We present our recent experimental results of monoenergetic protons accelerated from the interaction of an intense terawatt CO2 laser pulse with a near-critical hydrogen gas target, with its density profile tailored by a hydrodynamic shock. A 5-ns Nd:YAG laser pulse is focused onto a piece of stainless steel foil mounted at the front edge of the gas jet nozzle orifice. The ablation launches a spherical shock into the near-critical gas column, which creates a sharp density gradient at the front edge of the target, with ~ 6X local density enhancement up to several times of critical density within ~<100 microns. With such density profile, we have obtained monoenergetic proton beams with good shot-to-shot reproducibility and energies up to 1.2 MeV.
An intense laser pulse propagating through a near-critical density plasma is capable of generating a high peak current electron beam (~100kA) by means of a laser wakefield operating in the bubble regime. This beam drives surface currents on the sheath of the cavitation that produces large azithumal magnetic fields (~100 MG) and induces an on-axis electron current. These effects lead to a Z-pinch of the ambient plasma ions. Eventually the field confining the pinch relaxes and the ions explode radially due to self-repulsive forces. If this process occurs at a sharp exit gradient, the ions acquire some forward momentum consistent with the magnetic vortex acceleration mechanism. Fully 3D simulations indicate that the highest energy ions are emitted conically with a secondary lower energy ion beam accelerated on the axis.
This paper discusses an advanced target in the loop (ATIL) system with its performance based on a nonlinear phase conjugation scheme that performs rapid adjustment of the laser beam wavefront to mitigate effects associated with atmospheric turbulence along the propagation path. The ATIL method allows positional control of the laser spot (the beacon) on a remote imaged-resolved target. The size of this beacon is governed by the reciprocity of two counterpropagating beams (one towards the target and another scattered by the target) and the fidelity of the phase conjugation scheme. In this presentation we will present the results of the thorough analysis of ATIL operation, factors that affect its performance, its focusing efficiency and the comparison of laboratory experimental validation and computer simulation results.
A viable beam control technique is critical for effective laser beam transmission through turbulent atmosphere. Most
of the established approaches require information on the impact of perturbations on wavefront propagated waves.
Such information can be acquired by measuring the characteristics of the target-scattered light arriving from a small,
preferably diffraction-limited, beacon. This paper discusses an innovative beam control approach that can support
formation of a tight laser beacon in deep turbulence conditions. The technique employs Brillouin enhanced fourwave
mixing (BEFWM) to generate a localized beacon spot on a remote image-resolved target. Formation of the
tight beacon doesn’t require a wavefront sensor, AO system, or predictive feedback algorithm. Unlike conventional
adaptive optics methods which allow wavefront conjugation, the proposed total field conjugation technique is
critical for beam control in the presence of strong turbulence and can be achieved by using this non-linear BEFWM
technique. The phase information retrieved from the established beacon beam can then be used in conjunction with
an AO system to propagate laser beams in deep turbulence.
Blast waves ignited in a supersonic flow are capable of producing density profiles useful for laser acceleration of electrons and ions. By using a ≈0.1 joule nanosecond laser as an igniter, and controlling the angle of incidence and timing of the ultra-intense femtosecond drive pulse, one can produce a variety of gas density profiles. A profile with an abrupt up-ramp followed by a gradual down-ramp leads experimentally to stable generation of 40 MeV electrons from a 10 TW drive pulse. A profile with a narrow high density region, and a steep density ramp on one side, is useful for magnetic vortex acceleration of ions. Simulations predict that such a profile can be generated from a laser ignited blast wave, and that 35 MeV protons are produced when a 100 TW pulse is focused into the shock front.
This paper discusses a novel type of beam director for effective laser beacon formation in deep turbulence conditions. The concept of the proposed beam director is based on an innovative approach employing a Brillouin enhanced four-wave mixing (BEFWM) mechanism for generating a tight (small spot size) laser beacon on a remote image-resolved target. The BEFWM technique enables both amplification and total (phase and amplitude) conjugation of the beacon-forming beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Total conjugation is critical for beam control in the presence of strong turbulence, whereas conventional adaptive optics methods do not have this capability. The phase information retrieved from the beacon beam can be used in conjunction with an AO system to propagate laser beams in deep turbulence.
Recent theoretical work has provided new insight into the physics of Electro-Optic detection of ultrashort relativistic
electron beams.1 Typically, Electro-Optic detection has been restricted to bunches longer than ~ 100
fs. This limitation is due to the transverse optical (TO) phonon resonance that most Electro-Optic materials
exhibit in the THz range. Once the electron bunch profile becomes short enough so that a significant portion
of its frequency components reside above this resonance frequency, the temporal profile of the space charge field
begins to distort as it propagates through the crystal. This distortion becomes more significant as the bunch
becomes shorter and destroys the ability of current decoding techniques to resolve the original bunch profile.
It is possible to circumvent this issue by realizing that for these higher frequency components it is no longer
valid to rely on the formalism of Pockels effect. Instead, sum and difference frequency generation must be
taken into account. Using nonlinear three-wave mixing to describe the process, a new technique that promises
the order of magnitude increase in resolution necessary to measure the ultrashort bunches produced by laser
wakefield accelerators has been developed. This technique provides both phase and amplitude information about
the generated pulse from which, in principle, the temporal profile can be reconstructed.
In laser driven accelerators, the interaction of laser radiation with plasma leads to a variety of scattering mechansims.
The scattered radiation can be used to understand the wake structure and its effect on electron acceleration.
In the case of a resonantly driven quasi-linear wake, spectral broadening due to photon acceleration and deceleration
is related to the coupling of energy into plasma waves. Simultaneous time and frequency analysis of the laser
fields produces distinctive features in the photon phase space that give information on wake generation in long
plasma channels. The ponderomotive guiding center algorithm is advantageous for modeling such interactions
because it allows for averaging over optical cycles, and can be implemented in axisymmetric geometry. In the
case of the nonlinear wakes that are driven in the self-guided regime, a region of electron cavitation is formed,
which emits electro-optic shocks at the second harmonic of the drive laser. The form of this radiation can be
correlated with electron trapping.
A remote atmospheric breakdown (RAB) is a very rich source of ultraviolet (UV) and broadband visible light that could provide the early warning to the presence of CW/BW agents through spectroscopic detection, identification and quantification at extended standoff distances. A low-intensity negatively chirped laser pulse propagating in air compresses in time due to linear group velocity dispersion and focuses transversely due to non-linear effects resulting in rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof of principle laboratory experiments are being performed at the Naval Research Laboratory on the generation of RAB and the spectroscopic detection of mock BW agents. We have demonstrated pulse compression and focusing up to 105 meters in the laboratory using femtosecond pulses generated by a high power Ti:Sapphire laser. We observed nonlinear modifications to the temporal frequency chirp of the laser pulse and their effects on the laser compression and the positions of the final focus. We have generated third harmonics at 267 nm and white light in air from the compressed pulse. We have observed fluorescence emission from albumin aerosols as they were illuminated by the compressed femtosecond laser pulse.
The experimental results on generation of high gain-length product for 13.5 nm radiation from 2 - 1 transition in hydrogen-like Li III ions are presented for 1 micrometers subpicosecond pumping laser. The comparison with earlier results, obtained with 0.25 micrometers subpicosecond pumping laser, is discussed in terms of gain generation efficiency. The results for discharge created pre-plasma in L equals 4 mm and L equals 14 mm microcapillaries are also presented.
1407_46The generation of nonlinear plasma wakefields by an intense, short laser pulse and the relativistic optical guiding of intense laser pulses in plasmas are studied with a nonlinear, self-consistent model of laser-plasma interactions. Nonlinear steepening and period lengthening of the plasma waves are observed, and expressions are obtained for various nonlinear wakefield quantities. Relativistic focusing with the self-consistent plasma response shows that laser pulse fronts and laser pulses shorter than a plasma wavelength, 2(pi) c/(omega) (rho ), are not relativistically guided and will continuously erode due to diffraction.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.