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Amit Lal

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Centre National de la Recherche Scientifique / French National Centre for Scientific Research

Ucla school of engineering

Lawrence Berkeley National Laboratory

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Papers by Amit Lal

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

Proceedings of International Conference on Particle Accelerators

We describe the results from recent experiments1 on the plasma beat wave accelerator (PBWA) schem... more We describe the results from recent experiments1 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 CO2 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.

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

Review of Scientific Instruments, 1997

In laser-produced plasmas there are several effects which will scatter a longitudinally probing r... more 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) where j is the current density. We have successfully implemented this technique in the study of beat-excited laser plasmas.

Coupling between High-Frequency Plasma Waves in Laser-Plasma Interactions

Physical Review Letters, 1995

Experimental evidence for the coupling between two electron plasma waves having nearly the same f... more Experimental evidence for the coupling between two electron plasma waves having nearly the same frequency but greatly differing in wave number is presented using time and wave-number resolved spectra of Thomson scattered light from the plasma. The qualitative features of the measured w͑t, k͒ spectra are predicted by a Lagrangian fluid description and reproduced in particle simulations. These show that the daughter waves generated in this mode coupling process take the energy preferentially from the large k wave without significantly affecting the small k plasma wave.

Measurements of the beatwave dynamics in time and space

Proceedings Particle Accelerator Conference

We report on continuing experiments on the Plasma Beat Wave Accelerator, which uses an intense tw... more We report on continuing experiments on the Plasma Beat Wave Accelerator, which uses an intense twofrequency CO 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 [1]. Also, collective scattering of a probe laser beam has allowed us to measure ñ(ω,k) and ñ(ω 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 ñ(z,t), where z is the coordinate along the CO 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.

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

Resonant four-wave mixing of laser radiation in plasmas

Journal of the Optical Society of America B, 1991

Experimental evidence of resonant four-wave mixing of CO 2 laser radiation in a plasma is present... more Experimental evidence of resonant four-wave mixing of CO 2 laser radiation in a plasma is presented for the first time to our knowledge. Comparison of the experiment with theory indicates that, while collisions lead to a narrow spectral width of the ion acoustic resonance, convection and detuning owing to laser heating limit the enhancement of the signal reflectivity to below the expected value.

Transient Filamentation of a Laser Beam in a Thermal Force Dominated Plasma

Physical Review Letters, 1997

Experimental evidence for the transient phase of the filamentation instability of a laser beam in... more Experimental evidence for the transient phase of the filamentation instability of a laser beam in a thermal force dominated plasma is presented. When the laser beam is crossed with a weaker degenerate probe beam at a small angle, the interference of the two beams drives a thermally enhanced ion grating which during its transient phase acts to seed the filamentation/forward

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

Physics of Plasmas, 1994

The results from experiments in which a two-frequency COZ laser is used to beat-excite large-ampl... more The results from experiments in which a two-frequency COZ 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-l 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 the disparate time scales between the Thomson scattering and the electron scattering diagnostic. The whole beam deflection may be evidence for a Weibel-like instability at later times.

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 elec... more 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

Trapped electron acceleration by a laser-driven relativistic plasma wave

Nature, Apr 7, 1994

Ultrahigh-gradient acceleration of injected electrons by laser-excited relativistic electron plasma waves

Phys Rev Lett, 1993

A novel technique for probing the transverse interactions between an electron beam and a plasma

Proceedings Particle Accelerator Conference, 1995

In laser-plasma interaction experiments there are a variety of effects which will scatter or defl... more In laser-plasma interaction experiments there are a variety of effects which will scatter or deflect a probing electron beam. For example, a longitudinally probing electron beam is defocused by the ponderomotive force of a laser, but in a plasma the laser can set up fields which actually confine the electron beam. In the cases of plasma wave excitation via the beatwave or wakefield mechanisms, the thermalization of the electron distribution function can lead to large scale magnetic fields via the Weibel instability. A novel Cherenkov probe has been built and tested which measures the transverse current distribution of an electron beam with at least 50 ps time resolution. A solenoid lens is used to map angular deflections within the interaction region into x-y space at the location of the Cherenkov probe. This allows the cases mentioned above to be experimentally studied.

A Novel Technique for Probing theTransverse Interactions Between an Electron Beam and a Plasma

Spatial and temporal dynamics of relativistic plasma beat waves

<title>Experimental demonstration of laser acceleration of electrons via relativistic plasma waves</title>

Short-Pulse High-Intensity Lasers and Applications II, 1993

A two-frequency CO laser beam was used to beat-excite a large amplitude electron plasma wave in a... more A two-frequency CO 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 cutoff of the detection system. The corresponding average acceleration gradient is >0.7 GeV/m and the average wave amplitude n I n is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15-30% may have been achieved.

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

Review of Scientific Instruments, 1997

In the UCLA plasma beat wave accelerator, a high intensity two frequency CO 2 laser ͑ 1 ϭ10.6 m, ... more In the UCLA plasma beat wave accelerator, a high intensity two frequency CO 2 laser ͑ 1 ϭ10.6 m, 2 ϭ10.3 m͒ 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 ͑ 1 Ϫ p , 2 ϩ p ͒. 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 (n 1 /n 0 ϫL) of the wave. Since the Stokes and anti-Stokes signals are picosecond pulses at 10.9 and 10.0 m, 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 m light with visible light from a laser diode ͑670 nm͒ in a nonlinear crystal ͑AgGaS 2 ͒ 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 time resolved on a streak camera. Experimental results for the plasma wave amplitude, spatial length, and temporal length are shown.

Coupling between electron plasma waves in laser–plasma interactions

Physics of Plasmas, 1996

A Lagrangian fluid model ͑cold plasma, fixed ions͒ is developed for analyzing the coupling betwee... more A Lagrangian fluid model ͑cold plasma, fixed ions͒ is developed for analyzing the coupling between electron plasma waves. This model shows that a small wave number electron plasma wave (2 ,k 2) will strongly affect a large wave number electron plasma wave (1 ,k 1), transferring its energy into daughter waves or sidebands at (1 ϩn 2 ,k 1 ϩnk 2) 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 (1 ,k 1) can be completely transferred to the sidebands at (1 ϩn 2 ,k 1 ϩnk 2) by the presence of the electron plasma mode at (2 ,k 2). 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 CO 2 laser with a plasma.

Evolution of Stimulated Raman into Stimulated Compton Scattering of Laser Light via Wave Breaking of Plasma Waves

Physical Review Letters, 1995

Experimental evidence for the evolution of stimulated Raman backscatter instability of laser ligh... more Experimental evidence for the evolution of stimulated Raman backscatter instability of laser light into stimulated Compton scattering in an initially cold plasma is presented using time and wave number resolved spectra, w\(t\) and w\(k\), of Thomson scattered light from plasma fluctuations. Supporting particle simulations show that the key to the spectral evolution is wave breaking of Raman plasmons, which accelerates

Laser-Diode-Based Joint Transform Correlator for Fingerprint Identification

Optical Engineering, 1999

A laser-diode-based joint transform correlator (JTC) is reported here for the identification and ... more

$Research paper thumbnail of <title>Diffractively corrected Risley prism for infrared imaging</title>$

<title>Diffractively corrected Risley prism for infrared imaging</title>

by Robert Nichols and Amit Lal

Acquisition, Tracking, and Pointing XIV, 2000

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

Proceedings of International Conference on Particle Accelerators

We describe the results from recent experiments1 on the plasma beat wave accelerator (PBWA) schem... more We describe the results from recent experiments1 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 CO2 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.

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

Review of Scientific Instruments, 1997

In laser-produced plasmas there are several effects which will scatter a longitudinally probing r... more 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) where j is the current density. We have successfully implemented this technique in the study of beat-excited laser plasmas.

Coupling between High-Frequency Plasma Waves in Laser-Plasma Interactions

Physical Review Letters, 1995

Experimental evidence for the coupling between two electron plasma waves having nearly the same f... more Experimental evidence for the coupling between two electron plasma waves having nearly the same frequency but greatly differing in wave number is presented using time and wave-number resolved spectra of Thomson scattered light from the plasma. The qualitative features of the measured w͑t, k͒ spectra are predicted by a Lagrangian fluid description and reproduced in particle simulations. These show that the daughter waves generated in this mode coupling process take the energy preferentially from the large k wave without significantly affecting the small k plasma wave.

Measurements of the beatwave dynamics in time and space

Proceedings Particle Accelerator Conference

We report on continuing experiments on the Plasma Beat Wave Accelerator, which uses an intense tw... more We report on continuing experiments on the Plasma Beat Wave Accelerator, which uses an intense twofrequency CO 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 [1]. Also, collective scattering of a probe laser beam has allowed us to measure ñ(ω,k) and ñ(ω 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 ñ(z,t), where z is the coordinate along the CO 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.

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

Resonant four-wave mixing of laser radiation in plasmas

Journal of the Optical Society of America B, 1991

Experimental evidence of resonant four-wave mixing of CO 2 laser radiation in a plasma is present... more Experimental evidence of resonant four-wave mixing of CO 2 laser radiation in a plasma is presented for the first time to our knowledge. Comparison of the experiment with theory indicates that, while collisions lead to a narrow spectral width of the ion acoustic resonance, convection and detuning owing to laser heating limit the enhancement of the signal reflectivity to below the expected value.

Transient Filamentation of a Laser Beam in a Thermal Force Dominated Plasma

Physical Review Letters, 1997

Experimental evidence for the transient phase of the filamentation instability of a laser beam in... more Experimental evidence for the transient phase of the filamentation instability of a laser beam in a thermal force dominated plasma is presented. When the laser beam is crossed with a weaker degenerate probe beam at a small angle, the interference of the two beams drives a thermally enhanced ion grating which during its transient phase acts to seed the filamentation/forward

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

Physics of Plasmas, 1994

The results from experiments in which a two-frequency COZ laser is used to beat-excite large-ampl... more The results from experiments in which a two-frequency COZ 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-l 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 the disparate time scales between the Thomson scattering and the electron scattering diagnostic. The whole beam deflection may be evidence for a Weibel-like instability at later times.

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 elec... more 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

Trapped electron acceleration by a laser-driven relativistic plasma wave

Nature, Apr 7, 1994

Ultrahigh-gradient acceleration of injected electrons by laser-excited relativistic electron plasma waves

Phys Rev Lett, 1993

A novel technique for probing the transverse interactions between an electron beam and a plasma

Proceedings Particle Accelerator Conference, 1995

In laser-plasma interaction experiments there are a variety of effects which will scatter or defl... more In laser-plasma interaction experiments there are a variety of effects which will scatter or deflect a probing electron beam. For example, a longitudinally probing electron beam is defocused by the ponderomotive force of a laser, but in a plasma the laser can set up fields which actually confine the electron beam. In the cases of plasma wave excitation via the beatwave or wakefield mechanisms, the thermalization of the electron distribution function can lead to large scale magnetic fields via the Weibel instability. A novel Cherenkov probe has been built and tested which measures the transverse current distribution of an electron beam with at least 50 ps time resolution. A solenoid lens is used to map angular deflections within the interaction region into x-y space at the location of the Cherenkov probe. This allows the cases mentioned above to be experimentally studied.

A Novel Technique for Probing theTransverse Interactions Between an Electron Beam and a Plasma

Spatial and temporal dynamics of relativistic plasma beat waves

<title>Experimental demonstration of laser acceleration of electrons via relativistic plasma waves</title>

Short-Pulse High-Intensity Lasers and Applications II, 1993

A two-frequency CO laser beam was used to beat-excite a large amplitude electron plasma wave in a... more A two-frequency CO 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 cutoff of the detection system. The corresponding average acceleration gradient is >0.7 GeV/m and the average wave amplitude n I n is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15-30% may have been achieved.

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

Review of Scientific Instruments, 1997

In the UCLA plasma beat wave accelerator, a high intensity two frequency CO 2 laser ͑ 1 ϭ10.6 m, ... more In the UCLA plasma beat wave accelerator, a high intensity two frequency CO 2 laser ͑ 1 ϭ10.6 m, 2 ϭ10.3 m͒ 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 ͑ 1 Ϫ p , 2 ϩ p ͒. 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 (n 1 /n 0 ϫL) of the wave. Since the Stokes and anti-Stokes signals are picosecond pulses at 10.9 and 10.0 m, 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 m light with visible light from a laser diode ͑670 nm͒ in a nonlinear crystal ͑AgGaS 2 ͒ 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 time resolved on a streak camera. Experimental results for the plasma wave amplitude, spatial length, and temporal length are shown.

Coupling between electron plasma waves in laser–plasma interactions

Physics of Plasmas, 1996

A Lagrangian fluid model ͑cold plasma, fixed ions͒ is developed for analyzing the coupling betwee... more A Lagrangian fluid model ͑cold plasma, fixed ions͒ is developed for analyzing the coupling between electron plasma waves. This model shows that a small wave number electron plasma wave (2 ,k 2) will strongly affect a large wave number electron plasma wave (1 ,k 1), transferring its energy into daughter waves or sidebands at (1 ϩn 2 ,k 1 ϩnk 2) 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 (1 ,k 1) can be completely transferred to the sidebands at (1 ϩn 2 ,k 1 ϩnk 2) by the presence of the electron plasma mode at (2 ,k 2). 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 CO 2 laser with a plasma.

Evolution of Stimulated Raman into Stimulated Compton Scattering of Laser Light via Wave Breaking of Plasma Waves

Physical Review Letters, 1995

Experimental evidence for the evolution of stimulated Raman backscatter instability of laser ligh... more Experimental evidence for the evolution of stimulated Raman backscatter instability of laser light into stimulated Compton scattering in an initially cold plasma is presented using time and wave number resolved spectra, w\(t\) and w\(k\), of Thomson scattered light from plasma fluctuations. Supporting particle simulations show that the key to the spectral evolution is wave breaking of Raman plasmons, which accelerates

Laser-Diode-Based Joint Transform Correlator for Fingerprint Identification

Optical Engineering, 1999

A laser-diode-based joint transform correlator (JTC) is reported here for the identification and ... more

$Research paper thumbnail of <title>Diffractively corrected Risley prism for infrared imaging</title>$

<title>Diffractively corrected Risley prism for infrared imaging</title>

by Robert Nichols and Amit Lal

Acquisition, Tracking, and Pointing XIV, 2000