S. Eylon

Plasma lens focusing and plasma channel transport for heavy ion fusion

Abstract The capabilities of adiabatic, current-carrying plasma lenses for the final focus problem in heavy-ion-beam-driven inertial confinement fusion are explored and compared with the performance of non-adiabatic plasma lenses, and with that of conventional quadrupole lenses. A final focus system for a fusion reactor is proposed, consisting of a conventional quadrupole lens to prefocus the driver beams to the entrance aperture of the adiabatic lens, the plasma lens itself, and a high current discharge channel inside the chamber to transport the focused beam to the fusion pellet. Two experiments are described that address the issues of adiabatic focusing, and of transport channel generation and stability for ion beam transport. The test of the adiabatic focusing principle shows a 26-fold current density increase of a 1.5 MeV potassium ion beam during operation of the lens. The lens consist of a discharge of length 300 mm, filled with helium gas at a pressure of 1 Torr and is pulsed with a current between 5 and 15 kA. The investigations of discharge channels for ion beam transport show that preionization of the discharge channels with a UV laser can be an efficient way to direct and stabilize the discharge.

Beam dynamics studies with the heavy-ion linear induction accelerator MBE-4

Current amplification of heavy-ion beams is an integral feature of the induction linac approach to heavy-ion fusion (HIF). In this paper we report on amplification experiments conducted on a single beam of the Multiple Beam Experiment (MBE-4), a heavy-ion (Cs+) induction linac. Earlier MBE-4 experiments [H. Meuth et al., Nucl. Instrum. Methods Phys. Res. A 278, 153 (1989)] had demonstrated up-to-9× current amplification but had been accompanied by an up-to-2× increase of normalized transverse emittance. Experiments to pinpoint the causes of this emittance growth indicated various factors were responsible, including focusing aberrations and mismatch difficulties between the injector diode and the accelerator transport lattice, a localized quadrupole misalignment problem, and the interaction of transversely large beams with the nonlinear elements of the focusing lattice. Following ameliorative measures, new current amplification experiments, both with and without acceleration, showed that current amplificat...

Comparison of experimental data and three-dimensional simulations of ion beam neutralization from the Neutralized Transport Experiment

The Neutralized Transport Experiment at Lawrence Berkeley National Laboratory has been designed to study the final focus and neutralization of high perveance ion beams [E. Henestroza, S. Eylon, P. Roy, S. Yu, A. Anders, F. Bieniosek, W. Greenway, B. Logan, R. MacGill, D. Shuman et al., Phys. Rev. ST-Accel. Beams 7, 083501 (2004)]. Preformed plasmas in the last meter before the target of the scaled experiment provide a source of electrons which neutralize the ion current and prevent the space-charge-induced spreading of the beam spot. Neutralized Transport Experiment physics issues are discussed and experimental data are analyzed and compared with three-dimensional (3D) particle-in-cell simulations. Along with detailed target images, 4D phase-space data at the entrance of the neutralization region have been acquired. These data are used to provide a more accurate beam distribution with which to initialize the simulation. Previous treatments have used various idealized beam distributions which lack the deta...

Numerical simulation of intense-beam experiments at LLNL and LBNL

We present intense-beam simulations with the WARP code that are being carried out in support of the Heavy-Ion Fusion experimental programs at Lawrence Livermore National Laboratory (LLNL) and Lawrence Berkeley National Laboratory (LBNL). The WARP code is an electrostatic particle-in-cell code with an extensive hierarchy of simulation capabilities. Two experiments are analyzed. First, simulations are presented on an 80 keV, 2 mA K‘ bent transport channel at LLNL that employs an alternating-gradient lattice of magnetic quadrupoles for beam focusing and electric dipoles for beam bending. Issues on dispersion-induced changes in beam quality on the transition from straight- to bent-lattice sections are explored. The second experiment analyzed is a 2 MeV, 800 mA, driver-scale injector and matching section at LBNL that is based on a K‘ source and an alternating-gradient lattice of electrostatic quadrupoles biased to accelerate, focus, and match the beam. Issues on beam quality, space-charge waves, and beam hollowing are explored. Published by Elsevier Science B.V.

Progress toward a prototype recirculating induction accelerator for heavy-ion fusion

The US Inertial Fusion Energy (IFE) Program is developing induction accelerator technology toward the goal of electric power production using heavy-ion beam-driven inertial fusion (HIF). The recirculating induction accelerator promises driver cost reduction by repeatedly passing the beam through the same set of accelerating and focusing elements. We present plans for and progress toward a small (4.5-m diameter) prototype recirculator, which will accelerate K/sup +/ ions through 15 laps, from 80 to 320 keV and from 2 to 8 mA. Beam confinement is effected via permanent-magnet quadrupoles; bending is via electric dipoles. Scaling laws, and extensive particle and fluid simulations of the space-charge dominated beam behavior, have been used to arrive at the design. An injector and matching section are operational. Initial experiments are investigating intense-beam transport in a linear magnetic channel; near-term plans include studies of transport around a bend. Later experiments will study insertion/extraction and acceleration with centroid control.

2 MV injector as the Elise front-end and as an experimental facility

Abstract We report on progress in the preparation of the 2 MV injector at LBNL as the front end of Elise and as a multipurpose experimental facility for heavy ion fusion beam dynamics studies. Recent advances in the performance and understanding of the injector are described, and some of the ongoing experimental activities are summarized.

Focusing and neutralization of intense beams

In heavy ion inertial confinement fusion systems, intense beams of ions must be transported from the exit of the final focus magnet system through the target chamber to hit millimeter spot sizes on the target. Effective plasma neutralization of intense ion beams through the target chamber is essential for the viability of an economically competitive heavy ion fusion power plant. The physics of neutralized drift has been studied extensively with PIC simulations. To provide quantitative comparisons of theoretical predictions with experiment, the Heavy Ion Fusion Virtual National Laboratory has completed the construction and has begun experimentation with the NTX (Neutralized Transport Experiment) as shown in Figure 1. The experiment consists of 3 phases, each with physics issues of its own. Phase 1 is designed to generate a very high brightness potassium beam with variable perveance, using a beam aperturing technique. Phase 2 consists of magnetic transport through four pulsed quadrupoles. Here, beam tuning as well as the effects of phase space dilution through higher order nonlinear fields must be understood. In Phase 3, a converging ion beam at the exit of the magnetic section is transported through a drift section with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we present first results from all 3 phases of the experiment.

Electron-beam diagnostic for space-charge measurement of an ion beam

An electron beam diagnostic system for measuring the charge distribution of an ion beam without changing its properties is presently under development for Heavy Ion Fusion (HIF) beam physics studies. Conventional diagnostics require temporary insertion of sensors into the beam, but these capture it, or significantly alter its properties. In this new diagnostic a low energy, low current electron beam is scanned transversely across the ion beam; the measured electron beam deflection is used to calculate the line-integrated charge density of the ion beam, assuming at present a circular charge distribution that is functionally dependent only on radius. The initial application of this diagnostic is being made to the Neutralized Transport Experiment (NTX), which is exploring the physics of space charge dominated beam focusing through neutralizing plasma onto a small spot. The diagnostic system is able to scan an ion beam of up to 3 cm radius. Design and performance of this diagnostic system is presented.

Initial commissioning results of the RTA injector

The creation of the drive beam remains one of the most challenging technical endeavours in constructing two-beam accelerators. We have begun testing the 1.2-kA, 1.0-MeV electron induction injector for the RTA experiment. The electron source is a 3.5-inch diameter, thermionic, flat-surface cathode with a maximum shroud field stress of approximately 165 kV/cm. The pulse length of the injector is approximately 250 ns, with a 120-ns flattop region. We report here measurements of the pulsed power system performance, beam voltage and current. Plans to measure the emittance and current density profile are discussed.

Status of experiments leading to a small recirculator

A heavy ion linear induction accelerator is considered to be the leading driver candidate for an Inertial Fusion Energy reactor. To deliver a space-charge-dominated beam at the appropriate energy (several GeV), such an accelerator would be several kilometers in length. Since total length has a strong influence on accelerator cost, we are considering the potential advantages and practical implementation of a recirculating induction accelerator. To address the critical scientific and technical challenges of a recirculating space-charge-dominated heavy ion beam, we have begun to develop the elements of a scaled ``small recirculator``. An operating recirculator must demonstrate full beam control including multi-lap operation, beam insertion/extraction, acceleration and pulse compression. At present, experiments have been conducted using a 2mA, 80keV K{sup +} beam transported through a 45{degree} bend; experiments on a 90{degree} bend with five induction modulators will begin soon. This paper briefly summarizes the recirculator specifications and operational features and reports the latest experimental data as well as the developmental status of beam diagnostics.