Improved pulsed inductive plasma accelerators are undergoing development for use as long-life thrusters for propulsion of spacecraft on interplanetary missions and for maneuvering of spacecraft in orbit around the Earth. On Earth, plasma accelerators of this type could be used to produce a variety of atomic, molecular, and plasma beams for scientific experimentation and industrial processing (especially processing of materials in the fabrication of microcircuits). The principal advantages of the developmental accelerators over older pulsed inductive plasma accelerators arise from the use of advanced solid-state electronic pulse circuits, which operate at high repetition rates (as much as a couple of kilohertz), have long operating lives (1011 shots), deliver optimally shaped pulses, and recover reflected energy.
Some discussion of pulsed inductive plasma accelerators in general is prerequisite to a meaningful description of the present improved design concept. Figure 1 illustrates the basic principle of operation. A short pulse of electric current is sent through the coil by closing an electronic switch between a coil and a previously charged capacitor bank. The rapid rise of the axial magnetic field produces an azimuthal electric field, which electrically breaks down a propellant gas in a chamber adjacent to the coil, creating a plasma current ring in the direction opposite that of the current in the coil. The magnetic repulsion between the currents in the coil and the plasma drives the plasma away from the coil.
Pulsed inductive plasma accelerators offer advantages over electric plasma accelerators, in which electrodes supply currents to form the plasmas. Erosion of electrodes severely limits operational lifetimes and introduces electrode atoms into the plasmas. In the case of a plasma for industrial processing or scientific experimentation, contamination with electrode material can be unacceptable. Because a pulsed inductive plasma accelerator operates without electrodes and the plasma is pushed away out of contact with the induction coil immediately upon formation, there is much less erosion, and the plasma is much cleaner than in an electric plasma accelerator.
Until now, the performances of pulsed inductive plasma accelerators have been constrained by the unavailability of reliable, long-life, efficient pulse-power circuits. The gas spark-gap switches used in older pulsed inductive plasma accelerators are unreliable and short-lived (a few million shots maximum) because of erosion of spark-gap electrodes and contamination of gases. With regard to efficiency, two major issues arise:
- The pulse delivered by the power circuit in a typical older pulsed inductive plasma accelerator is not shaped to maximize energy efficiency. Before breakdown, the self-inductance of the coil limits the initial current rise; after breakdown the magnetic flux of the plasma current opposes that of the coil, so that the net inductance is reduced and the current rises rapidly. However, what is needed for efficient operation is a rapid initial current rise to obtain a strong electric field for uniform breakdown, followed by a slower current rise during acceleration to reduce resistive loss of energy in the plasma.
- Because of variation of the load impedance with acceleration of the plasma, some (typically 20 to 25 percent) of the energy fed to the accelerator is reflected. If the reflected energy can be recovered, then efficiency can be increased. The pulse-power circuits of older pulsed inductive plasma accelerators are not designed to recover the reflected energy.
In the developmental pulsed inductive plasma accelerators, the issues of optimal pulse shaping, recovery of reflected energy, and reliability of the switches are all addressed in the design of advanced pulsed driver circuits. These are compact, lightweight, modular solid-state circuits that contain (1) inductive and capacitive elements for storing energy and shaping pulses and (2) silicon controlled rectifiers (SCRs) for switching. Figure 2 shows a simplified version of a circuit of this type to illustrate the principle of operation. The input stage of the circuit is a command resonant charge (CRC) network controlled via SCRs. The CRC network delivers a preset quantity of charge and energy to intermediate storage capacitors between pulses. The pulses formed by the CRC are then compressed by a nonlinear inductance-and-capacitance network that includes a step-up transformer to obtain desired impedance levels.
The nonlinear circuit elements are inductors with saturable cores. The core of each following stage is allowed to saturate before a significant fraction of energy stored in the capacitors of the immediately preceding stage is transferred. The nonlinear saturation phenomenon increases the resonant frequency. Thus, energy is coupled progressively faster as it propagates through successive stages from the input to the output end. The circuit is efficient in transferring power in both directions because it not only upshifts frequency in the forward direction as described above but also downshifts frequency as a reflected pulse propagates back toward the input end. Energy reflected from a mismatched load can propagate all the way back to the SCR commutator. There, an energy-recovery circuit (omitted for clarity) that comprises inductors, capacitors, and SCRs stores some of the reflected energy, which is then added to the forward-propagating energy on the next pulse.
This work was done by Dennis A. Rally, Xing Chen, and William Guss of Science Research Laboratory, Inc., for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com, or circle no. 3 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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