Eusebio Garate

Novel cathode for field-emission applications

A CsI salt‐based cathode which is capable of producing a modest perveance, 10 s of A/cm2 electron beam for several microseconds pulse lengths, and has little susceptibility to diode closure has been experimentally characterized. This explosive field‐emission CsI‐coated carbon fiber cathode has operated in modest 10−5 Torr vacuums at voltages up to 160 kV, and can easily be configured to provide space‐charge‐limited solid or annular electron beams in arbitrarily large diameter configurations. The CsI cathode has demonstrated negligible closure for 2 μs pulses, and has operated for 200 shots with no degradation in cathode performance. Data on the operating performance of this salt cathode, including effective gap time history and streak photographs demonstrating uniformity of the current density, are presented. A comparison of CsI cathode performance with a velvet explosive field emitting cathode used in electron‐beam production is also presented.

200 MW S‐band Dielectric Cherenkov Maser oscillator

General Dynamics is investigating the Dielectric Cherenkov Maser as a rugged, compact, and tunable high‐power microwave source. This letter reports the test results of an S‐band device operating at 3.8 GHz. The device consisted of a dielectric (e=10) lined cylindrical waveguide with waveguide radius 3.63 cm, and liner thickness 0.60 cm; and an annular electron beam with radius 2.54 cm. The beam parameters were 700 keV, 12 kA, 100 ns and yielded an rf pulse of 200 MW for 20 ns. A 15 kG axial guide field was used. The measured results agree with the numerical solution of the dispersion relation which includes the effect of the beam and liner.

A new high performance field reversed configuration operating regime in the C-2 devicea)

Large field reversed configurations (FRCs) are produced in the C-2 device by combining dynamic formation and merging processes. The good confinement of these FRCs must be further improved to achieve sustainment with neutral beam (NB) injection and pellet fuelling. A plasma gun is installed at one end of the C-2 device to attempt electric field control of the FRC edge layer. The gun inward radial electric field counters the usual FRC spin-up and mitigates the n = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The combined effects of the plasma gun and of neutral beam injection lead to the high performance FRC operating regime, with FRC lifetimes up to 3 ms and with FRC confinement times improved by factors 2 to 4.

STUDY OF A PLASMA-FILLED X-BAND BACKWARD-WAVE OSCILLATOR

We present experimental studies of a plasma‐filled X‐band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement of a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ≤0.5 to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10–100 mT for 3 kA) as compared to a lower beam case (80–115 mT for 1.6 kA). The plasma‐filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength.

High-power dielectric Cherenkov maser oscillator

Results are presented of experiments conducted using the dielectric Cherenkov maser (DCM) oscillator. The device consists of a cylindrical metallic waveguide with an inner radius of 3.64 cm which is partially filled with a dielectric liner. Traveling through the lined waveguide is an annular relativistic electron beam. Liners of dielectric constant epsilon =10, 5, and 2.3 were investigated for liner thicknesses of 4 and 6 mm. The 6-mm-thick, epsilon =10 liner generated an RF output of 200 MW for 20 ns at 3.8 Hz with electron-beam parameters of 700 kV, 12 kA, and pulse duration of 100 ns. The maximum measured power output for the other configurations was 80 MW at a frequency of 7 GHz ( epsilon =5), with several megawatts of power output from the polyethylene liner ( epsilon =2.3) at a frequency of approximately 9 GHz. >

High gain metal grating free-electron laser

The small signal gain is calculated for a metal grating free‐electron laser consisting of a planar metallic grating and an opposing planar metal boundary. The TM01 mode of the metal grating waveguide interacts with a cold, dense, mildly relativistic electron beam which is constrained to move along the axis of the waveguide. Since the interaction fields evanesce away from the slow wave supporting structure, a beam‐grating gap is included in the analysis. Results indicate appreciable gain at lower mm wavelengths over a 30‐cm interaction length at electron accelerating voltages as low as 20 kV. Interaction between the forward wave region of the n=0 space harmonic and the electron beam slow‐space‐charge wave could result in output wavelengths as low as 400‐μm at 250‐kV accelerating voltages.

Ion flow measurements and plasma current analysis in the Irvine Field Reversed Configuration

Measurements of the Doppler shift of impurity lines indicate that there is an ion flow of ∼7 km/s in the Irvine Field Reversed Configuration. A charge-exchange neutral particle analyzer shows the peak energy is below the 20 eV minimum detectable energy threshold, which is in agreement with the spectroscopic data. By evaluating the collision times between the impurities and hydrogen, the dominant plasma ion species, it is concluded that the ions rotate with an angular frequency of ∼4×104 rad/s. Estimates of the ion current in the laboratory frame indicate it is one to two orders of magnitude larger than the measured plasma current of 15 kA. Electron drifts are expected to cancel most of the ion current based on the measured magnetic fields and calculated electric fields.

Operation of an X‐band dielectric Cherenkov maser amplifier

A dielectric Cherenkov maser has operated as an amplifier at a frequency of 9.8 GHz. The amplifier consisted of an electron beam interacting with the TM01 mode of a cylindrical, dielectric lined waveguide with the rf input provided by a tunable (9–10 GHz), 10 kW magnetron. The dielectric constant of the liner was 10 with inner and outer radii of 1.0 and 1.27 cm, respectively. At electron beam voltage and current of 190 kV and 90 A, respectively, the measured power gain of the amplifier was 11 dB over a 21 cm interaction length with a pulse length of approximately 1 μs.

Coaxial configuration of the dielectric Cherenkov maser

The linearized Lorentz force, continuity equation, and Maxwells equations are used to calculate the system dispersion relation for a coaxial configuration of the dielectric Cherenkov maser. The system consists of two coaxial conductors lined with dielectric and an annular relativistic electron beam, which propagates between the two liners. The dispersion relation for the beam and dielectric-lined coaxial waveguide structure and the no-beam system that describes the dependence of the generated frequency on the coaxial waveguide parameters are presented. Using the linearized dispersion relation, the growth rate for the beam-TM/sub 0n/ waveguide mode instability is calculated in the strong-coupling tenuous beam limit. >

Plasma and Ion Beam Injection into an FRC

Experiments on the transverse injection of intense (5–20 A/cm2), wide cross-section (10-cm), neutralized, ∼100-eV H+ plasma and 100-keV H+ ion beams into a preformed B-field reversed configuration (FRC) are described. The FRC background plasma temperature was ∼5 eV with densities of ∼1013 cm−3. In contrast to earlier experiments, the background plasma was generated by separate plasma gun arrays. For the startup of the FRC, a betatron-type “slow” coaxial source was used. Injection of the plasma beam into the preformed FRC resulted in a 30–40% increase of the FRC lifetime and the amplitude of the reversed magnetic field. As for the ion beam injection experiment into the preformed FRC, there was evidence of beam capture within the configuration.