Biomedical subjects

N J Fisch

Publications and source records attributed to N J Fisch.

At least 19 recordsLinked to original sources

PubMed2006

Alpha channeling in mirror machines.

The injection of radio frequency waves can cool charged particles trapped in a magnetic mirror. This cooling effect relies upon waves with azimuthal and axial phase velocities resonating with ions in different axial locations. The ions are then forced to diffuse along highly constrained orbits, such that they can only exit the magnetic trap at low energy. This cooling effect may have application to magnetic fusion mirror machines, where the free energy of the fusion by-products, the alpha particles, might be channeled into the waves that effect the cooling, thereby both extracting the fusion ash quickly and making that energy available in a convenient form for more useful purposes.

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PubMed2005

Current-drive efficiency in a degenerate plasma.

In a degenerate plasma, the rates of electron processes are much smaller than the classical model would predict, affecting the efficiencies of current generation by external noninductive means, such as by electromagnetic radiation or intense ion beams. For electron-based mechanisms, the current-drive efficiency is higher than the classical prediction by more than a factor of 6 in a degenerate hydrogen plasma, mainly because the electron-electron collisions do not quickly slow down fast electrons. Moreover, electrons much faster than thermal speeds are more readily excited without exciting thermal electrons. In ion-based mechanisms of current drive, the efficiency is likewise enhanced due to the degeneracy effects, since the electron stopping power on slow ion beams is significantly reduced.

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PubMed2005

Ponderomotive ratchet in a uniform magnetic field.

We show how a ratchet effect, generally used in systems with periodic potentials, can also be practiced on charged particles by an ac field alone, in a background magnetic field near the cyclotron resonance. The effect relies entirely on the spatial inhomogeneity of the high-frequency drive, which produces a deterministic asymmetric ponderomotive barrier for undamped particles. Such a barrier can reflect particles incident from one side while transmitting those incident from the opposite side, hence acting somewhat like a Maxwell demon. The necessary fields are perhaps most easily realized in a plasma, though the effect is more general.

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PubMed2005

Quantumlike dynamics of classical particles in ponderomotive potentials.

The average dynamics of a classical particle under the action of a high-frequency radiation resembles quantum particle motion in a conservative field with an effective de Broglie wavelength lambda equal to the particle average displacement on the oscillation period. In a quasiclassical field, with a spatial scale large compared to lambda, the guiding-center motion is adiabatic. Otherwise, a particle exhibits quantized eigenstates in ponderomotive potential wells, tunnels through "classically forbidden" regions, and experiences stochastic reflection from attractive potentials.

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PubMed2005

Backward Raman amplification in a partially ionized gas.

Compressing laser pulses to extremely high intensities through backward Raman amplification might be accomplished in a plasma medium. While the theory is relatively straightforward for homogeneous fully ionized plasma, a number of important effects enter when the plasma is not fully ionized. In particular, when a mixture of gases is employed to accomplish the coupling, there can be several thresholds for incremental ionization. The refraction of both the pump and the seed is then strongly affected by the plasma ionization. Moreover, in the case of Raman backscattering in partially ionized plasma, the degree of plasma ionization is particularly sensitive to the counterpropagating geometry. This idea is examined in light of data for a recent experiment on a Raman amplifier.

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PubMed2005

Compression of atomic phase space using an asymmetric one-way barrier.

We show how to construct asymmetric optical barriers for atoms. These barriers can be used to compress phase-space of a sample by creating a confined region in space where atoms can accumulate with heating at the single photon recoil level. We illustrate our method with a simple two-level model and then show how it can be applied to more realistic multilevel atoms.

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PubMed2005

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses.

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.

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PubMed2004

Pump side scattering in ultrapowerful backward Raman amplifiers.

Extremely large laser power might be obtained by compressing laser pulses through backward Raman amplification (BRA) in plasmas. Premature Raman backscattering of a laser pump by plasma noise might be suppressed by an appropriate detuning of the Raman resonance, even as the desired amplification of the seed persists with a high efficiency. In this paper we analyze side scattering of laser pumps by plasma noise in backward Raman amplifiers. Though its growth rate is smaller than that of backscattering, the side scattering can nevertheless be dangerous, because of a longer path of side-scattered pulses in plasmas and because of an angular dependence of the Raman resonance detuning. We show that side scattering of laser pumps by plasma noise in BRA might be suppressed to a tolerable level at all angles by an appropriate combination of two detuning mechanisms associated with plasma density gradient and pump chirp.

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PubMed2004

Finite-duration seeding effects in powerful backward Raman amplifiers.

In the process of backward Raman amplification (BRA), the leading layers of the seed laser pulse can shadow the rear layers, thus weakening the effective seeding power and affecting parameters of output pulses in BRA. We study this effect numerically and also analytically by approximating the pumped pulse by the "pi-pulse" manifold (family) of self-similar solutions. We determine how the pumped pulse projection moves within the pi-pulse manifold, and describe quantitatively the effective seeding power evolution. Our results extend the quantitative theory of BRA to regimes where the effective seeding power varies substantially during the amplification. These results might be of broader interest, since the basic equations are general equations for resonant three-wave interactions.

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PubMed2003

Relativistic electron acceleration in focused laser fields after above-threshold ionization.

Electrons produced as a result of above-threshold ionization of high-Z atoms can be accelerated by currently producible laser pulses up to GeV energies, as shown recently by Hu and Starace [Phys. Rev. Lett. 88, 245003 (2002)]. To describe electron acceleration by general focused laser fields, we employ an analytical model based on a Hamiltonian, fully relativistic, ponderomotive approach. Though the above-threshold ionization represents an abrupt process compared to laser oscillations, the ponderomotive approach can still adequately predict the resulting energy gain if the proper initial conditions are introduced for the particle drift following the ionization event. Analytical expressions for electron energy gain are derived and the applicability conditions of the ponderomotive formulation are studied both analytically and numerically. The theoretical predictions are supported by numerical computations.

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PubMed2003

Current drive in a ponderomotive potential with sign reversal.

Noninductive current drive can be accomplished through ponderomotive forces with high efficiency when the potential changes sign over the interaction region. The effect, which operates somewhat like a Maxwell demon, can be practiced upon both ions and electrons. The current-drive efficiencies, in principle, might be higher than those possible with conventional rf current-drive techniques. It remains, however, for us to identify how the effect might be implemented in a magnetic fusion device in a practical manner.

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PubMed2002

Raman amplification of ultrashort laser pulses in microcapillary plasmas.

Experimental evidences of Raman amplification of ultrashort pulses in microcapillary plasmas are presented. The amplification of 100-500 fs pulses was investigated in microcapillaries with different lengths. The experimental data, together with simulation results, indicate that the resonance condition for Raman amplification in high-density plasma, n(e) approximately 1-3x10(20) cm(-3), existed only in a very short plasma column. Such an assumption makes it possible to reconcile the experimental results and theoretical predictions. Investigations in very short microcapillaries (0.2-0.5 mm) with a broadband seed pulse further support this hypothesis and the amplification factor is in agreement with the linear growth rate.

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PubMed2002

Collective deceleration of relativistic electrons precisely in the core of an inertial-fusion target.

The energy deposition of a relativistic electron beam in a plasma can be managed through turning on or off fast beam-plasma instabilities in desirable regions. This management may enable new ways of realizing the fast-igniter scenario of inertial fusion. Collisional effects alone can decelerate electrons of at most a few MeV within the core of an inertial-fusion target. Beam-excited Langmuir turbulence, however, can decelerate even ultrarelativistic electrons in the core.

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PubMed2002

Suppression of superluminous precursors in high-power backward Raman amplifiers.

The very promising scheme for producing ultrapowerful laser pulses through Raman backscattering of pump lasers in plasmas can be jeopardized by superluminous precursors. Growing from the leading part of the Raman pumped seed pulse, these precursors can disturb the plasma and the pump ahead of the pumped pulse. These ruinous effects, however, might be averted by a detuning, large enough to suppress the precursors, yet small enough to allow the desired backscatter effect.

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PubMed2002

Storing, retrieving, and processing optical information by Raman backscattering in plasmas.

By employing stimulated Raman backscattering in a plasma, information carried by a laser pulse can be captured in the form of a very slowly propagating plasma wave that persists for a time long compared with the pulse duration. If the plasma is then probed with a short laser pulse, the information stored in the plasma wave can be retrieved in a second scattered electromagnetic wave. The recording and retrieving processes can conserve robustly the pulse shape, thus enabling the recording and retrieving with fidelity of information stored in optical signals.

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PubMed2002

Magnetic field generation through angular momentum exchange between circularly polarized radiation and charged particles.

The interaction between circularly polarized radiation and charged particles can lead to generation of magnetic field through an inverse Faraday effect. The spin of the circularly polarized electromagnetic wave can be converted into the angular momentum of the charged particles so long as there is dissipation. We demonstrate this by considering two mechanisms of angular momentum absorption relevant for laser-plasma interactions: electron-ion collisions and ionization. The precise dissipative mechanism, however, plays a role in determining the efficiency of the magnetic-field generation.

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PubMed2001

Motion of charged particles near magnetic-field discontinuities.

Charged particles near discontinuities in magnetic fields, so-called "boundary particles," can be constrained to remain near the discontinuity, even an arbitrarily fractured discontinuity, as the particle drifts along the fractured boundary. These particles are shown to exhibit new and interesting effects along broken and branching surfaces, including the wetting of fractured surfaces.

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PubMed2001

Parametric excitations of fast plasma waves by counterpropagating laser beams.

Short- and long-wavelength plasma waves can become strongly coupled in the presence of two counterpropagating laser pump pulses detuned by twice the cold-plasma frequency omega(p). What makes this four-wave interaction important is that the growth rate of the plasma waves occurs much faster than in the more obvious copropagating geometry.

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