A. Lal

Acceleration and scattering of injected electrons in plasma beat wave accelerator experiments

The results from experiments in which a two‐frequency CO2 laser is used to beat‐excite large‐amplitude, relativistic electron plasma waves in a tunnel‐ionized plasma are reported. The plasma wave is diagnosed by injecting a beam of 2 MeV electrons and observing the energy gain and loss of these electrons, as well as the scattering and deflection of the transmitted electrons near 2 MeV. Accelerated electrons up to 30 MeV have been observed. The lifetime of the accelerating structure as seen by small‐angle Thomson scattering is about 100 ps, whereas the injected electrons are seen to be scattered or deflected by the plasma for several ns, with diffuse scattering occurring 0.5–1 ns after forming the plasma wave and whole beam deflection occurring at later times. A simple model, which includes laser focusing, ionization, transit time, and relativistic saturation effects, suggests that the wave coherence may be short lived while the wave fields themselves persist for a longer time. This may be the reason for t...

Spatio-temporal dynamics of the resonantly excited relativistic plasma wave driven by a CO2 laser

The dynamics of a relativistic plasma wave (RPW) resonantly excited by a two frequency CO2 laser pulse and the effects of this wave on a co-propagating relativistic electron beam were studied through experiments and supporting simulations. The amplitude of the RPW and its harmonics were resolved in time and space with a Thomson scattering diagnostic. In addition, the plasma wave amplitude-length product and temporal duration were independently measured through time and frequency resolved forward scattering. The transverse electric and magnetic fields associated with the RPW were studied by the scattering of a 2 MeV electron beam, and the eventual heating of the plasma after the breakup of the RPW was measured from the x-ray radiation spectrum. The experiments and simulations show that the RPW reaches a peak amplitude of approximately 30%, with the amplitude limited by plasma blowout driven by the radial ponderomotive forces of the plasma wave.

Acceleration of injected electrons by the plasma beat wave accelerator

In this paper we describe the recent work at UCLA on the acceleration of externally injected electrons by a relativistic plasma wave. A two frequency laser was used to excite a plasma wave over a narrow range of static gas pressures close to resonance. Electrons with energies up to our detection limit of 9.1 MeV were observed when 2.1 MeV electrons were injected in the plasma wave. No accelerated electrons above the detection threshold were observed when the laser was operated on a single frequency or when no electrons were injected. Experimental results are compared with theoretical predictions, and future prospects for the plasma beat wave accelerator are discussed.

Demonstration of plasma beat wave acceleration of electrons from 2 MeV to 20 MeV

We describe the results from recent experimental on the plasma beat wave accelerator (PBWA) scheme at UCLA. A relativistic electron plasma wave (which is the accelerating structure) is resonantly excited in a plasma by the beating of two co-propagating electromagnetic waves (obtained from a CO/sub 2/ laser operating simultaneously on two wavelengths). A 2 MeV, 200 mA (peak-current) electron beam, roughly 1 nsec (FWHM) in duration is used as a source of test particles for measuring the longitudinal fields of the plasma wave which itself is moving with a relativistic Lorentz factor of about 34. Accelerated electrons are energy-selected with an imaging sector magnet and detected simultaneously with a cloud chamber and surface barrier detectors. Initial experiments show that electrons are accelerated up to 20 MeV over roughly 1 cm (the uniform length of plasma) indicating an gradient of acceleration of more than 1.8 GeV/m.<<ETX>>

DEGENERATE AND RESONANT FOUR-WAVE MIXING IN PLASMAS

The status of degenerate and resonant four-wave mixing in plasmas is reviewed. For the degenerate case in a collisional plasma, the theory predicts and experiments demonstrate that the thermal-force contribution to the signal reflectivity dominates over the ponderomotive-force contribution. In the resonant case, the reflectivity can be enhanced over the degenerate level. Experiments show that collisions can lead to a narrow spectral width of the ion-acoustic resonance, but the effects of convection and laser heating can limit the enhancement of the reflectivity below the expected value.

Exact forward scattering of a CO2 laser beam from a relativistic plasma wave by time resolved frequency mixing in AgGaS2

In the UCLA plasma beat wave accelerator, a high intensity two frequency CO2 laser (λ1=10.6 μm, λ2=10.3 μm) is used to drive a large amplitude relativistic plasma wave. The plasma wave acts as a moving phase grating and scatters the incident pump waves into Stokes and anti-Stokes sidebands (ω1−ωp, ω2+ωp). The observation of these sidebands in the forward direction confirms the presence of the relativistic plasmon, and also gives an estimate of the amplitude–length product (n1/n0×L) of the wave. Since the Stokes and anti-Stokes signals are picosecond pulses at 10.9 and 10.0 μm, respectively, this light cannot be time resolved directly on a conventional detector or streak camera. The forward scattered light can be analyzed, however, by mixing the 10 μm light with visible light from a laser diode (670 nm) in a nonlinear crystal (AgGaS2) to produce frequency shifted light at 630 nm. The intensity of the 630 nm light is proportional to the product of the intensities of the two incident laser pulses, and can be...

Coupling between electron plasma waves in laser–plasma interactions

A Lagrangian fluid model (cold plasma, fixed ions) is developed for analyzing the coupling between electron plasma waves. This model shows that a small wave number electron plasma wave (ω2,k2) will strongly affect a large wave number electron plasma wave (ω1,k1), transferring its energy into daughter waves or sidebands at (ω1+nω2,k1+nk2) in the lab frame. The accuracy of the model is checked via particle‐in‐cell simulations, which confirm that the energy in the mode at (ω1,k1) can be completely transferred to the sidebands at (ω1+nω2,k1+nk2) by the presence of the electron plasma mode at (ω2,k2). Conclusive experimental evidence for the generation of daughter waves via this coupling is then presented using time‐ and wave number‐resolved spectra of the light from a probe laser coherently Thomson scattered by the electron plasma waves generated by the interaction of a two‐frequency CO2 laser with a plasma.

Measurements of the beatwave dynamics in time and space

We report on continuing experiments on the plasma beat wave accelerator, which uses an intense two-frequency CO/sub 2/ laser pulse to resonantly drive a large amplitude, relativistically-propagating electron plasma wave suitable for electron acceleration. Previously, energy gains of a factor 15 (from 2 MeV to 30 MeV) have been obtained. Also, collective scattering of a probe laser beam has allowed us to measure n/spl tilde/(/spl omega/,k) and n/spl tilde/(/spl omega//sub m/,t) where m=1,2 are the fundamental and harmonic of the large amplitude plasma wave. This powerful Thomson scattering technique has now been extended to measure n/spl tilde/(z,t), where z is the coordinate along the CO/sub 2/ propagation direction. Nonlinear dynamics such as relativistic detuning and ponderomotive effects can complicate the longitudinal amplitude profile and coherence. Experimental results show a plasma wave with a peak amplitude of approximately 35%, with a FWHM of 100 ps, and extending for 1 cm in space. These wave parameters are consistent with the observed energy gains of accelerated electrons.

Experimental Demonstration of Laser Acceleration of Electrons via Relativistic Plasma Waves

A two-frequency CO2 laser beam was used to beat-excite a large amplitude electron plasma wave in a resonant density plasma. The accelerating fields of the relativistic plasma wave were probed with collinear injected 2.1 MeV electrons from an electron linac. Some electrons gained at least 7 MeV in traversing the approximately 1 cm length of the beat wave accelerator, with the measurement limited by the 9.1 MeV high energy cut-off of the detection system. The corresponding average acceleration gradient is > 0.7 GeV/m and the average wave amplitude n1/n0 is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15 - 30% may have been achieved.

Two-dimensional Cherenkov emission array for studies of relativistic electron dynamics in a laser plasma

In laser-produced plasmas there are several effects which will scatter a longitudinally probing relativistic electron beam. In vacuum, the laser itself will ponderomotively defocus the electron beam, while in plasma the ponderomotive force can dig an ion channel which would focus the electron beam. In the cases of plasma wave excitation via the beat-wave or wake-field mechanisms, the thermalization of the electron distribution function can lead to large scale magnetic fields via the Weibel instability. One way of studying such phenomena is to time resolve the transverse current distribution of the electron beam after it exits the plasma. A wire mesh has insufficient time resolution for this purpose, so we instead use a mesh of optical fibers. When the electron beam strikes the fiber mesh, Cherenkov radiation is generated within whichever fibers have current running across them. The Cherenkov radiation from all the fibers can then be analyzed on a streak camera. This allows the reconstruction of j(x,y,t) w...