev. Lett. \textbf{94}, 085002 (2005). \newline [3] J. Pratt and W. Horton, Phys. Plasmas \textbf{13}, 042513 (2006). \newline \newline Collaborators; W. Horton$^{1}$, J. Pratt$^{1}$, M. Hirata, J. Kohagura, T. Numakura, H. Hojo, M. Ichimura, A. Itakura, T. Kariya, I. Katanuma, R. Minami, Y. Nakashima, M. Yoshikawa, Y. Miyata, Y. Yamaguchi, T. Imai, V. P. Pastukhov$^{2}$, S. Miyoshi, GAMMA 10 Group ($^{1}$IFS, Univ. Texas at Austin, $^{2}$Kurchatov Institute, Russia)