R. Liou

Generation of ultrashort, discrete spectrum microwave pulses using the dc to ac radiation converter

The output radiation of a dc to ac radiation converter is characterized. A relativistic ionization front passing through a capacitor array of period d=1 cm produces short pulses of tunable radiation between 39 and 84 GHz with a gas pressure between 0 and 30 mT. The frequency spectra of the produced pulses are discrete and exhibit full widths at half maximum between 12% and 28%, consistent with the expected width for six cycles’ pulses. An upper bound of 750 ps (detection bandwidth limited) is placed on the pulse widths. These are the shortest pulses produced by a source of coherent radiation in this frequency range.

Generation of microwave pulses from the static electric field of a capacitor array by an underdense, relativistic ionization front

The dc to ac radiation converter is a new device in which a relativistic ionization front directly converts the static electric field of an array of alternatively biased capacitors into a pulse of tunable radiation. In a proof-of-principle experiment frequencies between 6 and 21 GHz were generated with plasma densities in the 1012 cm−3 range and a capacitor period 2d=9.4 cm. In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with 2d=2 cm. The frequency spectra of these pulses are measured using a diffraction grating. The spectra are discrete, and their center frequency varies linearly with the gas pressure prior to ionization (or plasma density), as expected from theory. Their relative spectral width is around 18%, consistent with the expected number of cycles (six) contained in the pulses. An upper limit of 750 psec (bandwidth detection limited) is placed on the pulses length. The emitted frequency increases from 53 to 93 GHz when the capacitors ar...

A study of the high current back lighted thyratron and pseudospark switch

High-current operation of the back-lighted thyratron (BLT) switch is reported. The cathode emission mechanism and discharge formation characteristics common to BLT and pseudospark switches are described. The BLT switch is an optically triggered low-pressure diffuse discharge closing switch that operates at high currents with a self-heated cathode. Operation for square 150 ns pulses of 1-10.5 kA, 2.5-20 kV, <25 Hz, 45 ns risetime and a 500 ns capacitor discharge up to 82 kA at 25 kV including operation at 5 Hz, 17.5 kV, 60 kA is reported. The switch exhibits a risetime and peak current limit at low currents but no high current limit (up to 82 kA). The low current limit is explained as being due to an insufficient heating of the self-heated cathode.<<ETX>>

Long‐pulse electron‐beam generation from the back‐lighted thyratron

A variable pulse‐length electron‐beam source capable of 100’s μs pulse is reported. Long‐pulse electron‐beam generation was based on the hollow cathode discharge mode of operation of the back‐lighted thyratron and achieved by the modification of circuit parameters that control the discharge. With 75 mTorr Ar and 20 kV applied voltage, the electron beam went through a transient phase before reaching a steady‐state long‐pulse generation. During the transient phase, a fast‐decaying voltage (20–2 kV) and a pulse of 2.5 A and 130 ns FWHM electron beam were observed. The self‐extracted long‐pulse electron beam has a duration ∼100 μs, energy ∼2 keV, and current density ∼10 A/cm2. The results demonstrate the feasibility of controlling the electron‐beam pulse length. The device is simple, robust, and compatible with a plasma environment.

High power radiation from ionization fronts in a static electric field in a waveguide

The radiation produced when a relativistically moving plasma/gas boundary (i.e., an ionization front) passes between alternatively biased capacitor electrodes is studied. Results of an experiment based on a design which incorporates the capacitor electrodes into an X band waveguide are presented. The waveguided design effectively couples nearly three orders of magnitude more power into the output than the previously unguided designs. Linear theory is extended to include the depletion of the laser energy as it propagates through the ionizable gas (i.e., laser depletion), and the effect of finite output pulse duration.

Emission of microwave and millimeter wavelength radiation during hollow cathode discharge operation of the back lighted thyratron

Experimental observation of pulsed radiation ranging from ∼20 GHz to above 100 GHz during the hollow cathode discharge phase of operation of a back‐lighted thyratron is reported. The discharge is operated with 120 mTorr Ar gas at 20 kV initial voltage. Pulsed radiation was observed for ∼50 ns, and an electron beam with energy of ∼20 keV was also observed. The observations are correlated with plasma processes predicted in recent computer simulations. The sudden turn‐off of the radiation is believed to be a result of plasma expansion and sheath contraction inside the hollow cathode region. A method for varying the pulse length is discussed.

An optically triggered, glow switch Marx bank

Operation of high-powers lasers, electron and other accelerators, and other devices is often limited by power modulator performance. Results of an investigation of a concept for modulator switching that has the potential to significantly extend these limitations presented. The switching device is a three-stage Marx bank based on back-lighted thyratrons that are triggered using optical fibers. Potential advantages include optical isolation of triggering, operation of the switches in a glow mode, no external cathode heater, reduced volume and supporting systems, and high-precision (subnanosecond) triggering. A three-stage proof-of-principle Marx bank has been operated at 105 kV with a rise time of >

Electron beam generation from a superemissive cathode

An experimental study of electron beams produced by a superemissive cathode in the Back-Lighted Thyratron (BLT) and the pseudospark is presented. This work is motivated by experiments demonstrating very high current densities (>or=10 kA/cm/sup 2/ over an area of 1 cm/sup 2/) from the pseudospark and BTL cathode. This high-density current is produced by field-enhanced thermionic emission from the ion beam-heated surface of a molybdenum cathode. This work reports the use of this cathode as a beam source, and is to be distinguished from previous work reporting hollow cathode-produced electron beams. An electron beam of more than 260 A peak current has been produced with 15 kV applied voltage. An efficiency of approximately=10% is estimated.<<ETX>>

Plasma-lens experiments at the Final Focus Test Beam

We intend to carry out a series of plasma lens experiments at the Final Focus Test Beam facility at SLAC. These experiments will be the first to study the focusing of particle beams by plasma focusing devices in the parameter regime of interest for high energy colliders, and is expected to lead to plasma lens designs capable of unprecedented spot sizes. Plasma focusing of positron beams will be attempted for the first time. We will study the effects of lens aberrations due to various lens imperfections. Several approaches will be applied to create the plasma required including laser ionization and beam ionization of a working gas. At an increased bunch population of 2.5/spl times/10/sup 10/, tunneling ionization of a gas target by an electron beam-an effect which has never been observed before-should be significant. The compactness of our device should prove to be of interest for applications at the SLC and the next generation linear colliders.<<ETX>>

Recent experimental studies of the blt switch

Recent results of experimental studies of the Back Lighted Thyratron BLT switch are reported. This optically triggered low pressure thyratron type closing switch has operated at voltages over 3OkV, currents over 80kA and can be triggered with subnanosecond jitter. Also reported here is a method for laser cleaning BLT windows.