R.F. Gribble

Oscillating field current drive for reversed field pinch discharges

Oscillating field current drive (OFCD), also known as F‐Θ pumping, is a steady‐state current drive technique proposed for the reversed field pinch (RFP). Unlike other current drive techniques, which employ high technology, invasive, and power intensive schemes using radio frequency or neutral particle injection, F‐Θ pumping entails driving the toroidal and poloidal magnetic field circuits with low‐frequency (audio) oscillating voltage sources. Current drive by this technique is a consequence of the strong nonlinear plasma coupling in the RFP. Because of its low frequency and efficient plasma coupling, F‐Θ pumping shows excellent promise as a reactor relevant current drive technique. A conceptual and computational study of this concept, including its experimental and reactor relevance, is explored in this paper.

Oscillating field current drive experiments in a reversed field pinch

Steady‐state current sustainment by oscillating field current drive (OFCD) utilizes a technique in which the toroidal and poloidal magnetic fields at the plasma surface are modulated at audio frequencies in quadrature. Experiments on the ZT‐40M reversed field pinch [Fusion Technol. 8, 1571 (1985)] have examined OFCD over a range of modulation amplitude, frequency, and phase. For all cases examined, the magnitude of the plasma current is dependent on the phase of the modulations as predicted by theory. However, evidence of current drive has only been observed at relatively low levels of injected power. For larger modulation amplitudes, the data suggest that substantial current drive is offset by increased plasma resistance as a result of modulation enhanced plasma–wall interactions. The initial experimental results and supporting theoretical interpretations of OFCD are discussed.

PLASMA FLOW SWITCH AND FOIL IMPLOSION EXPERIMENTS ON PEGASUS II

Pegasus II is the upgraded version of Pegasus, a pulsed power machine used in the Los Alamos AGEX (Above Ground EXperiments) program. A goal of the program is to produce an intense (> 100 TW) source of soft x-rays from the thermalization of the kinetic energy of a 1 to 10 MJ plasma implosion. The radiation pulse should have a maximum duration of several 10`s of nanoseconds and will be used in the study of fusion conditions and material properties. The radiating plasma source will be generated by the thermalization of the kinetic energy of an imploding cylindrical, thin, metallic foil. This paper addresses experiments done on a capacitor bank to develop a switch (plasma flow switch) to switch the bank current into the load at peak current. This allows efficient coupling of bank energy into foil kinetic energy.

Startup of reversed field pinches and current ramping using dynamo action

The different startup modes of a reversed-field pinch (RFP) are examined and compared. The RFP startup is not the same as startup of other toroidal devices such as the tokamak because of the spatial and temporal variations of the toroidal field and the reversed toroidal field in the outer region of the pinch near the wall. In matched mode startup, used in many RFP experiments today, the toroidal flux is held constant during the current rise, with the field reversal occurring before the peak current. This mode, with its short rise time, has a low volt-second (V⋅s) input but requires a high toroidal voltage to reach a specific current in a relatively short time. In a ramped current mode, alow current RFP discharge is ramped to its final peak value. The toroidal flux needed as the current rises is generated by a dynamo action. This slower startup mode can be driven by a lower voltage but requires more V⋅s input than the matched mode. Different startup modes in the ZT-40M experiment at Los Alamos are compared and an analytic expression is given for characterizing the V⋅s contributions. The resistive component of the toroidal loop voltage during the current rise in ramped discharges is found to depend on theta (Θ = Bpol(wall)/Bave)- At a theta of 1.45, the resistive voltage has a minimum and it has been possible to reduce the V⋅s input by as much as 40% in ramp discharges by keeping theta close to this value.

The Atlas High-Energy Density Physics Project

Atlas is a pulsed-power facility under development at Los Alamos National Laboratory to drive high-energy density experiments. Atlas will be operational in the summer of 2000 and is optimized for the study of dynamic material properties, hydrodynamics, and dense plasmas under extreme conditions. Atlas is designed to implode heavy-liner loads in a z-pinch configuration. The peak current of 30 MA is delivered in 4 µs. A typical Atlas liner is a 47-gram-aluminum cylinder with ∼ 4-cm radius and 4-cm length. Three to five MJ of kinetic energy will be delivered to the load. Using composite layers and a variety of interior target designs, a wide variety of experiments in ∼ cm3 volumes will be performed. Atlas applications, machine design, and the status of the project are reviewed.

Initial operation of a 10 ms, quasi‐steady multi‐megawatt, coaxial plasma thruster

The Los Alamos National Laboratory Coaxial Thruster Experiment (CTX) has been upgraded to enable the quasi‐steady operation of MPD type thrusters at power levels from 1 to 40 MW for 10 ms. Initial diagnostics include an 8 position, 3 axis magnetic field probe to measure magnetic field fluctuations during the pulse, a triple Langmuir probe to measure ion density, electron temperature and plasma potential and a time‐of‐flight neutral particle spectrometer to measure specific impulse. Here we report on preliminary investigation of long‐pulse quasi‐steady coaxial thruster performance.

Series fault limiting resistors for Atlas Marx modules

The proposed Atlas pulsed power supply design provides a current pulse to the experiment chamber from a set of 20, 3-Marx-unit-wide modules radially positioned around a rectangular disk transmission-line system (total of 60 Marxes in parallel). The Atlas circuit is designed to be a near-critically-damped network with a total erected capacitance of 200 /spl mu/F at 600 kV. The justification for the necessary circuit resistance in this approach is based on reliability, fault tolerance and operational maintenance. Also the use of high energy-density capacitors that have lower tolerance to voltage reversal is a primary reason for the damping provided by significant series resistance. To obtain the damping there are two system resistors in the Atlas design. One resistor is a shunt element designed to damp the resonance caused by the relatively high-Q disk transmission-line capacitance and the Marx bank inductance. The second, more significant resistor is a series, fault-current limiting element that also performs the necessary damping for voltage reversal at the bank capacitors. The series resistor is the subject of this paper.

The atlas power-flow system - A status report

The design requirements, design features, test results and status of the Atlas high-energy pulsed-power facility power flow system are described

Atlas transmission line breakdown analysis

The Atlas facility will use 24 radially converging, vertically oriented and tapered, oil insulated, triplate transmission lines between the Marx generators and the central load region. Among the requirements of the transmission lines are low inductance and high reliability. The inter-conductor gap is nominally 2 cm and the lines taper from a height of 1.75 m at the Marx end to 0.32 m at the output end. The aluminum conductors, held together by 20 insulating spacers, are assembled and inserted as a unit into radial oil-filled steel tanks. The negative, high-voltage, center conductor is 2.54-cm thick and the outer ground conductors are 1.59-cm thick. All 24 triplate transmission lines connect to a transition section at near 1 m radius that couples the transmission lines to a disk/conical solid-dielectric-insulated power flow channel transmission line terminating at the load. Peak operating voltage on the lines can be as high as 240 kV with an effective stress time of 0.8 /spl mu/s. Testing of small sections of the total area have been completed and the test results are analyzed to show that the probability of failure at these voltage levels is less than 1 in 1000 system shots.

Improved tuning methods for converter-modulators

The converter-modulator is a resonant power conditioning configuration that is optimized for a particular load impedance or parameter space. Although traction motor Insulated Gate Bipolar Transistors (IGBTs) are typically used for hard-switching application in the 1 kHz regime, the present use of high-power (10-15 MW) converter-modulators have used a 20 kHz resonant switching topology. This presents design challenges to maintain efficient and reliable switching characteristics for the IGBTs. Improved tuning methods and circuit topological changes now offer a significant reduction in IGBT switching losses as compared to those used on the spallation neutron source (SNS) design (perhaps by 10). These circuit and topology changes should also permit pulse width modulation (PWM) of the modulator output voltage to provide a regulated voltage without anomalous IGBT switching characteristics. This paper will review the results of this investigation based on models developed from the SNS converter-modulator operational data.