A. Fisher

Tunable, short pulse hard x‐rays from a compact laser synchrotron source

A compact laser synchrotron source (LSS) is proposed as a means of generating tunable, narrow bandwidth, ultra‐short pulses of hard x rays. The LSS is based on the Thomson backscattering of intense laser radiation from a counterstreaming electron beam. Advances in both compact ultra‐intense solid‐state lasers and high brightness electron accelerators make the LSS an attractive compact source of high brightness pulsed x rays, particularly at photon energies beyond ∼30 keV. The x‐ray wavelength is λ[A]=650 λ0[μm]/Eb2[MeV], where λ0 is the laser wavelength and Eb is the electron beam energy. For Eb=72 MeV and λ0=1 μm, x rays at λ=0.12 A (100 keV) are generated. The spectral flux, brightness, bandwidth, and pulse structure are analyzed. In the absence of filtering, the spectral bandwidth in the LSS is typically ≲1% and is limited by electron beam emittance and energy spread. Two configurations of the LSS are discussed, one providing high peak power and the other moderate average power x rays. Using present da...

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.

BACKSCATTERED SUPERCONTINUUM EMISSION FROM HIGH-INTENSITY LASER-PLASMA INTERACTIONS

We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0) > 1) extending from ~500 to >1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.

Observation of 20 eV x‐ray generation in a proof‐of‐principle laser synchrotron source experiment

A laser synchrotron source (LSS) [P. Sprangle, A. Ting, E. Esarey, and A. Fisher, J. Appl. Phys. 72, 5032 (1992)] was proposed to generate short‐pulsed, tunable x rays by Thomson scattering of laser photons from a relativistic electron beam. A proof‐of‐principal (p.o.p.) experiment on this LSS configuration is performed. An intense laser pulse (λ0=1.053 μm) is Thomson backscattered from a focused relativistic electron beam. Time integrated x‐ray signals from a photocathode/electron multiplier, at an electron beam energy of 650 keV and an x‐ray photon energy of 20 eV, indicate an increase in the x‐ray signals above the baseline by an amount comparable to the theoretically predicted value.

Measurements of gas preionization for plasma radiation sources

Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh–Taylor instabilities or other nonuniformities affecting the final implosion stage of the pinch, leading to an improvement in K-shell x-ray yield. We report on measurements of a flashover UV photoionization scheme, capable of ionizing the periphery of argon gas puffs to 1%–10% of the initial gas density. Measurements are made with a two-color, (1064 and 532 nm), high sensitivity (∼10−5λ), Mach–Zehnder type interferometer. Two methods of measuring preionization are investigated. The first uses a single laser wavelength, 1064 nm, to probe a chord of the cylindrical gas puff. The gas density is measured first without preionization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, de...

Demonstration experiment of a laser synchrotron source for tunable, monochromatic X-rays at 500 eV

A demonstration experiment of the laser synchrotron source (LSS) is being planned and constructed to generate short-pulsed, tunable X-rays in the range of ∼500 eV by Thomson scattering of laser photons from a relativistic electron beam. Laser photons of λ = 1.06 μm are Thomson backscattered by a 4.5 MeV electron beam from an S-band RF electron gun. The laser photons are derived from a 15 J, 3 ns Nd:glass laser. The RF electron gun is being constructed for initial operation using a thermionic cathode. It will be upgraded with a photocathode to produce high quality electron beams with high current and low emittance. The X-ray pulse structure consists of ∼10 ps long micropulses within a laser pulse width dependent macropulse. The estimated X-ray photon flux is ∼1018 photons/s, and the number of photons per macropulse is ∼108.

Measurement of gas distributions from PRS nozzles

A high-sensitivity laser interferometer has been used to measure gas distributions from nozzles used in high-power plasma radiation source experiments. These measurements are important for determining experimental parameters and for modeling implosions. The integral of the gas density along the laser beam line of sight is measured as a function of time at one axial distance, z, and one radial displacement, r. The nozzle is moved to scan the (r,z) cross section. The measurements are Abel-inverted to compute the local density n(r,z,t). Several examples are shown to illustrate the technique.

Long pulse electron beams produced from carbon fiber cathodes

Carbon fiber cathodes have been used to produce electron beams from 10s of nanoseconds to 10s of microseconds. Cathode areas can vary from a single fiber (/spl sim/10 micron diameter) to many hundreds of square cm. The micron size diameter of the fiber is responsible for the high electric field produced at the tips which allows operation of these cathodes at average fields as low as 10 kV/cm. During operation the emitting tips heat up and some plasma forms and erosion takes place. However, carbon has the highest sublimation energy per unit volume among all the elements. It does not melt and it sublimation temperature is high (4000 K). Therefore, no rounding of the tips take place (metals always melt and form a small sphere which turns off the emission). These properties allow producing cathodes with very million shot lifetimes at current densities of 10s to 1000s of A/cm/sup 2/. We describe our work on carbon fiber cathodes which range in operation from 6 kV to 500 kV anode-cathode voltage and up to a few microseconds pulse duration.

Tunable compact high power far-infrared grating free-electron laser

The linear and nonlinear operation of a multiwave grating free-electron laser is described. The radiation is generated by the passage of an annular electron beam through a coaxial waveguide, the central conductor being in the form of a corrugated cylinder. The model includes the effects of self-field forces, beam emittance and gyromotion of electrons in a guide magnetic field. The efficiency is determined by numerical simulation and compared with analytical estimates. A compact source utilizing, for example, a high brightness beam from a carbon fibre electron gun, that is tunable in the wavelength band 35-55 /spl mu/m with output in the kilowatt range, is shown to be feasible. >

A far-infrared grating free-electron laser

Abstract Design parameters for a compact radiation source operating in the infrared region of the spectrum are presented. The source is configured as a free-electron laser oscillator, utilizing an annular electron beam and a coaxial waveguide with a corrugated central conductor. A compact source that is tunable in the wavelength band 35–55 μm with output in the kilowatt range is shown to be feasible.