Search Results (50 total)

A classical particle oscillating in an arbitrary high-frequency or static field effectively exhibits a modified rest mass m_eff derived from the particle averaged Lagrangian. Relativistic ponderomotive and diamagnetic forces, as well as magnetic drifts, are obtained from the m_eff dependence on the guiding center location and velocity. The effective mass is not necessarily positive and can result in backward acceleration when an additional perturbation force is applied. As an example, adiabatic dynamics with m_∥>0 and m_∥<0 is demonstrated for a wave-driven particle along a dc magnetic field, m_∥ being the effective longitudinal mass derived from m_eff. Multiple energy states are realized in this case, yielding up to three branches of m_∥ for a given magnetic moment and parallel velocity.

Powerful x-ray pulses might be compressed to even greater powers by means of backward Raman amplification in ultradense plasmas produced by ionizing condensed matter by the same pulses. The pulse durations contemplated are shorter than the time for complete smoothing of the crystal lattice by thermal motion of ions. Although inhomogeneities are generally thought to be deleterious to the Raman amplification, the relic lattice might, in fact, be useful for the Raman amplification. The x-ray frequency band gaps can suppress parasitic Raman scattering of amplified pulses, while enhanced dispersion of the x-ray group velocity near the gaps can delay self-phase-modulation instability, thereby enabling further amplification of the x rays.

Intense laser waves can form a time-dependent gate, which transmits or reflects particles depending on their initial phases. When faced by a relativistic electron beam, such a barrier slices it by randomly scattering all but some particles, which nearly conserve their velocity. Subfemtosecond or attosecond periodic electron bunches are then formed downstream and can be used, for example, to generate coherent x rays via Thomson backscattering of the laser light.

Backward Raman amplification (BRA) in plasmas holds the potential for longitudinal compression and focusing of powerful x-ray pulses. In principle, this method is capable of producing pulse intensities close to the vacuum breakdown threshold by manipulating the output of planned x-ray sources. The minimum wavelength limit of BRA applicability to compression of laser pulses in plasmas is found.

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 α 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.

Localized regions of intense large-scale radiofrequency field are known to act like effective (“ponderomotive”) potential barriers, which scatter particles elastically and in the direction determined by the particle initial velocity rather than phase. In smaller-scale fields, transmission through a ponderomotive barrier is probabilistic and resembles tunneling of a quantum particle through a static potential. We derive asymptotic expressions for the phase-averaged transmission coefficient T as a function of the particle energy E₀. We show that, unlike for a truly quantum particle, T(E₀) is of algebraic form and has a threshold, below which transmission does not occur. We also find a threshold in E₀, above which all particles are transmitted regardless of their initial phase.

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.

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.

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 λ equal to the particle average displacement on the oscillation period. In a quasiclassical field, with a spatial scale large compared to λ, 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.

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.

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.

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.

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.

Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.

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 “π-pulse” manifold (family) of self-similar solutions. We determine how the pumped pulse projection moves within the π-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.

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.

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.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

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∼1-3×10²⁰ 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.

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.

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.

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.

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.

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.