R. Narang

Generation of 160-ps terawatt-power CO2 laser pulses

We have developed a three-stage CO(2) master-oscillator-amplifier system that produces 1.1 TW of peak power. The system generates 170 J of energy in a diffraction-limited 160+/-10ps pulse on the 10P(20) line. We also report the realization of a two-wavelength terawatt-peak-power CO(2) laser that can be tuned to an arbitrary pair of lines. A two-stage semiconductor switching system driven by a picosecond-pulse Nd:YAG laser was used to slice a short, low-power 10.6-mum pulse for amplification. A simple plasma shutter helped to compensate for gain narrowing in a final three-pass amplifier and to shorten the pulse.

Experiments on laser driven beatwave acceleration in a ponderomotively formed plasma channel

A 10 ps long beam of 12 MeV electrons is externally injected into a ∼3-cm long plasma beatwave excited in a laser ionized hydrogen gas. The electrons have been accelerated to 50 MeV with a gradient of ∼1.3 GeV/m. It is shown that when the effective plasma wave amplitude-length product is limited by ionization-induced defocusing (IID), acceleration of electrons is significantly enhanced by using a laser pulse with a duration longer than the time required for ions to move across the laser spot size. Both experiments and two-dimensional simulations reveal that, in this case, self-guiding of the laser pulse in a ponderomotively formed plasma channel occurs. This compensates for IID and drives the beatwave over the longer length compared to when such a channel is not present.

Optical Kerr switching technique for the production of a picosecond, multiwavelength CO 2 laser pulse

A wavelength-independent method for optical gating, based on the optical Kerr effect, has been demonstrated. Using this method, we produced 100-ps, 10-kW, two-wavelength pulses (10.3 and 10.6 microm) with a signal-to-background ratio contrast of 10(5) by slicing a long CO2 pulse. The capability of gating consecutive pulses separated on a picosecond time scale with this method is also shown.

Efficient shortening of self-chirped picosecond pulses in a high-power CO 2 amplifier

We report a factor-of-6 shortening of the 240-ps (FWHM) pulses in a triple-pass, 2.5-atm CO(2) amplifier. This technique is based on the self-phase modulation of a 10-mum pulse in a plasma after the first pass of amplification, followed by narrowing of this chirped pulse during further amplification. Subsequently, strong power broadening provides the necessary bandwidth to amplify 40-ps pulses to terawatt power levels.

Collinear Thomson scattering diagnostic system for the detection of relativistic waves in low-density plasmas

A Thomson scattering technique is used to study relativistic plasma waves at plasma densities as low as 2×1015 cm−3. A spatial-spectral filter is utilized to simultaneously attenuate the probe light ∼109 times and collect the weak scattered light which is shifted 3–24 A from the probe wavelength. The plasma waves, excited by a two-wavelength CO2 laser pulse with intensities up to 1015 W/cm2, are probed by a 532 nm laser pulse in a collinear geometry. Both the red- and blueshifted sidebands of Thomson scattered light are simultaneously resolved in time and frequency.

Plasma source test and simulation results for the underdense plasma lens experiment at the UCLA Neptune Laboratory

The planned plasma lens experiment at the UCLA Neptune Laboratory is described. In the experiment, electron beams with an energy of 16 MeV, a charge of 4 nC, and a pulse duration of 30 ps [full-width at half-maximum (FWHM)] are designed to be produced from the 1.625-cell photoinjector radio-frequency gun (f=2.856 GHz) and PWT linac in the Neptune. The generated beams are passed through a thin plasma with a density of low 10/sup 12/ cm/sup -3/ range and a thickness of a few centimeters. For this experiment, a LaB/sub 6/-based discharge plasma source was developed and tested. In this paper, the overview of the planned plasma lens experiment and the test results of the plasma source for various conditions are presented. In addition, computer simulations with a 2-1/2 dimensional particle-in-cell code (MAGIC) were performed and the simulation results are shown.

Underdense plasma lens experiment at the UCLA Neptune Laboratory

An underdense plasma-lens experiment is planned at the UCLA Neptune Laboratory. For this experiment, a LaB/sub 6/-based discharge plasma source was developed and tested. Test results of the plasma source show that it can provide satisfactory Ar plasma parameters for underdense plasma lens experiments, i.e., a density in the low 10/sup 12/ cm/sup -3/ range and a thickness of a few cm. In the plasma chamber a YAG slab and a Cherenkov radiator are placed for electron beam diagnostics so that both time-integrated and time-resolved information will be obtained and compared with the MAGIC code (2 and 1/2 dimensional particle-in-cell) simulations. In this paper, the planned experiment including test results of the plasma source, diagnostics and MAGIC simulation results is presented.

Interpretation of Resonant and Non‐Resonant Beat‐Wave Excitation: Experiments and Simulations

Beat‐wave excited relativistic plasma waves (RPWs) are investigated using collinear Thomson scattering. The RPWs are driven by a two‐wavelength CO2 laser pulse with a peak power of ∼1TW which also produces plasmas through tunneling ionization of He, Ar and H2. The beat frequency of the CO2 pulse is kept constant while the plasma density is changed from 1015cm−3 to 1017cm−3. RPWs are thus excited both at resonance, around 1016cm−3, and off‐resonance. The plasma waves are sampled with a 532‐nm probe pulse. The scattered light, comprised of red and blue shifted sidebands, is frequency and time resolved with a spectrometer and a streak camera. The amplitude of the RPWs is found from the amount of light scattered into the sidebands. We compare the experimental results with 2‐D PIC simulations. Furthermore, simulations are used to study and compare the RPWs excited in resonant and non‐resonant conditions.

Test results of the plasma source for underdense plasma lens experiments at the UCLA Neptune Lab

A plasma source was developed at UCLA for planned underdense plasma lens experiments, where the plasma density is less than the electron beam density. The argon plasma, produced by a discharge between a LaB6 cathode at 1330 °C and a tantalum anode, is confined by a solenoidal magnetic field and flows transversely across the electron beam path. Extensive test of the plasma source is under way for various conditions before it is assembled with the UCLA photocathode-based electron linac. In particular, different longitudinal (with respect to the electron beam) plasma profiles and effective plasma lengths can be obtained by adjusting the moveable sliding door between the plasma source and the transverse beamline. Test results of the plasma source are presented.

PIC Simulations of Plasma Beat‐Wave Acceleration Experiments at UCLA

The plasma beat‐wave accelerator (PBWA) in the Neptune Laboratory at UCLA utilizes a ∼1 terawatt two‐wavelength laser pulse to tunnel ionize hydrogen gas at conditions of resonance for driving relativistic plasma waves. This plasma wave is used as an accelerating structure for an externally injected ∼11 MeV electron beam from the Neptune Photo‐injector. Simulations in 2‐D have been done to model this experiment for laser ionized plasmas with mobile ions for two focusing geometries, f/3 and f/18. Simulations have shown that ion motions in the transverse direction for small spot size cases (f/3 case) cannot be neglected, and that the acceleration of electrons is therefore limited by shortening of the effective interaction length due to deviations from the resonant density. In the f/18 case, while ion motions are not as severe as in the f/3 case, ionization induced refraction begins to limit the peak intensity of the laser. In addition, injection of the electron beam into the plasma wave is modeled to determ...