Contains research objectives, summary of research and reports on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-030

Bekefi, George

Golden, Jeffry

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XV. RELATIVISTIC BEAMSAcademic Research StaffProf. G. BekefiProf. R. J. BriggsProf. J. E. McCuneGraduate StudentsJ. GoldenM. L. ViannaRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHThe use of intense electron beams in plasma research is a relatively new field ofvery considerable interest in several areas of research. This new program at theResearch Laboratory of Electronics has been made possible by a long-term loan of anEG&G facility. The past year has been spent in assembling the unit, building thedrift chamber for the beam and designing the diagnostic tools that will be used initially.Preparations for the initial experiment have been completed, and production of a100-kV, 35-kVA, electron beam lasting 30 ns has begun. The system has three parts:a Marx generator, a concentric cylinder capacitor and a field-emission diode. TheMarx generator, when triggered, delivers up to 500 J at up to 400 kV to the concentriccylindrical, water-filled capacitor at 108 W. When the voltage on the water capacitorrises sufficiently high, a precalibrated solid dielectric switch breaks and connects thediode to the water capacitor. The water capacitor then acts as a transmission lineapplying a potential of up to 200 kV for 30 ns to the diode. Electrons field-emitted fromthe cathode are accelerated and penetrate the anode (which is a very thin film) to emerge10as a 10 W beam in a drifting, field-free state.Our initial research objective concerns the interaction of the beam with anotherplasma. In particular, we wish to understand the various methods by which the beamdeposits some or all of its energy in the plasma and thus heats the ionized medium.The coupling involves collective (turbulent) interactions of waves and particles, but thedetailed nature of the instabilities is still unknown.G. BekefiA. PRELIMINARY DIAGNOSTICS OF THE COAXIAL GENERATORSYSTEM COGEN IIIInvestigation with the Cogen III System of a mildly relativistic high-intensity elec -tron beam injected into a neutral gas (helium) continues. Aside from the ever-presentmaintenance problems, our attention has focused on observation of the Marx generatordischarge characteristics, "stabbed-switch" techniques, "witness-plate" damage by thebeam, and some preliminary spectra.The analysis of Marx generator beam production1 has been extended to account forThis work is supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-0300.QPR No. 104(XV. RELATIVISTIC BEAMS)the transmission-line nature of the water dielectric capacitor load of the Marx generator.Measurements of the Marx generator discharge current and of the voltage rise on theload capacitor have been made. Using the time-integrated output from a dB/dt probe(a two-turn coil, of 1 in. diameter) situated near the second-stage capacitor of the Marx,the discharge current was found to have the expected shape and duration. This is shownin Fig. XV-1. The long-period oscillations represent ringing in the Marx generator;the short-period oscillations represent ringing of the load capacitor (which has a smallbut finite inductance). The voltage rise on the load capacitor was measured by means ofa capacitive voltage divider probe with a carbon-resistor voltage divider, a 50-ft lengthof coaxial cable and a 6 -dB attenuator producing a net reduction ratio of 8000:1. Whenthe voltage rises sufficiently to break down the "stabbed switch," the voltage drops fromthe peak, V , to 2. 5 V p/(2. 5+Zdiode). This drop can be seen in Fig. XV-2. Since thesquarest pulse on the diode results when the diode impedance, Zdiod e , matches the watercapacitor impedance (2. 5 2), this diagnostic method is useful in determining both the14TIMEc.- 600 ns -Fig. XV-1.V225 kV IHMarx generator dischargeas a function of time.current plottedFig. XV-2. Voltage on load capacitor plotted as afunction of time.QPR No. 104 192(XV. RELATIVISTIC BEAMS)voltage and mean impedance of the diode.Because the peak voltage depends crucially on the reliability by which the breakdownof the "stabbed switch" can be calibrated, the manufacture of the switch deserves muchconsideration. At present we are applying techniques learned at a meeting.2 Theswitches are two (1/16 in.) layers of low-density polyethylene sheet with approximately50 closely spaced holes, typically 25 mil deep, near the center of each sheet. The holesare made by a sewing-machine needle and are then covered with a sheet of aluminumfoil. In operation, when a spark channel forms in one layer, the other layerbecomes highly overvolted, and breaks down in more than one place. Having manychannels lowers the inductance.As the electrons leave the field-emission vacuum diode and enter the gas-filled drifttube they damage the anode foil (aluminized Mylar). This damage pattern isan indication of current distribution. (The anode foil acts as a "witness plate.") We findthat emission is strongest at the edge of our cylindrical disk cathode. This is consistentwith the expected high-fringing fields in this region. The beam is often filamentary, andprobably of very high current density, since most of the damage occupies an area muchsmaller (~3 cm ) than the cathode area.We use a 1. 5 m Wadsworth mounting spectrograph to observe the plasma resultingfrom the beam penetration of the helium target gas. Because of the long optical pathof the experimental arrangement and the short duration of the light emission, as manyas 6 shots are required to record high-quality spectra. (The turn-around time betweenshots is approximately 1 hour.) Spectra have been obtained for gas pressures of approx-imately 1 Torr using slit widths of 50 ym. This corresponds to an instrumental widthof approximately 1 /2 A. Thus either an unfolding of the density-produced linewidths oroperation at higher pressures may be necessary.We plan to refine the measurement of the diode voltage, expand "witness-plate"techniques, and begin x-ray self-photography. The spectroscopic analysis continues.J. GoldenReferences1. Quarterly Progress Report No. 103, Research Laboratory of Electronics, M.I. T.,October 15, 1971, pp. 103-107.2. J. Rome, Oak Ridge National Laboratory, and L. Bradley, Sandia Corporation,American Physical Society, Division of Plasma Physics, Meeting, Madison,Wisconsin, November 15-18, 1971.QPR No. 104 193

Relativistic Beams

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Relativistic Beams

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