J. Krall

Overview of plasma-based accelerator concepts

An overview is given of the physics issues relevant to the plasma wakefield accelerator, the plasma beat-wave accelerator, the laser wakefield accelerator, including the self-modulated regime, and wakefield accelerators driven by multiple electron or laser pulses. Basic properties of linear and nonlinear plasma waves are discussed, as well as the trapping and acceleration of electrons in the plasma wave. Formulas are presented for the accelerating field and the energy gain in the various accelerator configurations. The propagation of the drive electron or laser beams is discussed, including limitations imposed by key instabilities and methods for optically guiding laser pulses. Recent experimental results are summarized.

Self-focusing and guiding of short laser pulses in ionizing gases and plasmas

Several features of intense, short-pulse (/spl lsim/1 ps) laser propagation in gases undergoing ionization and in plasmas are reviewed, discussed, and analyzed. The wave equations for laser pulse propagation in a gas undergoing ionization and in a plasma are derived. The source-dependent expansion method is discussed, which is a general method for solving the paraxial wave equation with nonlinear source terms. In gases, the propagation of high-power (near the critical power) laser pulses is considered including the effects of diffraction, nonlinear self-focusing, ionization, and plasma generation. Self-guided solutions and the stability of these solutions are discussed. In plasmas, optical guiding by relativistic effects, ponderomotive effects, and preformed density channels is considered. The self consistent plasma response is discussed, including plasma wave effects and instabilities such as self-modulation. Recent experiments on the guiding of laser pulses in gases and in plasmas are briefly summarized.

Evidence of an Erupting Magnetic Flux Rope: LASCO Coronal Mass Ejection of 1997 April 13

A coronal mass ejection (CME) observed by LASCO exhibits evidence that its magnetic field geometry is that of a flux rope. The dynamical properties throughout the fields of view of C2 and C3 telescopes are examined. The results are compared with theoretical predictions based on a model of solar flux ropes. It is shown that the LASCO observations are consistent with a two-dimensional projection of a three-dimensional magnetic flux rope with legs that remain connected to the Sun.

Externally modulated intense relativistic electron beams

The physics of modulation of an intense relativistic electron beam by an external microwave source is studied in this paper via experiment, theory, and simulation. It is found that the self‐fields of the electron beam, in general, intensify the current modulation produced by the external source. The linear and nonlinear theory, together with the simulation, show that the classical klystron description in the drift tube region is substantially modified by the beam’s high density. In the modulating gap, electron bunches may be generated instantaneously without the necessity of propagating the beam through a long drift tube. These properties, which have no counterparts in low‐density beams, lead to the generation of large amplitude, coherent, and monochromatic current modulation on an intense beam. The excellent amplitude stability and the phase‐locking characteristics (<2°) of the modulated current, demonstrated in experiments, open new areas of research in high‐power microwave generation and compact partic...

Laser driven electron acceleration in vacuum, gases, and plasmas

In this paper we discuss some of the important issues pertaining to laser acceleration in vacuum, neutral gases, and plasmas. The limitations of laser vacuum acceleration as they relate to electron slippage, laser diffraction, material damage, and electron aperture effects, are discussed. An inverse Cherenkov laser acceleration configuration is presented in which a laser beam is self‐guided in a partially ionized gas. Optical self‐guiding is the result of a balance between the nonlinear self‐focusing properties of neutral gases and the diffraction effects of ionization. The stability of self‐guided beams is analyzed and discussed. In addition, aspects of the laser wakefield accelerator are presented and laser‐driven accelerator experiments are briefly discussed.

Magnetic Geometry and Dynamics of the Fast Coronal Mass Ejection of 1997 September 9

A coronal mass ejection (CME) was observed on 1997 September 9 by the Mauna Loa Solar Observatory Mark III K-coronameter (MK3) and by the LASCO C2/C3 and EIT instruments on board the SOHO spacecraft. Magnetograms and EIT images obtained on days leading up to the eruption show a neutral line that appears to correspond to the site of the eruption. Taken together, the data from these instruments provide a comprehensive, beginning-to-end record of the event within the 32 R☉ field of view. The motion of several features are tracked through the fields of view of MK3, C2, and C3. The CME exhibits the previously identified morphological features and dynamical properties consistent with those of an erupting magnetic flux rope with its legs connected to the Sun. The LASCO images and magnetograms indicate that the flux rope axis was aligned with the neutral line approximately 2 days behind the west limb. Its apparent orientation provides an oblique view of an erupting flux rope, a view that has not been discussed previously. A theoretical flux rope model is used to understand the forces responsible for the observed CME dynamics. Synthetic coronagraph images based on the model flux rope are constructed.

Erupting Solar Magnetic Flux Ropes: Theory and Observation

Measurements of coronograph (LASCO) and extreme-ultraviolet (EIT) images are presented for 11 coronal mass ejection (CME) events. Detailed measurements of these events, selected because they have flux-rope-like morphological features, show excellent agreement with results from a theoretical model of erupting flux-rope dynamics. Here, data are used to provide inputs and constraints on the model wherever possible. We conclude that flux rope CMEs constitute a distinct class of CMEs, characterized by specific morphological and dynamical properties.

Optically guided laser wake‐field acceleration*

The laser wake‐field acceleration concept is studied using a general axisymmetric formulation based on relativistic fluid equations. This formalism is valid for arbitrary laser intensities and allows the laser–plasma interaction to be simulated over long propagation distances (many Rayleigh lengths). Several methods for optically guiding the laser pulse are examined, including relativistic guiding, preformed plasma density channels and tailored pulse profiles. Self‐modulation of the laser, which occurs when the pulse length is long compared to the plasma wavelength and the power exceeds the critical power, is also examined. Simulations of three possible laser wake‐field accelerator (LWFA) configurations are performed and discussed: (i) a channel‐guided LWFA, (ii) a tailored‐pulse LWFA, and (iii) a self‐modulated LWFA.

Drive Mechanisms of Erupting Solar Magnetic Flux Ropes

The dynamics of magnetic flux ropes near the Sun and in interplanetary space are studied using a magnetohydrodynamic model of erupting magnetic flux ropes. In this model, the magnetic structure of a coronal mass ejection (CME) corresponds to a flux rope with footpoints that remain anchored below the photosphere. The model flux rope eruption can be driven by a rapid increase in poloidal flux (flux injection), a quasi-static increase in poloidal flux (photospheric footpoint twisting), a rapid release of stored magnetic energy (magnetic energy release), or a rapid increase in the amount of hot plasma within the flux rope (hot plasma injection). Model results are compared with Large-Angle Spectrometric Coronagraph data (from the CME of 1997 April 13) and with interplanetary magnetic cloud data over the range 0.4-5 AU. Of these mechanisms, only flux injection and magnetic energy release reproduce key features of the data both near the Sun and in the interplanetary medium and only flux injection obtains a detailed match to the near-Sun dynamics.

Efficient generation of multigigawatt rf power by a klystronlike amplifier

This article addresses the new development of high‐power rf klystronlike amplifiers using modulated intense relativistic electron beams. Development of these amplifiers follows earlier research in which the interaction between a high‐impedance (120‐Ω) intense relativistic electron beam and a low‐power rf pulse resulted in the generation of coherent bunches of electrons with excellent amplitude and phase stabilities. In the present experiment a low‐impedance (30‐Ω) large‐diameter (13.2‐cm) annular electron beam of power ∼8 GW was modulated using an external rf source (magnetron at 1.3 GHz) of 0.5 MW power. The interaction of the modulated electron beam with a structure generated a 3‐GW rf pulse that was radiated into the atmosphere. The self‐fields of the intense beam provided significant electrostatic insulation against vacuum breakdown at the modulating gaps and at the rf extraction gap.