J.R. Hoffman

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 1012u2009cm−3 range and a capacitor period 2d=9.4u2009cm. In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with 2d=2u2009cm. 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...

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.

Lithium plasma sources for acceleration and focusing of ultra-relativistic electron beams

A lithium (Li) plasma source is described that can be used for plasma acceleration as well as for electron beam focusing (plasma lens). The Li vapor with a density in the range is produced in a heat-pipe oven. The Li is ionized to Li/sup +/ through single UV photon absorption. For the plasma wake field acceleration experiment, the plasma density n/sub pe//spl ap/2-4/spl times/10/sup 14/ cm/sup -3/ is 1-meter long plasma. A shorter length of uniform plasma density can be used as an underdense plasma lens, or the plasma density can be tapered for an underdense tapered plasma lens.

On the possibility of a 10/sup 16/ per cc density, 1 meter long plasma for accelerator applications

Uniform, high density plasmas on the order of a few meters in length are needed for the next generation of plasma based particle accelerator boosters. Such plasmas are currently beyond the state of the art. In this poster, we present the results of a feasibility study for producing a one meter long, uniform plasma with a density of 10 16 cm -3

First experimental measurement of the free streaming plasma mode

Summary form only given, as follows. The free streaming mode is magneto-static (w=0) plasma eigenmode sustained by steady currents flowing in the plasma in alternate directions. For this reason it has also been referred to as the picket fence mode. The existence of this mode has been theorized for more than 30 years, but has never been observed experimentally. This mode is excited in the DARC source where a static electric field alternating with a spatial wavenumber. k/sub 0/ is transmitted into a plasma through a relativistic ionization front. We present the first experimental measurement of the free streaming mode. This was accomplished by placing an array of microstrip coils, (whose impedance is matched to a 50 ohm load from zero to 1 GHz), inside of the proof of principle DC to AC radiation convertor (DARC source). With this arrangement the phasing of the interaction region and the microstrip coil array is held constant, thus allowing measuring each coil in the array independently, instead of simultaneously. The wavenumber of the free streaming mode, variations of the magnetic field amplitude with the incident electric field strength and the plasma density were measured, and are consistent with theoretical predictions. The implications of the existence of the free streaming mode is the validation of the assumption that the electrons born via laser ionization are essentially at rest, or at most, born with an isotropic velocity distribution.

Experiments on new plasma concepts for enhanced microwave vacuum electronics

Summary form only given. Recently new schemes have been proposed for plasma based microwave sources that could lead to output power increases by orders of magnitude, as well as offer new possibilities such as an broad band tuning and frequency chirping, ultra-short pulse generation, pulse design, etc. In the first scheme, the static field of an alternatively biased capacitor is directly converted into short pulses of tunable electromagnetic (em) radiation upon transmission through a relativistic, underdense ionization front. The structure presently under investigation consist of pin pairs (capacitors) inserted into an X-band waveguide through its narrow side wall and separated by 1.134 cm. In the second scheme, a fraction of the large amplitude electrostatic (es) wave generated in a plasma beat wave acceleration (PBWA) experiment (up to 3 GeV/m) is converted into em radiation by applying a static magnetic field perpendicularly to the driving laser beam. The two-frequency CO/sub 2/ laser beam resonantly drives the es wave, and couples to the L branch of the XO mode of the magnetized plasma through Cherenkov radiation. The radiation is emitted predominantly in the forward direction (direction of the laser beam), and is at the plasma frequency (n/sub e/=10/sup 16/ cm/sup -3/, f/spl ap/1 THz).

Photon acceleration-based radiation sources

The acceleration and deceleration of photons in a plasma provides the means for a series of new radiation sources. Previous work on a DC to AC Radiation Converter (DARC source) has shown variable acceleration of photons having zero frequency (i.e., an electrostatic field) to between 6 and 100 GHz (1–3). These sources all had poor guiding characteristics resulting in poor power coupling from the source to the load. Continuing research has identified a novel way to integrate the DARC source into a waveguide. The so called “pin structure” uses stainless steel pins inserted through the narrow side of an X band waveguide to form the electrostatic field pattern (k≠0, ω=0). The pins are spaced such that the absorption band resulting from this additional periodic structure is outside of the X band range (8–12 GHz), in which the normal waveguide characteristics are left unchanged. The power of this X band source is predicted theoretically to scale quadratically with the pin bias voltage as −800 W/(kV)2 and have a ...

Simulation and experimental results of the excitation of the free streaming mode in the plasma wake created by a moving ionization front

Summary form only given, as follows. The excitation of a static (w=0) magnetic field mode in a plasma has been theorized for close to 30 years, but has never been observed experimentally. This mode has also been referred to alternatively as the picket fence or free streaming mode, reflecting the fact that it is supported by steady state current loops with alternating sense of direction (k*0) in the plasma. We present PIC simulations and preliminary experimental results of work being done at USC and UCLA to measure the free streaming mode when a moving gas/plasma boundary (e.g., created by a short laser pulse) moves either through a static electric field (e.g., an electric wiggler) or an incident electromagnetic wave. Comparisons of the experimental and simulations results with analytical results are also presented.

Novel DARC sources

Summary form only given, as follows. In the DC to AC radiation converter (DARC source), a relativistic, underdense ionization front (plasma/vacuum boundary) directly converts the static electric field of an array of alternatively biased capacitors into a short pulse of tunable radiation. In previous experiments, frequencies continuously tunable from 6 to 21 GHz, and 39 to 93 GHz were produced with devices consisting of capacitor plates. In order to increase the power of the pulses produced by the DARC source, a new device in the microwave frequency range has been designed in which pins inserted in the X-band waveguide through its narrow side wall replace the capacitor of the previous devices. Cold tests show that between the stopbands created by the periodic pins the insertion loss of the structure is much smaller (<0.2 dB) than that measured with previous structures (>30 dB). The stopbands appear at frequencies given by /spl omega//sub sb,l/=(/spl omega//sup 2//sub c/+(l/sup 1//d)/sup 2/c/sup 2/)/sup 1/2/ l=1, 2, ... where /spl omega//sub c/=((m/sup 1//a)/sup 2/+(n/sup 1//b)/sup 2/)/sup 1/2/ is the cut-off frequency of the TE/sub m,n/ mode in a waveguide of transverse dimension a and b, and d is the distance between the pins along the waveguide axis. The stored electrostatic energy in the pin device is only a factor of 3 smaller than that of the plate case. Frequencies in the X-band and above (/spl omega/=(k/sub 0//sup 2/c/sup 2/+/spl omega//sub c//sup 2/+/spl omega//sub pe//sup 2/)/sup 1/2//2k/sub 0/c, k/sub 0/=1/d) can be generated with plasma densities in the 10/sup 11/ cm/sup -3/ range. We plan to operate a DARC source in the therahertz frequency range by using a capacitor array with d=300 /spl mu/m, and plasma densities in the 10/sup 17/ cm/sup -3/ range. Nitrogen or xenon gas will be used and will be field-ionized by a 150 fs laser pulse.