Zhehui Wang

Hypervelocity dust beam injection for internal magnetic field mapping

Injecting neutral atoms into high-temperature plasmas forms the basis for several important diagnostics, such as motional Stark effect and charge exchange recombination spectroscopy. We describe an alternative approach to seeding the plasma with neutrals, via “hypervelocity dust beam injection” (HDBI), using micron-sized dusts. Among its many potential applications, HDBI mapping of two-dimensional internal magnetic fields inside medium-sized (50–500 eV) plasmas is discussed in detail. Electrostatic acceleration at ∼100–200 kV will launch a stream of (0.2–10 μm-sized) dust grains of lithium or carbon to hypervelocities (1–10 km/s). Each dust grain, acting as a “microcomet” in the plasma, forming a plume (tail), which if photographed, will reveal the direction of the local magnetic field, with anywhere from 10–100 microcomets in the plasma at any time, a full profile of the B-field direction could be obtained per high resolution image. Due to the small dust grain size, the perturbation to the plasma will be...

Plasma dragged microparticles as a method to measure plasma flows

The physics of microparticle motion in flowing plasmas is studied in detail for plasmas with electron and ion densities ne,i∼1019m−3, electron and ion temperatures of no more than 15eV, and plasma flows on the order of the ion thermal speed, vf∼vti. The equations of motion due to Coulomb interactions and direct impact with ions and electrons, of charge variation, as well as of heat exchange with the plasma, are solved numerically for isolated particles (or dust grains) of micron sizes. It is predicted that microparticles can survive in plasma long enough, and can be dragged in the direction of the local ion flow. Based on the theoretical analysis, we describe a new plasma flow measurement technique called microparticle tracer velocimetry (mPTV), which tracks microparticle motion in a plasma with a high-speed camera. The mPTV can reveal the directions of the plasma flow vectors at multiple locations simultaneously and at submillimeter scales, which is hard to achieve by most other techniques. Thus, mPTV ca...

Plasmadynamic hypervelocity dust injector for the National Spherical Torus Experiment

The design and construction of a plasmadynamic device to accelerate dust to hypervelocities is presented. High speed dust will be used to measure magnetic field lines in the National Spherical Torus Experiment. The plasma gun produces a high density (ne≈1018cm−3) and low temperature (a few eV) deuterium plasma, ejected by J×B forces which provide drag on the dust particles in its path. The dust will be entrained by the plasma to velocities of 1–30km∕s, depending on the dust mass. Carbon dust particles will be used, with diameters from 1to50μm. The key components of the plasmadynamic accelerator are a coaxial plasma gun operated at 10kV (with an estimated discharge current of 200kA), a dust dispenser activated by a piezoelectric transducer, and power and remote-control systems.

Equilibrium and magnetic properties of a rotating plasma annulus

Local linear analysis shows that magneto-rotational instability can be excited in laboratory rotating plasmas with a density of 1019m−3, a temperature on the order of 10eV, and a magnetic field on the order of 100G. A laboratory plasma annulus experiment with a dimension of ∼1m, and rotation at ∼0.5 sound speed is described. Correspondingly, magnetic Reynolds number of these plasmas is ∼1000, and magnetic Prandtl number ranges from about one to a few hundred. A radial equilibrium, ρUθ2∕r=d(p+Bz2∕2μ0)∕dr=K0, with K0 being a nonzero constant, is proposed for the experimental data. Plasma rotation is observed to drive a quasisteady diamagnetic electrical current (rotational current drive) in a high-β plasma annulus. The rotational energy depends on the direction and the magnitude of the externally applied magnetic field. Radial current (Jr) is produced through biasing the center rod at a negative electric potential relative to the outer wall. Jr×Bz torque generates and sustains the plasma rotation. Rotationa...

Dust trajectories and diagnostic applications beyond strongly coupled dusty plasmas

Plasma interaction with dust is of growing interest for a number of reasons. On the one hand, dusty plasma research has become one of the most vibrant branches of plasma science. On the other hand, substantially less is known about dust dynamics outside the laboratory strongly coupled dusty-plasma regime, which typically corresponds to 1015m−3 electron density with ions at room temperature. Dust dynamics is also important to magnetic fusion because of concerns about safety and potential dust contamination of the fusion core. Dust trajectories are measured under two plasma conditions, both of which have larger densities and hotter ions than in typical dusty plasmas. Plasma-flow drag force, dominating over other forces in flowing plasmas, can explain the dust motion. In addition, quantitative understanding of dust trajectories is the basis for diagnostic applications using dust. Observation of hypervelocity dust in laboratory enables dust as diagnostic tool (hypervelocity dust injection) in magnetic fusion....

Plasma jet acceleration of dust particles to hypervelocities

A convenient method to accelerate simultaneously hundreds of micron-size dust particles to a few km/s over a distance of about 1m is based on plasma drag. Plasma jets which can deliver sufficient momentum to the dust particles need to have speeds of at least several tens of km/s, densities of the order of 1022m−3 or higher, and low temperature ∼1eV, in order to prevent dust destruction. An experimental demonstration of dust particles acceleration to hypervelocities by plasma produced in a coaxial gun is presented here. The plasma flow speed is deduced from photodiode signals while the plasma density is measured by streaked spectroscopy. As a result of the interaction with the plasma jet, the dust grains are also heated to high temperatures and emit visible light. A hypervelocity dust shower is imaged in situ with a high speed video camera at some distance from the coaxial gun, where light emission from the plasma flow is less intense. The bright traces of the flying microparticles are used to infer their ...

Microparticle probes for laboratory plasmas

Two applications of microparticles (micron-size particles) for laboratory plasma diagnosis are discussed. The first application is about injecting hypervelocity microparticles [(HDI) for hypervelocity dust injection] for internal magnetic field measurement in high-temperature plasmas. Since the concept of HDI has already been examined in details in our previous works, the primary focus here is to compare different schemes of microparticle acceleration. A new design of HDI based on plasma-dynamic accelerator is described to inject multiple microparticles to velocities around 10 km/s simultaneously. The other application is about using microparticles to measure plasma flow [(mPTV) for microparticle tracer velocimetry]. Directions of plasma flow at multiple locations can be measured simultaneously using mPTV because ion drag dominates over other forces inside laboratory plasmas of order 10/sup 19/ m/sup -3/ in density and a few electron volts in temperature. In addition to complex interactions between a microparticle with plasma, the magnitude of plasma flow may not be obtained directly from the microparticle velocity because of the time it takes for each microparticle to relax to local plasma velocity. In summary, microparticles are naturally small objects in all three dimensions and can, therefore, become useful diagnostics for laboratory plasmas with minimal perturbation.

A Penning-assisted subkilovolt coaxial plasma source

A Penning-assisted 20MW coaxial plasma source (plasma gun), which can achieve breakdown at sub-kV voltages, is described. The minimum breakdown voltage is about 400V, significantly lower than previously reported values of 1–5kV. The Penning region for electrons is created using a permanent magnet assembly, which is mounted to the inside of the cathode of the coaxial plasma source. A theoretical model for the breakdown is given. A 900V 0.5F capacitor bank supplies energy for gas breakdown and plasma sustainment from 4to6ms duration. Typical peak gun current is about 100kA and gun voltage between anode and cathode after breakdown is about 200V. A circuit model is used to understand the current-voltage characteristics of the coaxial gun plasma. Energy deposited into the plasma accounts for about 60% of the total capacitor bank energy. This plasma source is uniquely suitable for studying multi-MW multi-ms plasmas with sub-MJ capacitor bank energy.

Hypervelocity dust beam injection for national spherical torus experiment

An internal magnetic field measurement technique using hypervelocity dust beam injection is described for eventual implementation on the National Spherical Torus experiment. The principle of the diagnostic has been described previously [Wang and Wurden, Rev. Sci. Instrum. 74, 1887 (2003)]. Approximately 200u2002kV of electrostatic potential will be used to accelerate multiple dust particles to velocities in the range of 1.0–10u2002km/s. A fast framing camera will be used to visualize and map two-dimensional internal magnetic field structure from the orientation of the visible light emissions generated by the plumes associated with the hypervelocity dust.

Exact solutions to magnetized plasma flow

Exact analytic solutions for steady-state magnetized plasma flow (MPF) using ideal magnetohydrodynamics formalism are presented. Several cases are considered. When plasma flow is included, a finite plasma pressure gradient ∇p can be maintained in a force-free state J×B=0 by the velocity gradient. Both incompressible and compressible MPF examples are discussed for a Taylor-state spheromak B field. A new magnetized nozzle solution is given for compressible plasma when U∥B. Transition from a magnetized nozzle to a magnetic nozzle is possible when the B field is strong enough. No physical nozzle would be needed in the magnetic nozzle case. Diverging-, drum- and nozzle-shaped MPF solutions when U⊥B are also given. The electric field is needed to balance the U×B term in Ohm’s law. The electric field can be generated in the laboratory with the proposed conducting electrodes. If such electric fields also exist in stars and galaxies, such as through a dynamo process, then these solutions can be candidates to expla...