Session GI3: Basic Plasma Physics: LAPD, TORPEX, and Non-Neutral Plasmas

Control of pressure-gradient-driven instabilities using shear Alfv\'en Beat-Waves

A new technique for manipulation and control of gradient-driven instabilities through nonlinear interaction with shear Alfven waves (SAW) in a laboratory plasma is presented [1]. A narrow field-aligned density depletion is created in the background plasma of the Large Plasma Device (LAPD), resulting in coherent unstable fluctuations on the periphery of the depletion. Two independent SAWs are launched along the depletion at separately controlled frequencies, creating a nonlinear beat-wave response at or near the frequency of the original instability. Resonant drive of the instability is observed when the beat frequency matches the frequency of the unstable mode. More interestingly, when the beat-wave is driven at a frequency slightly above the instability frequency, the m=1 instability is suppressed in favor of (or synchronized to) an m=2 mode at the beat frequency. An amplitude threshold is observed for this behavior, and the frequency width of the region of beat wave control increases with beat wave amplitude. Although power is being added to the system by broadcasting into the gradient region, the low-frequency fluctuations (both broadband noise and the dominant coherent mode) are reduced when the instability is suppressed in favor of the m=2 mode. This behavior is only observed for long-wavelength beat waves driven by co- propagating SAWs; the interaction is not observed with short wavelength beat waves from counter propagating SAWs.\\[4pt] [1] D. W. Auerbach {\it et al.}, arXiv:1005.0647v2 [physics.plasm-ph].   

Structure of an Exploding Laser-Produced Plasma

The behavior of expanding dense plasmas has long been a topic of interest in space plasma research, particularly in the case of expansion within a magnetized background plasma. Expansion perpendicular to B causes a wide range of effects, including a ``diamagnetic bubble'' or localized reduction of the background field, as well as visible periodic structures on the expanding plasma surface. A recent series of experiments at the UCLA Large Plasma Device (LaPD) studied these phenomena via a laser-produced plasma immersed in a large magnetized background plasma. The structure of the expanding plasma is diagnosed in three dimensions via a high-resolution ($\Delta L/L_{\rm{plasma}} \sim 0.03$) in-plasma probe drive. Currents within the expanding plasma are found to have complex structure in three dimensions; in particular, an unexpected current system along the background field was discovered at the cavity surface. In addition to measurement of the plasma structure, the time behavior of large-scale ($L \sim L_{\rm{plasma}}$) periodic structures on the plasma surface was investigated via two-probe correlation analysis, revealing that the structures are static and translate with the bubble across the background field.   

Toward validation of a 3-D plasma turbulence model using LAPD data

Detailed results from a 3-D fluid simulation of plasma turbulence are compared with experimental data from the Large Plasma Device (LAPD) at UCLA. LAPD is a magnetized plasma column experiment with a high repetition rate, allowing detailed time-and-space resolved probe data on plasma turbulence and transport. The large amount of data allows a thorough comparison with the simulation results. For the observed drift-type modes, LAPD plasmas are strongly collisional ($\omega_{*}/\nu_{ei} \ll$1 and $\lambda_{ei}/L\ll$1), providing justification for a fluid treatment. Accordingly, the model is based on reduced Braginskii equations and is implemented in the framework of the BOUT code, originally developed at LLNL for tokamak edge plasmas. Analysis of linear plasma instabilities shows that resistive drift modes, rotation-driven interchange modes, and Kelvin-Helmholtz modes can all be important in LAPD and have comparable frequencies and growth rates. In nonlinear simulations using measured LAPD density profiles, evolution of instabilities and self-generated zonal flows results in a saturated turbulent state. Comparisons of these simulations with measurements in LAPD plasmas reveal good agreement, in particular in the frequency spectrum, spatial correlation, and amplitude probability distribution function of density fluctuations. Also, consistent with the experiment, the simulations indicate a great deal of similarity between plasma turbulence in LAPD and some features of tokamak edge turbulence. Similar to tokamak edge plasmas, density transport appears to be predominantly carried by large particle-flux events. Despite the intermittent character of the calculated turbulence, as indicated by fluctuation statistics, the turbulent particle flux is consistent with a diffusive model with diffusion coefficient close to the Bohm value.   

Blob motion and control in simple magnetized plasmas

Intermittent convective transport caused by coherent structures, or blobs, are universally observed in the edge of laboratory plasmas. Besides being of fundamental physics interest, the dynamics of these structures in fusion reactors influence the density scale-length in the scrape-off layer, its impurity screening characteristics, wall-recycling and possibly the overall confinement properties. In TORPEX simple magnetized plasmas, blobs are generated from interchange turbulence and, driven by gradB and curvature-induced charge separation, propagate radially outwards. The magnitude of their velocity depends on the current paths to damp charge separation. Regimes dominated by either parallel or cross-field currents are achieved by varying the ion mass. An analytical expression for the blob velocity including cross-field ion polarization currents, cross-field ion currents due to neutral friction and parallel currents to the sheath is derived and shows good quantitative agreement with the experimental data. To confirm this interpretation, direct measurements of the 2D structure of the blob-induced parallel currents have been obtained using magnetic probes. Parallel blob dynamics are further studied with a Mach probe, revealing the convection of parallel momentum by blobs. Methods to influence and control blob motion are also being explored, such as the variation of the connection length or the use of poloidal arrays of biased electrodes. This study is part of a more general project of code validation on TORPEX. Methodology and results of a comparison of 2D and global 3D fluid simulations with experiments will be presented. (Co-authors: A. Fasoli, I. Furno, D. Iraji, B. Labit, P. Ricci, M. Spolaore (RFX-Padua), N. Vianello (RFX-Padua))   

Formation of High-Beta Plasma and Stable Confinement of Toroidal Electron Plasma in RT-1

The Ring Trap 1 (RT-1) device is a laboratory magnetosphere generated by a levitated superconducting magnet. The goals of RT-1 are to realize stable formation of ultra high-beta plasma suitable for burning advanced fusion fuels, and confinement of toroidal non-neutral plasmas including antimatter particles. RT- 1 has produced high-beta plasma in the magnetospheric configuration. The effects of coil levitation and geomagnetic field compensation [Y. Yano et al., Plasma Fusion Res. {\bf 4}, 039] resulted drastic improvements of the plasma properties, and a maximum local beta value exceeded 70\%. Because plasma is generated by electron cyclotron resonance heating (ECH) in the present experiment, the plasma pressure is mainly due to hot electrons, whose bremsstrahlung was observed with an x-ray CCD camera. The pressure profiles have rather steep gradient near the superconducting coil in the strong field region. The decay rates of magnetic probe and interferometer signals have different time constants, suggesting multiple temperature components. The energy confinement time estimated from the input RF power and stored magnetic energy is on the order of 1s, which is comparable to the decay time constant of the density of hot electron component. Pure electron plasma experiments are also conducted in RT-1. Radial profiles of electrostatic potential and electron density showed that the plasma rigidly rotates in the toroidal direction in the stable confinement phase. Long time confinement of toroidal non- neutral plasma for more than 300s and inward particle diffusion to strong field regions, caused by the activation of the diocotron (Kelvin-Helmholtz) instability, have been realized [Z. Yoshida et al., Phys. Rev. Lett. {\bf 104}, 235004].   

Plasmas of arbitrary neutrality

The physics of partially neutralized plasmas is largely unexplored, partly because of the difficulty of confining such plasmas. Plasmas are confined in a stellarator without the need for a plasma current, and regardless of the degree of neutralization. The Columbia Non-neutral Torus (CNT) is a stellarator dedicated to the study of non-neutral plasmas, and partially neutralized plasmas. CNT is currently conducting the first systematic studies of plasmas of arbitrary neutrality. The degree of neutralization of the plasma can be parameterized through the quantity $\eta \equiv (n_{e}-Z n_{i})/(n_{e}+Zn_{i})$. In CNT, $\eta$ can be varied continuously from pure electron ($\eta = 1$) to quasi-neutral ($\eta \approx 0$) by adjusting the neutral pressure in the chamber, which controls the volumetric ionization rate. Pure electron plasmas are in macroscopically stable equilibria, and have strong self electric potentials dictated by the emitter filament bias voltage on the magnetic axis. As $\eta$ decreases, the plasma potential decouples from the emitter, and fluctuations begin to appear. For $\eta \approx 0.5$, the plasma oscillates at a single dominant mode (40 - 130 kHz) related to the $\mathbf{E \times B}$ rotation of the plasma [1]. For $\eta \approx 0.01$, the plasma exhibits multiple mode oscillations. And when the plasma becomes quasi-neutral ($\eta < 10^{-4}$), it reverts to single mode behavior (2 - 18 kHz). The spatial structure of these low frequency oscillations in the quasi-neutral regime has been studied using a high speed camera, showing that the detected mode travels approximately perpendicularly to the field, but it is not perfectly aligned with the field lines. A parametric characterization of the modes detected in plasmas of arbitrary neutrality will be presented along with measurements of the spatial structure of the oscillations, with an aim to characterize the physics of the modes observed. \\[4pt] [1] Q. Marksteiner, PRL 100 (2008) 065002