David Q. Hwang

Design and operation of a passively switched repetitive compact toroid plasma accelerator

The design and operation ofa spheromak-like compact toroid (SCT) plasma accelerator is described. As an example application, some principles are presented for using the device as a plasma injector to fuel a tokamak plasma. The device forms and accelerates an SCT plasma. The SCT is a self-contained structure of plasma with embedded poloidal and toroidal magnetic fields and their associated currents that provide plasma confinement and structural integrity. The SCT is formed in a magnetized coaxial plasma gun and then accelerated within coaxial electrodes. The typical mass of an SCTfor tokamak fueling is from several tens to several hundreds of micrograms and is accelerated up to a velocity of ∼2 × 10 5 m/s. Larger-mass SCTs can be produced, and higher velocities are possible. This is important for other applications such as space propulsion, X-ray generation, fast-opening plasma switches, and low-temperature high-density plasma simulators. The novel features of the device are as follows: (a) it can be operated in a repetitive mode, (b) the high-energy capacitor bank to form the SCT is switched by initiating breakdown with fast gas injection, (c) the required delay between formation and acceleration is achieved passively with saturable inductors that switch the high-energy accelerator capacitor bank, and (d) a drift section has been added within the toroidal field region to study cross-field propagation prior to tokamak penetration. With the device installed on the Davis Diverted Tokamak (DDT), measurements are taken to study tokamak fueling. Typical rep-rated parameters are as follows: the SCT poloidal magnetic field at the outer electrode = 0.4 T, the stored formation bank energy = 1600 J, and the stored accelerator bank energy = 3600 J. The lower bound on SCT kinetic energy leaving the accelerator = 40 J (inferred from electron line density measurements). Typical SCT velocity is 15 to 20 cm/μs. The maximum rep-rate achieved so far with the device is 0.2 Hz and is currently limited by vacuum pumping capacity. Reliable operation has been demonstrated for 1000 consecutive shots. Higher-energy single shots have also been taken to study SCT propagation through an open guide tube and to study penetration of the SCT into the DDT tokamak vessel with both tokamak plasma discharges and vacuum toroidal magnetic field only.

A simple fast pulse gas valve using a dynamic pressure differential as the primary closing mechanism

In this article we describe a simple fast pulse gas valve developed for use in a plasma discharge experiment. The valve delivers 1017–1019 molecules per pulse varied by changing the voltage on the electromagnetic driver power supply. Valve pulse widths are observed to be less than 300 μs full width at half maximum with a rise time of less than 100 μs resulting in a maximum gas flow rate of ∼1022 molecules per second. An optical transmission technique was used to determine the mechanical opening and closing characteristics of the valve piston. A fast ionization gauge (FIG) was used for diagnosis of the temporal character of the gas pulse while the total gas throughput was determined by measuring the change in pressure per pulse in a small test chamber with a convectron tube gauge. Calibration of the FIG was accomplished by comparing the net change in pressure in a large chamber as measured by the FIG to the net change in pressure in a small test chamber as measured by the convectron tube gauge.

Plasma density gradient measurement using laser deflection

For a given chord through a plasma, changes in the line integrated index of refraction as a result of a transverse density gradient can be observed by measuring the angle of deflection of a laser beam. In contrast to laser interferometers, this method of density profile measurement places modest requirements on laser quality and alignment procedures, allowing measurements to be conducted with short coherence length commercial laser diodes and segmented photodiode detectors. A prototype implementation of this scheme has been constructed and tested on the compact toroid injection experiment (CTIX). At densities comparable to magnetic fusion plasmas, laser deflections in the nanoradian range were measured. By assuming a particular density profile, a sensitivity of ∼1012cm−3∕nrad was obtained. This produced estimates of CTIX peak density in reasonable agreement with conventional interferometry data. The final goal of this diagnostic is a simple, reliable, array deployable density profile diagnostic.

Penetration of a compressible magnetized plasma object into a low beta target plasma

The effects of the shape and compressibility of a perfectly conducting spheromaklike compact toroid (SCT) propagating into a low beta plasma have been studied. Simple tools that allow one to identify conditions for the optimum penetration of the SCT into the low beta target plasma have been developed. The main results obtained are (1) squeezed (prolate) SCT’s penetrate more easily; (2) including SCT compressibility reduces injection power requirement, and (3) there exists a broad parameter range where the SCT injection can maintain tokamak particle inventory while simultaneously reducing external auxiliary heating requirements.

Interaction of a spheromak-like compact toroid with a high beta spherical tokamak plasma

Recent experiments using accelerated spheromak-like compact toroids (SCTs) to fuel tokamak plasmas have quantified the penetration mechanism in the low beta regime; i.e. external magnetic field pressure dominates plasma thermal pressure. However, fusion reactor designs require high beta plasma and, more importantly, the proper plasma pressure profile. Here, the effect of the plasma pressure profile on SCT penetration, specifically, the effect of diamagnetism, is addressed. It is estimated that magnetic field pressure dominates penetration even up to 50% local beta. The combination of the diamagnetic effect on the toroidal magnetic field and the strong poloidal field at the outer major radius of a spherical tokamak will result in a diamagnetic well in the total magnetic field. Therefore, the spherical tokamak is a good candidate to test the potential trapping of an SCT in a high beta diamagnetic well. The diamagnetic effects of a high beta spherical tokamak discharge (low aspect ratio) are computed. To test the penetration of an SCT into such a diamagnetic well, experiments have been conducted of SCT injection into a vacuum field structure which simulates the diamagnetic field effect of a high beta tokamak. The diamagnetic field gradient length is substantially shorter than that of the toroidal field of the tokamak, and the results show that it can still improve the penetration of the SCT. Finally, analytic results have been used to estimate the effect of plasma pressure on penetration, and the effect of plasma pressure was found to be small in comparison with the magnetic field pressure. The penetration condition for a vacuum field only is reported. To study the diamagnetic effect in a high beta plasma, additional experiments need to be carried out on a high beta spherical tokamak.

Validating cylindrical Langmuir probe techniques

Several methods for estimating the plasma potential and density using cylindrical Langmuir probes are compared to the self‐consistent solutions of the Vlasov–Poisson equations calculated by Laframboise (J. G. Laframboise, Ph. D. dissertation, University of Toronto, 1966). Measurements are made during the decay of a magnetic‐field‐free plasma in which the mean‐free path of the electron is shorter than the dimensions of the vacuum vessel (the electrons, therefore, have a Maxwellian velocity distribution). The measurements are made in a parameter range in which exact analytical solutions do not exist for the ion and electron saturation currents, 0.5≤R/λDe≤5, where R is the probe radius and λDe is the electron Debye length (kTe/4πne2)1/2. An iterative procedure is used to fit the data at probe voltages both above and below the plasma potential while constraining the curves to be continuous at the plasma potential. The measured curves could be represented extremely well by the numerical results. It is therefor...

Compact Toroid Fueling for ITER

Experimental and theoretical work indicates that deep fueling of ITER may be possible by Compact Toroid (CT) injection. CT velocities sufficient for center fueling of a reactor have been demonstrated in the RACE device. CT injections into the TdeV tokamak have achieved central penetration at 1.4 T, and have increased the particle inventory by more than 30% without disruption. Tests on the MARAUDER device have achieved CT mass-densities suitable for injection into 5 T tokamaks. Techniques for producing multiple-shot CTs with passive electric switching are being tested on CTIX. The advantages of deep fueling by CT injection include profile peaking to reach ignition, profile control, low tritium inventory and others. In this paper, the CT experimental results are summarized, a conceptual design of CT fueler for ITER is presented, and the implications on ITER operation and fuel cycle are discussed.

Beat wave excitation of electron plasma waves in a toroidal magnetized plasma

Electrostatic waves are driven in a toroidal plasma by counterpropagating microwave beams with a difference in frequency approximately equal to the electron plasma frequency. Energetic electrons are detected when the phase velocity of the electrostatic waves are 3ve< vph < 7ve, where ve is the electron thermal velocity. Experiments are performed in the Davis Diverted Torus (DDT) [Bull. Am. Phys. Soc. 33, 2049 (1988)] operating in a high repetition rate (15 Hz), low‐density (7×107–2×109 cm−3) mode with only a toroidal magnetic field (∼110 G). The microwaves are triggered 30 μsec after the pulsed discharge ends. At this time the energetic electrons have left the system and the velocity distribution is Maxwellian (Te ∼ 1 eV). The microwaves have tunable frequencies over the range 8.5–9.5 GHz, and peak powers ∼180 kW (400 nsec). Bounded plasma modes are excited when the electron cyclotron frequency is larger than the electron plasma frequency. Direct measurements of the wave vector have been made with a doubl...

Experimental research of a multipass transverse excitation atmospheric-pressure CO2 laser

Abstract. We have designed and constructed a transverse excitation atmospheric-pressure CO2 laser using a multipass configuration. We compare the measured results of both laser output power and energy from various modes of operation. We have measured the maximum laser output energy to be 19 J and the peak output power to be greater than 100 MW.

Simulated and experimental compression of a compact toroid

We present simulation results and experimental data for the compression of a compact toroid by a conducting nozzle without a center electrode. In both simulation and experiment, the plasma flow is obstructed by even modest magnetic fields. A simple mechanism for this obstruction is suggested by our simulations. The configuration of the plasmoid’s magnetic field plays a significant role in its compression. We analyze two types of plasma configurations under compression and demonstrate that the results from the simulations match those from the experiments, and that the mechanism predicts the different behaviors observed in the two cases.