A two stage experiment on optically injected Laser Wakefield Acceleration (LWFA) was performed at... more A two stage experiment on optically injected Laser Wakefield Acceleration (LWFA) was performed at the Naval Research Laboratory. Two temporally and spatially synchronized laser beams at 2 TW and 10 TW respectively are each focused collinearly into two adjacent gas jets. Electrons at <0.5 MeV generated from the 2 TW laser beam are injected into the wakefields generated by the 10 TW laser beam. Accelerated electrons >20 MeV are observed, implying an acceleration gradient of 20 GeV/m for the ˜1 mm acceleration distance. Apparent peaks and structures are observed in the energy spectrum. This is a clear demonstration of substantial laser wakefield acceleration of externally and optically produced injection electrons. Details of the experiment will be presented.
Aps Division of Plasma Physics Meeting Abstracts, Nov 1, 1999
Most laser wakefield accelerator (LWFA) experiments to date have operated in the high plasma dens... more Most laser wakefield accelerator (LWFA) experiments to date have operated in the high plasma density self-modulated (SM) regime in which the laser pulse length c τL is much longer than the plasma wavelength λp = 2 π c / ω_p. These experiments rely on a self-guiding effect to overcome diffraction of the laser pulse and extend the interaction distance. In a channel-guided SM-LWFA, a preformed plasma channel provides the guiding. Simulations indicate that for a wide range of laser and channel parameters, the laser pulse will become modulated at λ_p, eventually producing accelerating gradients which may exceed 10 GV/m. The resulting wakefields are generally more regular than those produced in the self-guided regime, and the accelerating gradients are usually much higher than in the resonant (c τL ~ λp / 2) LWFA regime. Simulations of a recent channel guiding experiment at NRL(D. Kaganovich, et al., Phys. Rev. E 59), R4769 (1999) show that self-modulation and high accelerating gradients may have been achieved in that experiment.
Intense laser pulses may be focused, compressed, chirped, or shaped using a variety of techniques... more Intense laser pulses may be focused, compressed, chirped, or shaped using a variety of techniques. This pulse control is normally accomplished using optical elements such as lenses and gratings that are placed where the laser intensity or fluence is low enough to avoid damage. This paper describes several techniques for using plasmas to control pulses at locations where the laser intensity is far above conventional optical damage limits. For example, short plasma channels may be used as lenses or spatial filters. Longer channels may be used for pulse compression or frequency chirping. Potential applications to laser wakefield accelerators and advanced radiation sources will be discussed.
ABSTRACT There has been considerable progress on laser-plasma acceleration. Presentations on lase... more ABSTRACT There has been considerable progress on laser-plasma acceleration. Presentations on laser-plasma acceleration concepts at the 2002 Advanced Accelerator Concepts Workshop are reviewed, and the status and future directions of research in this area are discussed.
Multiple diagnostics for simultaneous plasma wave characterization and high energy electron measu... more Multiple diagnostics for simultaneous plasma wave characterization and high energy electron measurement have been implemented in the LWFA at the NRL. These diagnostics allow determination of the optimal plasma wave characteristics for acceleration of background electrons. The plasma wave diagnostics include: Coherent Thomson scattering of a sub-picosecond variable-delay probe laser, forward and backward Raman scattering of the pump laser, and 90 degree imaging of Thomson scattered light from the plasma. Coincident with the plasma wave diagnostics, the acceleration of background electrons is investigated with a magnetic electron spectrometer. The results of these experiments will be presented. The incorporation of a 4.5 MeV RF electron gun (currently under construction) for injection into the wakefields will also be discussed.
Propagtion of intense short laser pulses in plasmas is strongly affected by relativistic correcti... more Propagtion of intense short laser pulses in plasmas is strongly affected by relativistic corrections to the plasma index of refraction. If the peak laser power P0 is slightly above the critical power Pr for relativistic focusing, the center of the pulse will be focused, and the front and back will expand due to diffraction. Pondermotive expulsion of plasma electrons substantially enhances the radial focusing effect, while self-phase modulation and anomalous dispersion may enhance longitudinal compression. Simulations in this regime often produce reductions in the focused laser spot size and pulse length by a factor of 5 or more. Proper tailoring of the plasma radial profile, as can be done with a capillary discharge, may produce substantial pulse shortening without the reduction in spot size. Possible future experiments on the NRL T^3 laser and potential applications will be discussed.
The traditional long term strategy for producing high quality electron beams in a single stage LW... more The traditional long term strategy for producing high quality electron beams in a single stage LWFA involves three elements: operation in the resonant or standard regime, the use of optical guiding to extend the acceleration region, and external injection of a precisely-phased, high quality injection electron bunch. The standard regime and optical guiding has been studied by many research groups and promise good results for the acceleration. The creation of the electron beam for external injection is still a very problematic issue. Most experiments to date have operated in the self modulated (SM) regime, which produces very large accelerating gradients but poor quality electron beams with large energy spread. More recently, quasi-monoenergetic acceleration of particles from the background plasma has been observed in simulations and experiments operating in a shorter pulse regime. Such quasi-monoenergetic electrons could be a candidate for injection into a following stage of standard LWFA if not for the relatively poor shot to shot reproducibility. We are in the initial stage of experiments to generate injection electrons using the HD-LIPA schemes with a 10 TW 50 fs laser system. The second stage accelerator will be a capillary discharge plasma channel for extended acceleration distance. Preliminary results, including statistics on the stability of quasi-monoenergetic acceleration, will be presented. Supported by DOE and ONR.
The generation of high quality electron beams from a laser wakefield accelerator (LWFA) is expect... more The generation of high quality electron beams from a laser wakefield accelerator (LWFA) is expected to require external injection of electrons. Several all-optical electron injection schemes have been proposed and will be employed in upcoming LWFA experiments at the Naval Research Laboratory. Some of these schemes are theoretically capable of producing extremely short, precisely timed (phased) injected electron bunches. Simulations demonstrate that such ideal bunches may be accelerated by the LWFA to high energies with excellent beam emittance and small energy spread. Simulations using nonideal bunches with longer bunch length or energy spread have also been carried out. Some of these simulations show that phased, ultrashort injection may not be necessary for a high quality accelerated beam if the injected energy is carefully chosen. In these cases, the small energy spread and bunch length in the final accelerated beam is a result of removal (pruning) of electrons that move into defocusing portions of the wakefield, combined with strong phase bunching and rapid acceleration.
A 1-D numerical model has been developed which treats propagation of intense laser pulses in a no... more A 1-D numerical model has been developed which treats propagation of intense laser pulses in a nonuniform plasma when the laser frequency ω0 is comparable with the maximum plasma frequency ω_pm. The plasma density may be specified analytically or generated self-consistently assuming that tunneling ionization is the dominant ionization mechanism. This allows the code to treat wakefield generation in laser wakefield accelerators self-consistently. Results will also be presented for cases with a stationary plasma and a second laser pulse with amplitude a1 << a0 and frequency ω1 ≈ ω0 - ω_pm < ω_pm. A localized instability may be generated at the location z where ω_p(z) = ω_0/2. This may also result in modulation and partial reflection of the high frequency pulse as it propagates into the region where ω_p(z) ≈ ω_pm. Reducing the gradient scale length can suppress the instability while leading to enhanced penetration of the low frequency pulse.
Patent Application Department of the Navy Washington Dc, Dec 1, 1991
The invention pertains to plasma physics, and in particular to and transportation and use of posi... more The invention pertains to plasma physics, and in particular to and transportation and use of positively charged particle beams. Particle beams of positively charged atoms have potentially a number of useful applications, among which are thin film deposition, semiconductor doping, use as a general laboratory tool, a source of pulsed neutrons (i.e., by impacting a suitable target), shock hardening of materials, advanced military weaponry, and, most interestingly, as the trigger in a nuclear fusion reactor. Unfortunately, a number of these applications require that after the beams are created in particle accelerators, they must travel a considerable distance to their targets.
A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dram... more A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dramatic photon deceleration while propagating a distance on the order of a dephasing length. The deceleration of photons is localized to the back of the pulse and is accompanied by compression and explosive growth of the ponderomotive potential. Fully explicit particle-in-cell simulations are applied to the problem and are compared with ponderomotive guiding center simulations. A numerical Wigner transform is used to examine local frequency shifts within the pulse and to suggest an experimental diagnostic of plasma waves inside a capillary.
An injector and accelerator is analyzed that uses three collinear laser pulses in a plasma: an in... more An injector and accelerator is analyzed that uses three collinear laser pulses in a plasma: an intense pump pulse, which generates a large wake field \(&amp;amp;amp;amp;amp;amp;gt;=20 GV/m\), and two counterpropagating injection pulses. When the injection pulses collide, a slow phase velocity beat wave is generated that injects electrons into the fast wake field for acceleration. Particle tracking simulations in 1D
To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary ... more To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in plasma without disruption. A 3D envelope equation for a laser pulse in a tapered plasma channel is derived, which includes wakefields and relativistic and nonparaxial effects, such as finite pulse length and group velocity dispersion. It is shown that electron energies of ~GeV in a plasma-channel LWFA can be achieved by using short pulses where the forward Raman and modulation nonlinearities tend to cancel. Further energy gain can be achieved by tapering the plasma density to reduce electron dephasing.
Optical guiding of intense short laser pulses in plasmas is important in applications ranging fro... more Optical guiding of intense short laser pulses in plasmas is important in applications ranging from x-ray lasers to laser driven electron accelerators. The dynamics of short intense laser pulses propagating in channels is analyzed. The model describing the pulse envelope dynamics includes nonlinear, finite pulse length and group velocity dispersion effects. The intense short laser pulse propagating in the channel undergoes self phase modulation resulting in a large frequency spread. The nonlinear evolution of the pulse's frequency spread is analyzed. In addition to ultra broadband radiation generation, laser driven electron acceleration in plasma channels will be discussed.
A two stage experiment on optically injected Laser Wakefield Acceleration (LWFA) was performed at... more A two stage experiment on optically injected Laser Wakefield Acceleration (LWFA) was performed at the Naval Research Laboratory. Two temporally and spatially synchronized laser beams at 2 TW and 10 TW respectively are each focused collinearly into two adjacent gas jets. Electrons at <0.5 MeV generated from the 2 TW laser beam are injected into the wakefields generated by the 10 TW laser beam. Accelerated electrons >20 MeV are observed, implying an acceleration gradient of 20 GeV/m for the ˜1 mm acceleration distance. Apparent peaks and structures are observed in the energy spectrum. This is a clear demonstration of substantial laser wakefield acceleration of externally and optically produced injection electrons. Details of the experiment will be presented.
Aps Division of Plasma Physics Meeting Abstracts, Nov 1, 1999
Most laser wakefield accelerator (LWFA) experiments to date have operated in the high plasma dens... more Most laser wakefield accelerator (LWFA) experiments to date have operated in the high plasma density self-modulated (SM) regime in which the laser pulse length c τL is much longer than the plasma wavelength λp = 2 π c / ω_p. These experiments rely on a self-guiding effect to overcome diffraction of the laser pulse and extend the interaction distance. In a channel-guided SM-LWFA, a preformed plasma channel provides the guiding. Simulations indicate that for a wide range of laser and channel parameters, the laser pulse will become modulated at λ_p, eventually producing accelerating gradients which may exceed 10 GV/m. The resulting wakefields are generally more regular than those produced in the self-guided regime, and the accelerating gradients are usually much higher than in the resonant (c τL ~ λp / 2) LWFA regime. Simulations of a recent channel guiding experiment at NRL(D. Kaganovich, et al., Phys. Rev. E 59), R4769 (1999) show that self-modulation and high accelerating gradients may have been achieved in that experiment.
Intense laser pulses may be focused, compressed, chirped, or shaped using a variety of techniques... more Intense laser pulses may be focused, compressed, chirped, or shaped using a variety of techniques. This pulse control is normally accomplished using optical elements such as lenses and gratings that are placed where the laser intensity or fluence is low enough to avoid damage. This paper describes several techniques for using plasmas to control pulses at locations where the laser intensity is far above conventional optical damage limits. For example, short plasma channels may be used as lenses or spatial filters. Longer channels may be used for pulse compression or frequency chirping. Potential applications to laser wakefield accelerators and advanced radiation sources will be discussed.
ABSTRACT There has been considerable progress on laser-plasma acceleration. Presentations on lase... more ABSTRACT There has been considerable progress on laser-plasma acceleration. Presentations on laser-plasma acceleration concepts at the 2002 Advanced Accelerator Concepts Workshop are reviewed, and the status and future directions of research in this area are discussed.
Multiple diagnostics for simultaneous plasma wave characterization and high energy electron measu... more Multiple diagnostics for simultaneous plasma wave characterization and high energy electron measurement have been implemented in the LWFA at the NRL. These diagnostics allow determination of the optimal plasma wave characteristics for acceleration of background electrons. The plasma wave diagnostics include: Coherent Thomson scattering of a sub-picosecond variable-delay probe laser, forward and backward Raman scattering of the pump laser, and 90 degree imaging of Thomson scattered light from the plasma. Coincident with the plasma wave diagnostics, the acceleration of background electrons is investigated with a magnetic electron spectrometer. The results of these experiments will be presented. The incorporation of a 4.5 MeV RF electron gun (currently under construction) for injection into the wakefields will also be discussed.
Propagtion of intense short laser pulses in plasmas is strongly affected by relativistic correcti... more Propagtion of intense short laser pulses in plasmas is strongly affected by relativistic corrections to the plasma index of refraction. If the peak laser power P0 is slightly above the critical power Pr for relativistic focusing, the center of the pulse will be focused, and the front and back will expand due to diffraction. Pondermotive expulsion of plasma electrons substantially enhances the radial focusing effect, while self-phase modulation and anomalous dispersion may enhance longitudinal compression. Simulations in this regime often produce reductions in the focused laser spot size and pulse length by a factor of 5 or more. Proper tailoring of the plasma radial profile, as can be done with a capillary discharge, may produce substantial pulse shortening without the reduction in spot size. Possible future experiments on the NRL T^3 laser and potential applications will be discussed.
The traditional long term strategy for producing high quality electron beams in a single stage LW... more The traditional long term strategy for producing high quality electron beams in a single stage LWFA involves three elements: operation in the resonant or standard regime, the use of optical guiding to extend the acceleration region, and external injection of a precisely-phased, high quality injection electron bunch. The standard regime and optical guiding has been studied by many research groups and promise good results for the acceleration. The creation of the electron beam for external injection is still a very problematic issue. Most experiments to date have operated in the self modulated (SM) regime, which produces very large accelerating gradients but poor quality electron beams with large energy spread. More recently, quasi-monoenergetic acceleration of particles from the background plasma has been observed in simulations and experiments operating in a shorter pulse regime. Such quasi-monoenergetic electrons could be a candidate for injection into a following stage of standard LWFA if not for the relatively poor shot to shot reproducibility. We are in the initial stage of experiments to generate injection electrons using the HD-LIPA schemes with a 10 TW 50 fs laser system. The second stage accelerator will be a capillary discharge plasma channel for extended acceleration distance. Preliminary results, including statistics on the stability of quasi-monoenergetic acceleration, will be presented. Supported by DOE and ONR.
The generation of high quality electron beams from a laser wakefield accelerator (LWFA) is expect... more The generation of high quality electron beams from a laser wakefield accelerator (LWFA) is expected to require external injection of electrons. Several all-optical electron injection schemes have been proposed and will be employed in upcoming LWFA experiments at the Naval Research Laboratory. Some of these schemes are theoretically capable of producing extremely short, precisely timed (phased) injected electron bunches. Simulations demonstrate that such ideal bunches may be accelerated by the LWFA to high energies with excellent beam emittance and small energy spread. Simulations using nonideal bunches with longer bunch length or energy spread have also been carried out. Some of these simulations show that phased, ultrashort injection may not be necessary for a high quality accelerated beam if the injected energy is carefully chosen. In these cases, the small energy spread and bunch length in the final accelerated beam is a result of removal (pruning) of electrons that move into defocusing portions of the wakefield, combined with strong phase bunching and rapid acceleration.
A 1-D numerical model has been developed which treats propagation of intense laser pulses in a no... more A 1-D numerical model has been developed which treats propagation of intense laser pulses in a nonuniform plasma when the laser frequency ω0 is comparable with the maximum plasma frequency ω_pm. The plasma density may be specified analytically or generated self-consistently assuming that tunneling ionization is the dominant ionization mechanism. This allows the code to treat wakefield generation in laser wakefield accelerators self-consistently. Results will also be presented for cases with a stationary plasma and a second laser pulse with amplitude a1 << a0 and frequency ω1 ≈ ω0 - ω_pm < ω_pm. A localized instability may be generated at the location z where ω_p(z) = ω_0/2. This may also result in modulation and partial reflection of the high frequency pulse as it propagates into the region where ω_p(z) ≈ ω_pm. Reducing the gradient scale length can suppress the instability while leading to enhanced penetration of the low frequency pulse.
Patent Application Department of the Navy Washington Dc, Dec 1, 1991
The invention pertains to plasma physics, and in particular to and transportation and use of posi... more The invention pertains to plasma physics, and in particular to and transportation and use of positively charged particle beams. Particle beams of positively charged atoms have potentially a number of useful applications, among which are thin film deposition, semiconductor doping, use as a general laboratory tool, a source of pulsed neutrons (i.e., by impacting a suitable target), shock hardening of materials, advanced military weaponry, and, most interestingly, as the trigger in a nuclear fusion reactor. Unfortunately, a number of these applications require that after the beams are created in particle accelerators, they must travel a considerable distance to their targets.
A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dram... more A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dramatic photon deceleration while propagating a distance on the order of a dephasing length. The deceleration of photons is localized to the back of the pulse and is accompanied by compression and explosive growth of the ponderomotive potential. Fully explicit particle-in-cell simulations are applied to the problem and are compared with ponderomotive guiding center simulations. A numerical Wigner transform is used to examine local frequency shifts within the pulse and to suggest an experimental diagnostic of plasma waves inside a capillary.
An injector and accelerator is analyzed that uses three collinear laser pulses in a plasma: an in... more An injector and accelerator is analyzed that uses three collinear laser pulses in a plasma: an intense pump pulse, which generates a large wake field \(&amp;amp;amp;amp;amp;amp;gt;=20 GV/m\), and two counterpropagating injection pulses. When the injection pulses collide, a slow phase velocity beat wave is generated that injects electrons into the fast wake field for acceleration. Particle tracking simulations in 1D
To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary ... more To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in plasma without disruption. A 3D envelope equation for a laser pulse in a tapered plasma channel is derived, which includes wakefields and relativistic and nonparaxial effects, such as finite pulse length and group velocity dispersion. It is shown that electron energies of ~GeV in a plasma-channel LWFA can be achieved by using short pulses where the forward Raman and modulation nonlinearities tend to cancel. Further energy gain can be achieved by tapering the plasma density to reduce electron dephasing.
Optical guiding of intense short laser pulses in plasmas is important in applications ranging fro... more Optical guiding of intense short laser pulses in plasmas is important in applications ranging from x-ray lasers to laser driven electron accelerators. The dynamics of short intense laser pulses propagating in channels is analyzed. The model describing the pulse envelope dynamics includes nonlinear, finite pulse length and group velocity dispersion effects. The intense short laser pulse propagating in the channel undergoes self phase modulation resulting in a large frequency spread. The nonlinear evolution of the pulse's frequency spread is analyzed. In addition to ultra broadband radiation generation, laser driven electron acceleration in plasma channels will be discussed.
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Papers by Richard Hubbard