H.C. Kirbie

Development of solid-state induction modulators for high PRF accelerators

Lawrence Livermore National Laboratory (LLNL) and EG&G Energy Measurements are developing a new solid-state power system for two proposed accelerators. One of the accelerators is a circular arrangement of induction cells called a recirculator for accelerating heavy ions for inertial fusion, the other is a linear induction accelerator for electron beams called the Advanced Radiographic Machine (ARM). The need for complex pulse agility in these accelerators led us to examine solid-state switching components that have an on/off capability. The intrinsic speed of solid-state switching satisfies our high PRF requirements, while the on/off switching action of some semiconductor devices enables us to select an arbitrary pulse width. To accommodate these requirements, we selected field effect transistors (FETs) as the preferred switching elements. The same FET switching technology applies to both accelerators due to their similar pulse requirements. For our research, two full-scale prototypes were built-a 5 kV induction recirculator cell and a single 15 kV induction modulator for the ARM accelerator. We discuss the general network features that are common to both machines, followed by performance and modeling data.

A FET-SWITCHED INDUCTION ACCELERATOR CELL

Abstract : An array of 24 field-effect transistors (FETs) is being used to switch a nominal4-kV, 1-microsecond pulse onto a Metglas induction core at pulse rates exceeding 100kHz. Each transistor receives isolated gate power from a dc/dc converter and analog pulse control via an optical fiber. The array is part of a specialized circuit architecture that generates bursts of pulses while providing for core reset between pulses. The circuit will accommodate variations in pulse width, repetition frequency (prf), pulse amplitude, burst length and reset interval. The various circuit elements are assembled directly onto the core structure to yield a compact, low-impedance package. Two prototype machines are presently under development. A 24- FET machine is in operation and capable of 4.2-kV, 1-microsecond pulses (max.) at a 120-kHz prf for short bursts. Pulse rise and fall times are 25 ns and 65 ns respectively. A 128-FET machine is under construction which should be capable of 6-kV, 1-microsecond pulses (max.) at a 150-kHz prf for long bursts.

MHz repetition rate solid-state driver for high current induction accelerators

A research team from the Lawrence Livermore National Laboratory and Bechtel Nevada Corporation is developing an all solid-state power source for high-current induction accelerators. The original power system design, developed for heavy-ion fusion accelerators, is based on the simple idea of using an array of field effect transistors (FETs) to switch energy from a pre-charged capacitor bank to an induction accelerator cell. Recently, that idea has been expanded to accommodate the greater power needs of a new class of high-current electron accelerators for advanced radiography. For this purpose, we developed a 3-stage induction adder that uses over 4000 field effect transistors to switch peak voltages of 45 kV at currents up to 4.8 kA, with pulse repetition rates of up to 2 MHz. This radically advanced power system can generate a burst of five or more pulses that vary from 200 ns to 2 /spl mu/s at a duty cycle of up to 25%. Our new source is precise, robust, flexible, and exceeds all previous drivers for induction machines by a factor of 400 in repetition rate and a factor of 1000 in duty cycle.

A magnetically switched trigger source for fxr

This paper describes the design and performance of a three-stage magnetic pulse compressor used to trigger the Flash X-Ray Accelerator at Lawrence Livermore National Laboratory. Each of two compressors generates a pulse onto a bundle of thirteen parallel cables. The cables carry the pulse to a network of spark gaps that initiates the accelerator charge and discharge sequence. The performance data illustrate the compression of a 7.7-/spl mu/s, dual-resonant waveform into a 232-ns (FWHM) unipolar pulse with an 80-ns risetime and a maximum dV/dt of 1.98 kV/ns. Additional data are also presented to show network jitter, efficiency, and sensitivity to small changes in charge voltage and reset current.

An all solid state pulse power source for high PRF induction accelerators

Researchers at the Lawrence Livermore National Laboratory (LLNL) are developing a flexible, all solid-state pulsed power source that will enable an induction accelerator to produce multikiloampere electron beams at a maximum pulse repetition frequency (PRF) of 2 MHz. The prototype source consists of three, 15 kV, 4.8 kA solid-state modulators stacked in an induction adder configuration. Each modulator contains over 1300 field-effect transistors (FETs) that quickly connect and disconnect four banks of energy storage capacitors to a magnetic induction core. The FETs are commanded on and off by an optical signal that determines the duration of the accelerating pulse. Further electronic circuitry is provided that resets the magnetic cores in each modulator immediately after the accelerating pulse. The system produces bursts of five or more pulses with an adjustable pulse width that ranges from 200 ns to 2 /spl mu/s. The pulse duty factor within a burst can be as high as 25% while still allowing time for the induction core to reset. The solid-state modulator described above is called ARM-II and is named for the Advanced Radiographic Machine (ARM)-a powerful radiographic accelerator that will be the principal diagnostic device for the future Advanced Hydrotest Facility (AHF).

Optical control, diagnostic and power supply system for a solid state induction modulator

A new high speed optical control, diagnostic and power supply system has been developed for a solid state induction modulator. The modulator consists of a large array of field effect transistors (FETs) that switch a high-voltage pulse across a tape-wound magnetic core. The FETs within the modulator are mounted on numerous circuit boards that are stacked in series for high-voltage operation. The new optical system overcomes the issue of voltage isolation by supplying each circuit board with optically coupled control power and high bandwidth signal information. An optical fiber is used to transmit laser light to a custom photovoltaic cell that provides DC power to the on-board control circuits. Optical fiber technology is again used to convey a pulse that contains detailed analog features to the FET gate controls. Diagnostic data and status information are also obtained from each board by similar optical methods.

Integrated power conditioning for laser diode arrays

The authors describe a compact pulsed power modulator which has demonstrated its ability to efficiently and accurately drive a laser diode array. The addition of a crowbar protection circuit is an invaluable addition to the integrated system and is capable of protecting the laser diode array against severe damage. The authors show that the correlation between measured data and simulation indicates that their modulator model is valid and can be used as a tool in the design of future systems. The spectrometer measurements that they conducted underline the importance of current regulation to stable laser operation.

SPICE MODELING OF A FET-SWITCHED INDUCTION ACCELERATOR CELL

Abstract : A PSpice model of an induction accelerator cell switched by field-effect transistors (FETs) has been developed to simulate the modulators circuit performance and induction core flux behavior. A FET switched induction cell has been built that generate 4-kV, 1 microsecond pulses at pulse rates exceeding 100 kHz. The circuit architecture provides for core reset between pulses and produces bursts of pulses that are variable in amplitude, pulse width and prf. The transistor switching array, energy storage capacitors, reset circuit, and cell core are all combined into a compact, low-impedance package. This high-prf induction cell is being developed as the accelerating element for a proposed heavy-ion recirculator, which is an arrangement of many small induction cells in a 30-m diameter circle. The recirculator will accept 10-MeV ions from a linear ion accelerator, under development at the Lawrence Berkeley Laboratory, and continue their acceleration to 60-MeV by repeatedly passing the ion beam through the many 5-kV cells. As the ions gain speed, the cell prf must also keep pace by increasing from 70 kHz to 200 kHz. Simple PSpice models have been used to predict B-H loop behavior in the magnetic core and to analyze circuit performance. Simulations of the induction cell will be presented and compared with experimental data.

Development of FET-switched induction accelerator cells for heavy-ion fusion recirculators

The recirculator, a recirculating heavy-ion induction accelerator, has been identified as a promising approach for an inertial fusion driver. One of the technical challenges to building a recirculator is the requirement for a modulator that can drive the induction accelerator cells at repetition rates /spl ges/100 kHz with variable pulse width and pulse repetition rate capability. A high repetition rate modulator and cell is presently being developed for use on a proposed heavy-ion recirculator. The goal is to develop an array of field-effect transistors to switch 5 kV, 1 /spl mu/s pulses onto a Metglas induction core at pulse rates exceeding 100 kHz. Each transistor in the array is driven by a fiber-optic isolated gate signal that is powered by a dc/dc converter. The circuit architecture provides for core reset between pulses and produces bursts of pulses that are variable in pulse width and prf. The transistor switching array, energy storage capacitors, reset circuit and cell core are all combined into a single compact, low-impedance package. Progress of this development work is presented with supporting data.<<ETX>>

MAGNETICALLY DELAYED LOW-PRESSURE GAS DISCHARGE SWITCHING

Abstract : We have investigated the properties of a magnetically delayed, low-pressure gas discharge switch. We performed measurements of the closure and recovery properties of the switch; performed quantitative erosion measurements; and observed the onset of x-ray production in order to compare switch properties with and without delay. Further, we performed qualitative optical measurements of transition line spectra to correlate our electrical recovery measurements with plasma deionization.