Frank Hegeler

A durable gigawatt class solid state pulsed power system

A unique all solid-state pulsed power system has been tested at the Naval Research Laboratory that produced 200 kV, 4.5 kA, and 300 ns pulses, continuously for more than 11,500,000 shots into a resistive load at a repetition rate of 10 pps. The Marx has an efficiency of 80% based on calorimetric measurements. This pulser is used to evaluate components and advance solid state designs for a next generation solid-state pulsed power system to drive an electron beam pumped KrF laser system for inertial fusion energy. The solid state pulser, designed and constructed by PLEX LLC, consists of a 12 stage Marx, coupled with a 3rd harmonic stage to sharpen the Marx output waveforms, a main magnetic switch, a compact pulse forming line used as a transit time isolator, and a resistive load. Each Marx stage uses an APP Model S33A compact high voltage switch that consists of 12 series connected thyristors. A life test on individual thyristors showed operation of >; 300 M shots at 20 Hz without failure.

Electron density measurements during microwave generation in a high power backward-wave oscillator

Laser interferometry is used for the first time to measure plasma electron density along the slow wave structure (SWS) wall during microwave generation in a vacuum, long pulse, high power backward-wave oscillator (BWO). The University of New Mexico long pulse backward-wave oscillator, which displays the characteristic pulse shortening phenomenon, is investigated in these studies. Although pulse shortening is observed across a wide class of high power microwave devices, its origin is not definitively understood. Many hypotheses suggest that the unintentional introduction of plasma into the interaction region near the walls of the SWS is one of several likely causes of pulse shortening in intense electron beam driven slow wave devices. This article presents initial measurements of the line-integrated, temporally resolved plasma density between an intense, relativistic, annular electron beam and SWS walls for a variety of radiated microwave peak power levels. Line-integrated electron densities, , between 9.10/sup 15/ and 2.410/sup 16/ cm/sup -2/ for radiated microwave powers between 20 and 120 MW have been measured. The two main sources of the measured electron density are postulated to be (i) plasma generated from the cutoff neck due to beam scrape off, and (ii) material removed and ionized from the SWS walls during microwave generation.

Ozone Generation by Positive and Negative Wire-to-Plate Streamer Discharges

A UV absorption technique is used to investigate the ozone generation after a positive or negative streamer discharge in air at atmospheric pressure. The discharge apparatus consists of a 45 cm long wire-to-plate configuration with an electrode gap distance of 2.5 cm. After either positive or negative streamer discharges, the highest ozone density occurs in an area just below the wire electrode. Towards the ground plate, the ozone density decreases rapidly. Positive streamer discharges produce about twice as much ozone density as negative streamer discharges with the same pulse duration.

Studies of relativistic backward-wave oscillator operation in the cross-excitation regime

We first reported the operation of a relativistic backward-wave oscillator (BWO) in the so-called cross-excitation regime in 1998. This instability, whose general properties were predicted earlier through numerical studies, resulted from the use of a particularly shallow rippled-wall waveguide [slow wave structure (SWS)] that was installed in an experiment to diagnose pulse shortening in a long-pulse electron beam-driven high-power microwave (HPM) source. This SWS was necessary to accommodate laser interferometry measurements along the SWS during the course of microwave generation. Since those early experiments, we have studied this regime in greater detail using two different SWS lengths. We have invoked time-frequency analysis, the smoothed-pseudo Wigner-Ville distribution in particular, to interpret the heterodyned signals of the radiated power measurements. These recent results are consistent with earlier theoretical predictions for the onset and voltage scaling for this instability. This paper presents data for a relativistic BWO operating in the single-frequency regime for two axial modes, operating in the cross-excitation regime, and discusses the interpretation of the data, as well as the methodology used for its analysis. Although operation in the cross-excitation regime is typically avoided due to its poorer efficiency, it may prove useful for future HPM effects studies.

Forced Convective Cooling of Foils in a Repetitively Pulsed Electron-Beam Diode

Electron-beam (e-beam)-pumped high-power gas lasers require the use of a transmission window/foil to separate the vacuum diode from the laser cell. Under repetitive operation, the foil is subject to an e-beam heat load and would eventually fail without cooling. This paper investigates forced convective cooling of a foil in the main amplifier of the Electra KrF laser by flowing the laser gas around a closed loop. The experimental data were taken with one of the two diodes operating at 500 kV, 110 kA, a full-width at half-maximum of 140 ns, and with an external axial magnetic field of 0.14 T. Type-T thermocouples are used to measure the temperature of the foil under a variety of conditions including flow-velocity enhancement due to louver inserts, repetition rate, cathode configuration, gas composition, and height along the foil. A first-order model that considers cooling due to turbulent flow, as well as internal foil thermal conduction and radiation, reproduces the general trends observed in the data. The goal is to keep the temperature of a 25-mum-thick stainless steel foil below the tensile strength and long-term thermal fatigue limits when operating at 5 Hz. The data, in combination with the model, predict that this goal can be achieved by diverting the laser gas to flow at high velocity along the foil surface.

Observation of the cross-excitation instability in a relativistic backward wave oscillator

Experimental observation of the cross-excitation instability in a relativistic backward wave oscillator is reported. The transition between single frequency operation and the cross-excitation regime is measured. The initial mode excited in the cross-excitation regime has a relatively low efficiency, while the second mode has considerably higher efficiency and a slightly higher frequency. The two modes are observed to be beating for a considerable fraction of the radiated microwave pulse. These results are consistent with earlier theoretical predictions for the onset and voltage scaling for this instability.

Low jitter, high voltage, repetitive laser triggered gas switches

The Electra pulsed power system at the Naval Research Laboratory is capable of supplying 16 kJ to a low impedance load within 140 ns, and it allows continuous operation of up to 5 pulses per second for several hours. Four laser triggered SF6 gas switches transfer the stored pulse forming line energy to the load. Each switch has a hold-off voltage of more than 1 MV and transfers a charge of 10 mC per shot. This paper describes the redesign of the gas switch with hemispherical electrodes to a flat electrode configuration, which led to an improvement in switch reliability. A one sigma switch jitter of ±1.2 ns has been achieved for tens of thousands of continuous shots, with an electrode erosion rate as low as 1 mg/C. Detailed statistical analyses are provided when the switches are operated at a SF6 pressure of 0.36 - 0.69 MPa, with a laser trigger energy of 1 - 18 mJ at 266 nm, and a switch hold-off voltage ranging from 0.7 - 1.2 MV.

Development of a Continuous Multi-Thousand Shot Electron Beam Pumped KrF Rep-Rate Laser for Fusion Energy

Abstract The Electra laser system is currently being developed at the Naval Research Laboratory to serve as a test bed for laser driver technologies needed for an inertial fusion energy power plant. The main amplifier has produced 730 J of laser light operating in an oscillator mode. These results as well as advancement of the laser physics, electron beam deposition, and the pulse power technologies give us projections of >7% wall plug efficiency for an IFE system. The Electra main amplifier in oscillator configuration has run continuously at 1 Hz, 2.5 Hz, and 5 Hz for multi-thousand shot runs. This paper will discuss recent results of the Electra program at the Naval Research Laboratory including integrating the Electra main amplifier into a complete laser amplifier system. Issues addressed will include development paths for the cathode, window coating, and foil longevity to attain the durability required for a fusion power plant.

ELECTRA: AN ELECTRON BEAM PUMPED KrF REP-RATE LASER SYSTEM FOR INERTIAL FUSION ENERGY

Electra is a high average power KrF laser system at the Naval Research Laboratory funded under the HAPL program. The goal of Electra is to develop the laser driver technologies needed for an inertial fusion energy power plant. When run in an oscillator configuration the 500 kV, 100 kA e-beam pumped main amplifier produces 730 J with a 100ns pulse width at 248 nm. KrF lasers have been shown to have intrinsic efficiencies of 12%leading to a projected wall plug efficiency of >7% for an IFE system with demonstrated improvements in laser physics and pulse power technologies. As an oscillator the Electra main amplifier has run continuously at 1 Hz,2.5 Hz, and 5 Hz for multi-thousand shot runs. This paper will discuss recent results from Electra including operation as a complete laser amplifier system, first demonstration of a new method to efficiently cool the hibachi foil with indications of a reduced penalty in laser uniformity, and design modifications to increase durability.

Recent advances in the study of a long pulse relativistic backward wave oscillator

Laser interferometry was recently used to diagnose plasma formation and evolution in the slow wave structure (SWS) of a relativistic backward wave oscillator (BWO) during the course of microwave generation. The results indicated that plasma from the cutoff neck inlet contributed to the termination of the high power microwave pulse. In an effort to mitigate this pulse shortening effect, the authors have replaced the cutoff neck with a Bragg reflector. As part of these studies, they have observed the cross-excitation instability because of the particularly shallow-ripple SWS used. They present results from recent experiments performed with the long pulse relativistic BWO, including the implementation of a hybrid-hard tube BWO at the University of New Mexico (UNM).