Search Results (7 total)

Currently ongoing at Los Alamos National Laboratory is a program to develop high-power, planar 100–300 GHz traveling-wave tubes. A necessary part of this effort is a sheet electron beam source. Previously, we have described a novel asymmetric solenoid lens concept for transforming the circular beam from a high-perveance electron gun to a planar configuration. The lens is a standard electromagnetic solenoid with elliptical, instead of circular, pole apertures. The elliptical pole openings result in asymmetric focusing, which in turn forms an elliptical sheet beam suitable for our planar structures. Here we report the first experimental demonstration of this lens.

Two-plane focusing of sheet electron beams will be an essential technology for an emerging class of high-power, 100 to 300 GHz rf sources [Carlsten , IEEE Trans. Plasma Sci. 33, 85 (2005)]. In these devices, the beam has a unique asymmetry in which the transport is emittance dominated in the sheet’s thin dimension and space-charge dominated in the sheet’s wide dimension. Previous work has studied the stability of the transport of beams in the emittance-dominated regime for both wiggler and periodic permanent magnet (PPM) configurations with single-plane focusing, and has found that bigger envelope scalloping occurs for equilibrium transport, as compared to space-charge dominated beams [Carlsten , this issue, Phys. Rev. ST Accel. Beams 8, 062001 (2005)]. In this paper, we describe the differences in transport stability when two-plane focusing is included. Two-plane wiggler focusing degrades the transport stability slightly, whereas two-plane PPM focusing greatly compromises the transport. On the other hand, single-plane PPM focusing can be augmented with external quadrupole fields to provide weak focusing in the sheet’s wide dimension, which has stability comparable to two-plane wiggler transport.

A sheet-beam traveling-wave amplifier has been proposed as a high-power generator of rf from 95 to 300 GHz, using a microfabricated rf slow-wave structure [Carlsten , IEEE Trans. Plasma Sci. 33, 85 (2005)], for emerging radar and communications applications. The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power, high-average power, and wide bandwidths. Simulations of nominal designs using a vane-loaded waveguide as the slow-wave structure have indicated gains in excess of 1   dB/mm, with extraction efficiencies greater than 20% at 95 GHz with a 120-kV, 20-A electron beam. We have identified stable sheet-beam formation and transport as the key enabling technology for this type of device. In this paper, we describe sheet-beam transport, for both wiggler and periodic permanent magnet (PPM) magnetic field configurations, with natural (or single-plane) focusing. For emittance-dominated transport, the transverse equation of motion reduces to a Mathieu equation, and to a modified Mathieu equation for a space-charge dominated beam. The space-charge dominated beam has less beam envelope ripple than an emittance-dominated beam, but they have similar stability thresholds (defined by where the beam ripple continues to grow without bound along the transport line), consistent with the threshold predicted by the Mathieu equation. Design limits are derived for an emittance-dominated beam based on the Mathieu stability threshold. The increased beam envelope ripple for emittance-dominated transport may impact these design limits, for some transport requirements. The stability of transport in a wiggler field is additionally compromised by the beam’s increased transverse motion. Stable sheet-beam transport with natural focusing is shown to be achievable for a 120-kV, 20-A, elliptical beam with a cross section of 1 cm by 0.5 mm, with both a PPM and a wiggler field, with magnetic field amplitude of about 2.5 kG.

The problem of the neutralization of one-dimensional ion beams moving through a source of cold electrons into free space has been studied with the use of a computer simulation of the electron dynamics. The results indicate that neutralized high-intensity beams of ions appropriate for inertial-confinement fusion may be able to propagate to a focus through vacuum with negligible limits imposed by the electron pressure.

The production, focusing, and numerical simulation of a 0.5-TW proton beam is reported. This beam is produced with a spherical, magnetically insulated, ion diode fed symmetrically by the dual-pulse-line Proto I generator. The ions are accelerated with electric fields due to a virtual cathode supported by magnetic field surfaces. Approximately 75% of the diode electrical power is delivered to ions and 25% of the ion beam is focused upon thin, 1-cm-diam, 1-cm-long conical targets to produce the first experimental ion-driven implosions.

We have obtained initial geometric-focusing results using a high-current (≳ 100 kA) magnetically insulated diode. At the line focus, current densities over 300 A/cm² have been obtained with a radial compression of about 10. Results for propagation of the intense beams are in excellent agreement with geometric single-particle predictions.