摘要:

The self‐focusing of relativistically intense laser light pulses is analyzed, where the pulse length is short enough that ion inertia prevents any significant motion of ions. Self‐focusing occurs as a result of an increase of the wave refractive index arising from two effects: the mass increase of electrons caused by their relativistic quiver velocity in the light wave, and the reduction of the electron density as a result of ponderomotive force expulsion of the electrons. The latter effect is significant even for rather small values of (P−PL)/PL, wherePis the laser beam power andPLis the critical power above which self‐focusing occurs. In fact, for (P−PL)/PL≳0.1 the effect is so strong thatallelectrons are expelled within a core radial region of the self‐focused laser light channel (this new phenomenon is calledelectroncavitation).

ISSN:0031-9171    

DOI:10.1063/1.866349

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Filamentation has been examined in laser‐produced plasmas in which magnetic fields are present, using a two‐fluid model of the plasma. Configurations in which the low‐frequency perturbation is in turn parallel and perpendicular to the magnetic field have been considered and the instability examined in the two cases. In the latter it is shown that the instability is quenched by large enough fields, while for the ion‐acoustic perturbation the behavior is more complex.

ISSN:0031-9171    

DOI:10.1063/1.866350

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A general theory of nontenuous cold relativistic helically symmetric steady flows is developed. Both self‐fields and radial effects are included. The fluid and the Maxwell equations are reduced to a system of equations for three scalar functions: a stream function, a flux function, and an electrostatic potential. These equations are expanded under the assumption of small perpendicular momenta and a simplified set of ordinary differential equations is obtained. Two cases are studied in detail. The first case is of a system externally driven by a magnetic wiggler. The second is the self‐excited system, with no externally applied wiggler. The equilibria described here are of a practical importance for free‐electron lasers which employ high‐density thick beams.

ISSN:0031-9171    

DOI:10.1063/1.866351

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The problem of the instability of counterstreaming beams of charged particles is extended to cylindrical and spherical geometries. For well‐focused configurations it can be solved by complex contour integral representations. The effects of the convergence of the flow and the density gradient along the trajectories of the particles are considered. The linear spectrum for the cylindrical case is obtained, together with the proof that the solution has finite energy and satisfies two physical matching conditions through the origin. The properties of the special functions which solve this problem are presented. Although the density of the ideally focused model diverges as 1/rat the origin, the growth rate of the instability, for a system of radiusR, is given by &ohgr;2pR/V02&xgr;n, whereV0is the beam velocity, &xgr;nare the zeros of the Bessel function of zeroth order, and the plasma frequency &ohgr;pis evaluated at one‐half the average density of particles.

ISSN:0031-9171    

DOI:10.1063/1.866352

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Use is made of the single‐particle orbit equations together with Maxwell’s equations and appropriate statistical averages to investigate detailed properties of the sideband instability for a helical‐wiggler free‐electron laser with wiggler wavelength &lgr;0=2&pgr;/k0=const and normalized wiggler amplitudeaw=eBˆw/mc2k0=const. The model describes the nonlinear evolution of a right‐circularly polarized primary electromagnetic wave with frequency &ohgr;s, wavenumberks, and slowly varying amplitudeaˆs(z,t) and phase &dgr;s(z,t) (eikonal approximation). The orbit and wave equations are analyzed in the ponderomotive frame (‘‘primed’’ variables) moving with velocityvp=&ohgr;s/(ks+k0) relative to the laboratory. Detailed properties of the sideband instability are investigated for small‐amplitude perturbations about a quasisteady equilibrium state characterized byaˆ0s=const (independent ofz’andt’). Two cases are treated. The first case assumes constant equilibrium wave phase &dgr;0s=const, which requires (for self‐consistency) both untrapped‐ and trapped‐electron populations satisfying ⟨∑j exp[ik’pzj0(t’) +i&dgr;0s]/&ggr;’j⟩=0.Herekp=(ks+k0)/&ggr;pis the wavenumber of the ponderomotive potential,z’j0(t’) is the equilibrium orbit, and &ggr;jmc2is the electron energy. The second case assumes that all of the electrons are deeply trapped, which requires a slow spatial variation of the equilibrium wave phase, ∂&dgr;0s/∂z’=2&Ggr;0(&Ggr;0ck0/&OHgr;B)2k’p≠0. The resulting dispersion relations and detailed stability properties are found to be quite different in the two cases. Both the weak‐pump and strong‐pump regimes are considered.

ISSN:0031-9171    

DOI:10.1063/1.866353

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

High‐current, fast‐rise‐time electron beams in air often are subject to powerful hollowing instabilities. Previous work indicates that the instability is triggered by avalanche ionization of the air, which occurs for a ratio of inductive electric field strength to air density exceeding 130 kV/cm‐atm. Here, computer simulations are used to recast this threshold criterion in terms of the beam current rise rate, which is more readily accessible experimentally. The threshold dependence on beam radius and peak current is much weaker. The current rise rate tends to increase during propagation caused by erosion of the beam front, and this behavior is explored with additional simulations. Evidence for hollowing stabilization by electron energy spread also is presented. Numerical algorithms employed in these calculations are included for completeness.

ISSN:0031-9171    

DOI:10.1063/1.866354

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Intense electron beam hollowing instabilities, although routinely observed in axisymmetric computer simulations of beam propagation in air, are not often seen in experiments because of competing nonaxisymmetric effects. Recent experiments with the IBEX accelerator 4 MeV, 70 kA electron beam [R. B. Miller, M. G. Mazarakis, J. W. Poukey, and R. J. Adler, IEEE Trans. Nucl. Sci.30, 2722 (1983)] provided detailed measurements of hollowing instability scaling over the pressure range of 10–80 Torr. Simulations performed in support of those experiments are reported here. Both experiment and simulation show large amplitude hollowing at the lowest pressures, decreasing in magnitude as the pressure is increased. The instability is suppressed above about 120 Torr. In addition, the simulations indicate that hollowing can be reduced by matching the beam radius at injection with its equilibrium value in air. The good agreement between experiment and simulation validates existing hollowing instability scaling laws.

ISSN:0031-9171    

DOI:10.1063/1.866355

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The plasma potential is measured near the edge of an electrically floating obstacle placed in a steady‐state, supersonic, unmagnetized, neutral plasma flow. Equipotential contours show the sheath of the upstream side of the obstacle wrapping around the edge of the obstacle and fanning out into the near wake. Both fluid theory and the data find the near‐wake plasma potential to be self‐similar when ionization, charge exchange, and magnetic field can be neglected. The theory also finds that fluid velocity is self‐similar, the near wake is nonneutral, and plasma density is not self‐similar. Strong electric fields are found near the obstacle and equipotential contours are found to conform to all boundaries.

ISSN:0031-9171    

DOI:10.1063/1.866356

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Experimental studies of the forward scattered spectrum and stimulated Raman backscattering from foam targets are presented. An attempt has been made to isolate the effects of the presence of the quarter critical density on the Raman spectrum by creating plasmas with various peak plasma densities. The plasmas created had a length larger than 600 &mgr;m and a variable peak density between 0.11ncand slightly higher than 0.25nc. The total Raman reflectivity in the backward direction is of 0.3%, with 80% of its energy being scattered in a range of frequencies between 470 nm and 500 nm for all the targets used. The scattered intensity near the half‐harmonic region shows a weak dependence on the presence of thenc/4 density layer. The forward scattered spectrum obtained from targets with average densities between 0.11ncand 0.22ncshows a broadband of frequencies similar to the backscattered spectrum indicating that the forward spectrum is probably being seeded by the beating of the plasma wave from stimulated Raman backscattering (SRS‐B) with the ion acoustic wave from stimulated Brillouin scattering (SBS).

ISSN:0031-9171    

DOI:10.1063/1.866357

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Ion turbulence generated in a CO2laser‐heated gas target plasma has been studied using ruby laser Thomson scattering. Considerable enhancement of the ion fluctuations over the thermal level was observed for two high‐density plasma regimes (short and long density scale lengths). In the short scale length regime the magnitude of density fluctuations, together with temporal and spectral features of the scattered ruby light, indicate that ion‐turbulence levels are sufficient to account for anomalous absorption of CO2laser energy. In the long scale length regime, measurements of the ion‐fluctuation wavenumber spectrum induced in the plane of the CO2laser electric field, together with high‐speed streak camera measurements of the Thomson scattered ruby laser light, were performed to determine general features of strong ion fluctuations in laser heated plasma. Possible mechanisms for generating the observed ion turbulence are discussed.

ISSN:0031-9171    

DOI:10.1063/1.866358

出版商:AIP    

年代:1987

数据来源: AIP