Shreekrishna Tripathi

The upgraded Large Plasma Device, a machine for studying frontier basic plasma physics

In 1991 a manuscript describing an instrument for studying magnetized plasmas was published in this journal. The Large Plasma Device (LAPD) was upgraded in 2001 and has become a national user facility for the study of basic plasma physics. The upgrade as well as diagnostics introduced since then has significantly changed the capabilities of the device. All references to the machine still quote the original RSI paper, which at this time is not appropriate. In this work, the properties of the updated LAPD are presented. The strategy of the machine construction, the available diagnostics, the parameters available for experiments, as well as illustrations of several experiments are presented here.

Dynamics of exploding plasmas in a large magnetized plasma

The dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma was investigated with magnetic flux probes and Langmuir probes. Debris-ions expanding at super-Alfvenic velocity (up to MA=1.5) expel the ambient magnetic field, creating a large (>20 cm) diamagnetic cavity. We observe a field compression of up to B/B0=1.5 as well as localized electron heating at the edge of the bubble. Two-dimensional hybrid simulations reproduce these measurements well and show that the majority of the ambient ions are energized by the magnetic piston and swept outside the bubble volume. Nonlinear shear-Alfven waves (δB/B0>25%) are radiated from the cavity with a coupling efficiency of 70% from magnetic energy in the bubble to the wave.

Co- and counter-helicity interaction between two adjacent laboratory prominences

The interaction between two side-by-side solar prominence-like plasmas has been studied using a four-electrode magnetized plasma source that can impose a wide variety of surface boundary conditions. When the source is arranged to create two prominences with the same helicity (co-helicity), it is observed that helicity transfer from one prominence to the other causes the receiving prominence to erupt sooner and faster than the transmitting prominence. When the source is arranged to create two prominences with opposite helicity (counter-helicity), it is observed that upon merging, prominences wrap around each other to form closely spaced, writhing turns of plasma. This is followed by appearance of a distinct bright region in the middle and order of magnitude higher emission of soft x rays. The four-electrode device has also been used to change the angle of the neutral line and so form more pronounced S-shapes.

Turbulent transport of fast ions in the Large Plasma Device

Strong drift wave turbulence is observed in the Large Plasma Device [H. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by a plate limiter. Energetic lithium ions orbit through the turbulent region. Scans with a collimated ion analyzer and with Langmuir probes give detailed profiles of the fast ion spatial distribution and the fluctuating fields. The fast ion transport decreases rapidly with increasing fast ion gyroradius. Unlike the diffusive transport caused by Coulomb collisions, in this case the turbulent transport is nondiffusive. Analysis and simulation suggest that such nondiffusive transport is due to the interaction of the fast ions with stationary two-dimensional electrostatic turbulence.

Laser-driven, magnetized quasi-perpendicular collisionless shocks on the Large Plasma Devicea)

The interaction of a laser-driven super-Alfvenic magnetic piston with a large, preformed magnetized ambient plasma has been studied by utilizing a unique experimental platform that couples the Raptor kJ-class laser system [Niemann et al., J. Instrum. 7, P03010 (2012)] to the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. This platform provides experimental conditions of relevance to space and astrophysical magnetic collisionless shocks and, in particular, allows a detailed study of the microphysics of shock formation, including piston-ambient ion collisionless coupling. An overview of the platform and its capabilities is given, and recent experimental results on the coupling of energy between piston and ambient ions and the formation of collisionless shocks are presented and compared to theoretical and computational work. In particular, a magnetosonic pulse consistent with a low-Mach number collisionless shock is observed in a quasi-perpendicular geometry in both experiments and simulations.

Dependence of fast-ion transport on the nature of the turbulence in the Large Plasma Device

Strong turbulent waves (δn/n ∼0.5, f ∼5-40 kHz) are observed in the upgraded Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by an annular obstacle. Energetic lithium ions (Efast/Ti≥300, ρfast/ρs~10) orbit through the turbulent region. Scans with a collimated analyzer and with probes give detailed profiles of the fast ion spatial distribution and of the fluctuating wave fields. The characteristics of the fluctuations are modified by changing the plasma species from helium to neon and by modifying the bias on the obstacle. Different spatial structure sizes (Ls) and correlation lengths (Lcorr) of the wave potential fields alter the fast ion transport. The effects of electrostatic fluctuations are reduced due to gyro-averaging, which explains the difference in the fast-ion transport. A transition from super-diffusive to sub-diffusive transport is observed when the fast ion interacts with the waves f...

Current drive by toroidally bounded whistlers

Low-frequency bounded whistlers have been launched in a toroidal system to produce a high-density plasma. These waves generate a completely different wave field pattern in the toroidal plasma cavity as compared to helicon waves in thin cylindrical plasma columns. Based on the excitation of toroidal bounded whistlers, which sustain the discharge, a new approach to the problem of current drive is discussed. 1.0?1.5?kA of plasma current is driven with 1.0?kW of input radiofrequency (RF) power. The same RF source sustains steady-state plasma with 1012?cm?3 peak density and 18?eV electron temperature. Along with studies on the dependence of plasma current on wave and plasma parameters, resonant and nonresonant contributions to the total plasma current are estimated.

Normal modes of bounded whistler produced toroidal plasmas

In tight aspect ratio toroidal devices, characteristics of bounded whistler sustained plasma filled resonant cavities are shown to have significant differences as compared to characteristics of cylindrical cavities in helicon plasma sources. In the present article the analytical-numerical treatment of the problem in electronmagnetohydrodynamic regime is based on wave-field computation and modified dispersive properties of wave for toroidal devices. In the limit of tight aspect ratio, the parallel wavelength of the excited whistler becomes comparable to the system dimension and, thus, strong poloidal mode coupling introduces new features to radially bounded whistlers. Smooth transition of toroidal bounded whistler modes towards conventional helicon modes is observed as the major radius increases.

Measurements of interactions between waves and energetic ions in basic plasma experiments

To measure the transport of fast ions by various types of waves, complementary experiments are conducted in linear and toroidal magnetic fields in the large plasma device and the toroidal plasma experiment. Lithium sources that are immersed in the plasma provide the energetic ions. Spatial scans of collectors measure the transport. Techniques to find the beam and optimize the spatial sensitivity are described. Measurements of Coulomb scattering, resonant ) (

Development of a radio-frequency ion beam source for fast-ion studies on the large plasma device

A helium ion beam source (23 kV/2.0 A) has been constructed for studying fast-ion physics in the cylindrical magnetized plasma of the large plasma device (LAPD). An inductive RF source produces a 10(19) m(-3) density plasma in a ceramic dome. A multi-aperture, rectangular (8 cm × 8 cm) three-grid system extracts the ion beam from the RF plasma. The ion beam is injected at a variety of pitch angles with Alfvénic speeds in the LAPD. The beam current is intense enough to excite magnetic perturbations in the ambient plasma. Measurements of the ion beam profile were made to achieve an optimum beam performance and a reliable source operation was demonstrated on the LAPD.