Denis Keefe

Measurements of Stability Limits for a Space-Charge-Dominated Ion Beam in a Long a. G. Transport Channel

The Single Beam Transport Experiment at LBL consists of 82 electrostatic quadrupole lenses arranged in a FODO lattice. Five further lenses provide a matched beam from a high-current high-brightness cesium source for injection into the FODO channel. We call the transport conditions stable if both the emittance and current remain unchanged between the beginning and end of the channel, and unstable if either the emittance grows or the current decreases because of collective effects. We have explored the range of single-particle betatron phase advance per period from ¿0 = 45° to 150° to determine the stability limits for the space-charge depressed phase advance, ¿. No lower limit for ¿ (down to 7°) has been found at ¿0= 60°, whereas limits have clearly been identified and mapped in the region of ¿0 above 90°.

Heavy-Ion Linear Induction Accelerators as Drivers for Inertial Fusion Power Plants

A linear induction accelerator that produces a beam of energetic heavy ions (T approx. = 10 GeV, A approx. = 200 am..mu..) is a prime candidate as a driver for an inertial fusion power plant. Some early perceptions were that heavy-ion driven fusion would not be cost-competitive with other power sources because of the high cost of the accelerators. However, improved understanding of the physics of heavy-ion transport and acceleration (supported by experimental results), combined with advances in accelerator technology, have resulted in accelerator design costs -- 50% of previous estimates. As a result, heavy-ion driven fusion power plants conceptual fusion power plants. A brief formulation of transport and acceleration physics is presented here, along with a description of the induction Linac cost optimization code LIACEP. Cost trends are presented and discussed, along with specific cost estimates for several accelerator designs matched to specific inertial fusion target yields. Finally, a cost-effective strategy using heavy-ion induction Linacs in a development scenario for inertial fusion is presented.

Shattering Rock with Intense Bursts of Energetic Electrons

It has been successfully demonstrated that intense bursts of energetic electrons cause significant rock spalling for modest energy inputs. The corresponding temperature rise per pulse in the bombarded volume of rock is only ~ 200° C, or so. Some analytical predictions and experimental evidence of this novel accelerator application are presented. The promise of this technique for more rapid and economical tunneling through rock is also examined.

Design/Cost Study of an Induction Linac for Heavy Ions for Pellet-Fusion

The physics of the pellet implosion sets stringent conditions on the accelerator driver. The beam energy should be > 1 MJ, the beam power > 100 TW (implying a pulse length approx. = 10 ns), and the specific energy deposition in the pellet > 20 MJ/g. Thus, considerable current amplification is required, e.g. from some 10 amps at the source to perhaps 10 kiloamps at the pellet. Most of this amplification can be accomplished continuously along the accelerator and the remainder achieved at the end by bunching in the final transport lines to the target chamber. A conceptual schematic of an Induction Linac Fusion Driver is shown, which includes an injector, an accelerator-buncher, and a final transport system. Here only the accelerator portion of the driver is discussed.

Long-Pulse Induction Acceleration of Heavy Ions

A long-pulse induction acceleration unit has been installed in the high-current Cs+ beam line at LBL and has accelerated heavy ions. A maximum energy gain of 250 keV for 1.5 ¿s is possible. The unit comprises 12 independent modules which may be used to synthesize a variety of waveforms by varying the triggering times of the low voltage trigger generators.

A Quadrupole Beam Transport Experiment for Heavy Ions under Extreme Space Charge Conditions

A Cs ion-beam transport experiment is in progress to study beam behavior under extreme space charge conditions. A five-lens section matches the beam into a periodic electrostatic quadrupole FODO channel and its behavior is found to agree with predictions. With the available parameters (¿200 keV, ¿20 mA, ¿¿n ¿ 10-7 ¿ rad-m, up to 41 periods) the transverse (betatron) oscillation frequency (¿) can be depressed down to one-tenth of its zero current value (¿o), where ¿2 = ¿2o - ¿2p/2, and ¿p is the beam plasma frequency. The current can be controlled by adjustment of the gun and the emittance can be controlled independently by means of a set of charged grids.

MBE-4, a Heavy Ion Multiple-Beam Experiment

MBE-4, a heavy-ion multiple beam induction linac being built at LBL in FY85/86, will model many features of a much longer device. It will accelerate four spacecharge-dominated Cesium ion beams from, for example, 0.2 MeV, 5 mA/beam, 3.0 ¿sec, 1.6 m length at injection to ~0.8 MeV, 15 mA/beam, 1.0 ¿sec, 1.1 m length at the exit. It will permit study of simultaneous focussing, acceleration, current amplification and emittance growth of multiple space-charge-dominated ion beams. Some features of this accelerator are described.

The cost of induction Linac drivers for inertial fusion for various target yields

The cost of induction linac accelerators for inertial fusion using mass 200 ions at a charge state of +3 for target yields of 300, 600, and 1200 MJ is presented. The ions are injected into the accelerator at 3 MV, and accelerated to the required voltage appropriate to the desired target yield. A cost comparison of the low voltage portion of the accelerator (3–50 MV) is made between a system with 64 and one with 16 superconducting quadrupoles. The design of the low voltage portion which yields the minimum‐cost accelerator designs for several target yields and a fusion power of 3000 MW is presented.

Experiments and prospects for induction linac drivers

In the last three years, the US program in Heavy Ion Fusion has concentrated on understanding the induction linac approach to a power-plant driver. In this method it is important that the beam current be maximized throughout the accelerator. Consequently, it is crucial to understand the space-charge limit in the AG transport system in the linac and, also, to achieve current amplification during acceleration to keep pace with the kinematical increase of this limit with energy. Experimental results on both these matters and also on the use of multiple beams (inside the same accelerating structure) will be described. A new examination of the most attractive properties of the induction linac for a fusion driver has clearly pointed to the advantage of using heavy ions with a charge-state greater than unity - perhaps q = 3 may be an optimum. This development places even greater importance on understanding space-charge limits and mechanisms for emittance growth; also, it will require a new emphasis on the development of a suitable ion source.

Developments in Accelerators for Heavy Ion Fusion

The long term goal of Heavy Ion Fusion (HIF) is the development of an accelerator with the large beam power, large beam stored-energy, and high brightness needed to implode small deuterium-tritium capsules for fusion power. While studies of an rf linac/storage ring combination as an inertial fusion driver continue in Japan and Europe, the US program in recent times has concentrated on the study of the suitability of linear induction acceleration of ions for this purpose. Novel features required include use of multiple beams, beam current amplification in the linac, and manipulation of long beam bunches with a large velocity difference between head and tail. Recent experiments with an intense bright beam of cesium ions have established that much higher currents can be transported in a long quadrupole system than was believed possible a few years ago. A proof-of-principle ion induction linac to demonstrate beam current amplification with multiple beams is at present being fabricated at LBL.