Emittance growth of beams during transverse combining has been studied computationally and experimentally for Heavy Ion Fusion applications, and the theory and results are presented. A hardware design is also discussed.

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+ beam transported through a 45° bend; experiments on a 90° 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.

The Heavy Ion Fusion Group at Lawrence Livermore National Laboratory has for several years been developing the worlds first circular ion induction accelerator. This machine has recently been extended to 90 degrees, or 10 half-lattice periods (HLP) with full beam transport. In addition, induction cores have been installed on five of the HLPs, each with an independent arbitrary waveform pulser. An arbitrary waveform pulser for the bending electrostatic dipoles has also been enabled. Together, they have allowed the first attempts at coordinated bending and acceleration of the beam. The results of these first attempts will be reported on in the paper by examining the output of various diagnostic devices, such as the capacitive Beam Probes (C-probes), slit scanners, and the Gated Beam Imager(GBI).

Recirculating induction accelerators (recirculators) have been investigated as possible drivers for inertial fusion energy production because of their potential cost advantage over linear induction accelerators. Point designs were obtained by Barnard et al. (UCRL-LR-108095, 1991; Phys. Fluids B Plasma Phys. 5 (1993), 2698) and many of the critical physics and technology issues that would need to be addressed were detailed. A collaboration (Friedman et al., 32-33 (1996) 235) involving Lawrence Livermore National Laboratory and Lawrence Berkeley National Laboratory researchers is now developing a small prototype recirculator in order to demonstrate an understanding of nearly all the critical beam dynamics issues that have been raised by Barnard et al. and subsequently.We review the design equations for recirculators (which have been incorporated into a MATHEMATICA-based design code) and demonstrate how, by keeping crucial dimensionless quantities constant, a small prototype recirculator was designed which will simulate the essential beam physics of a driver. We further show how important physical quantities such as the sensitivity to errors of optical elements (in both field strength and placement), insertion-extraction, vacuum requirements and emittance growth scale from small prototype to driver-size accelerator.

The U.S. Inertial Fusion Energy (IFE) Program is developing induction accelerator technology toward the goal of electric power production using Heavy-Ion beamdriven 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.[ 11 We present plans for and progress toward a small (4.5-m diameter) prototype recirculator,[2] which will accelerate K+ 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 insertiodextraction and acceleration with centroid control.

The USA is developing the physics and technology of induction accelerators for heavy ion beam-driven inertial fusion. The recirculating induction accelerator repeatedly passes beams through the same set of accelerating and focusing elements, thereby reducing both the length and gradient of the accelerator structure. This promises an attractive driver cost if the technical challenges associated with recirculation can be met. Point designs for recirculator drivers were developed in multiyear study by LLNL, LBNL and FM Technologies and that work is briefly reviewed here. To validate major elements of the recirculator concept, we are developing a small (4.5 m diameter) prototype recirculator which will accelerate a space-charge-dominated beam of K+ ions through 15 laps from 80 to 320 keV and from 2 to 8 mA. Transverse beam confinement is effected via permanent magnet quadrupoles; bending is via electric dipoles. This 'small recirculator' is being developed through a sequence of experiments. An injector, matching section and linear magnetic channel using seven half-lattice periods of permanent magnet quadrupole lenses are operational. A prototype recirculator half-lattice period is being fabricated. This paper outlines the research program and presents initial experimental results.

We review the status of the experimental campaign being carried out at Lawrence Livermore National Laboratory, involving scaled investigations of the acceleration and transport of space‐charge dominated heavy ion beams. The ultimate goal of these experiments is to help lay the groundwork for a larger scale ion driven inertial fusion reactor, the purpose of which is to produce inexpensive and clean electric power.

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+ 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

A two-year study of recirculating induction heavy-ion accelerators (recirculators) as low-cost drivers for inertial-fusion energy power plants has recently been completed. A summary of that study and other recent work on recirculators is presented.

A two-year study of recirculating induction heavy ion accelerators as low-cost driver for inertial-fusion energy applications was recently completed. The projected cost of a 4 MJ accelerator was estimated to be about $500 M (million) and the efficiency was estimated to be 35%. The principal technology issues include energy recovery of the ramped dipole magnets, which is achieved through use of ringing inductive/capacitive circuits, and high repetition rates of the induction cell pulsers, which is accomplished through arrays of field effect transistor (FET) switches. Principal physics issues identified include minimization of particle loss from interactions with the background gas, and more demanding emittance growth and centroid control requirements associated with the propagation of space-charge-dominated beams around bends and over large path lengths. In addition, instabilities such as the longitudinal resistive instability, beam-breakup instability and betatron-orbit instability were found to be controllable with careful design.

In ILSE (Induction Linac Systems Experiment), sixteen intense parallel ion beams are to be transversely combined into four by dispersionless double bends. Emittance growth due to electrostatic energy redistribution and to the geometry is evaluated. Most bending elements are electric, and alternate with AG electrostatic quadrupoles similar to those upstream. The final elements are magnetic, combining focusing and `unbending'. Electrode shapes and pulsed-current arrays (with very small clearances), as well as mechanical and electric features of the combiner, are described

Sixteen intense parallel ion beams are to be transversely combined into four by dispersionless double bends. Emittance growth due to electrostatic energy redistribution and to the geometry is evaluated. Most bending elements are electric, and alternate with AG electrostatic quadrupoles similar to those upstream. The final elements are magnetic, combining focusing and ''unbending''. Electrode shapes and pulsed-current arrays (having very small clearances), and mechanical and electric features of the combiner, and described. 1 ref., 7 figs.

This may make an array of such beams unacceptably large. A sketch of a possible combining section is shown in iig. 11 Emittance growth of beams during transverse combining has been studied computationally and experimentally for tieavy Ion Fusion applications, and the theory and results are presented. A hardware design is also discussed.

An experiment to study the physics of merging and of focusing ion beams is under development at the Lawrence Berkeley Laboratory. In this design, parallel beams of ions (C+, Al+, or Al++) are accelerated to several MV and merged transversely. The merged beams are then further accelerated and the growth in transverse and longitudinal emittance is determined for comparison with theory. The completed apparatus will be used to study problems associated with focusing ion beams to a small spot as required for heavy ion fusion. Details of the accelerator design and considerations of the physics of combining beams are presented.

The multiple-beam induction linac approach to a heavy ion fusion driver features continuous current amplification along the accelerator and a minimum of transverse beam manipulation from source to pellet. Current amplification and bunch length control require careful shaping of the accelerating voltages. This driver approach exploits developments in electron induction linac technology that have occurred within the last 15 years at LBL, LLNL and NBS. MBE-4 is a four beam induction linac that models much of the accelerator physics of the electrostatically focused section of a considerably longer induction accelerator. Four parallel Cs/sup +/ beams are electrostatically focussed and will be accelerated from 200 keV to approximately one MeV when the experiment is complete in the spring of 1987. The current in each of the four beams will increase from 10 to 40 mA due to both increase in beam speed and shortening of the bunch length. Results of experiments with the injector and first eight accelerating gaps are presented.

In the earlier stages of acceleration in a heavy‐ion‐induction linac, acceleration and bunching rates are constrained by the allowable value of head‐to‐tail velocity tilt at a given location. If focusing parameters at a given location are fixed, the velocity tilt should be less than a certain upper bound to avoid too much envelope variation and consequent beam losses. For space charge dominated beams, we found some favorable particle distributions in longitudinal phase space for which the maximum‐matched‐beam envelope at a given location is almost constant with respect to time, in spite of the presence of a large velocity tilt. Mismatch oscillations can be reduced by slow variation of the velocity tilt and slow current amplification. Under these circumstances, the velocity tilt can be as large as allowed by the usable range of σ0. Behavior of Cs ion beams with very large velocity tilts (up to 40%) are studied experimentally in MBE‐4 and the results are presented.

At the low energy end of an induction linac HIF driver the beam current is limited by our ability to control space charge by a focusing system. As a consequence, HIF induction accelerator designs feature simultaneous acceleration of many beams in parallel within a single accelerator structure. As the speed of the beams increase, the focusing system changes from electrostatic to magnetic quadrupoles with a corresponding increase in the maximum allowable current. At that point the beams are merged thereby decreasing the cost of the subsequent accelerator structure. The LBL group is developing an experiment to study the physics of merging and of focusing ion beams. In the design, parallel beams of ions (C/sup +/, Al/sup +/, or Al/sup + +/) are accelerated to several MV and merged transversely. The merged beams are then further accelerated and the growth in transverse and longitudinal emittance is determined for comparison with theory. The apparatus will then be used to study the problems associated with focusing ion beams to a small spot. Details of the accelerator design and considerations of the physics of combining beams are presented.

Preliminary results are presented from a scaled experimental multiple beam induction linac. This experiment is part of a program of accelerator research for heavy ion fusion. It is shown that multiple beams can be accelerated without significant mutual interaction. Measurements of the longitudinal dynamics of a current‐amplifying induction linac are presented and compared to calculations. Coupling of transverse and longitudinal dynamics is discussed.