J.W. Kwan

A merging preaccelerator for high current H− ion beams

The high power ion beams used in the next generation thermonuclear fusion reactors require high current negative ion beams accelerated to high energy, with high efficiency. One way to meet these requirements is to merge multiple low current density H− beamlets into a single high current beam. The feasibility of a high current merging preaccelerator was demonstrated in this experiment by merging 19 beamlets of H− ions distributed over a circular area 80 mm in diameter from a Japan Atomic Energy Research Institute negative ion source. H− ions were extracted at a current density exceeding 10 mA/cm2 at the ion source which operates at 0.13 Pa (1 mTorr), with a low arc power density (70 V×250 A). Spherically curved grids (with built‐in magnetic electron suppression) were used in the preaccelerator to focus the extracted beamlets into a single 104 mA, 100 keV beam. The merged beam has a diameter of 23 mm and a converging angle of ±30 mrad at the beam envelope. The rms emittance of the 104 mA merging beam was 1....

Operation of a dc large aperture volume‐production H− source

In testing a multicusp volume‐production H− ion source (20 cm diameter, 23 cm long), we optimized the gas pressure, the plasma electrode bias potential, and the magnetic filter. At the optimum pressure of 9 mTorr, the H− beam output increased linearly with discharge power. The maximum H− beam, measured with a current transformer downstream of the accelerator, was 100 mA while using a 6.67‐cm2 aperture. Presently we are limited by overheating of the cathodes by the plasma ions. Under similar discharge conditions the maximum H− current density was found to vary as a−0.7, where a is the aperture radius. Results from emittance measurements showed that the effective H− ion temperature increased with a for a>0.8 cm. Thus, the brightness of the beam decreased with increasing aperture radius. Operating the source with cesium would increase the H− output; however, our accelerator must be improved to avoid breakdowns caused by the cesium contamination.

Planning for an integrated research experiment

We describe the goals and research program leading to the Heavy Ion Integrated Research Experiment (IRE). We review the basic constraints which lead to a design and give examples of parameters and capabilities of an IRE. We also show design tradeoffs generated by the systems code IBEAM.

Development of a RF-Driven Neutron Generator for Associated Particle Imaging

We present recent work in the development of a compact, radio frequency (RF) D-T neutron generator with a maximum neutron yield of 108 n/s for the application of associated particle imaging (API) used in explosive and contraband detection. API makes use of alpha particles produced in conjunction with 14 MeV neutrons in the D-T reaction to locate the neutron interaction and reduce background noise. To achieve high spatial resolution in API, a beam diameter of les1 mm on target is desired. For portable neutron generators used in API, the ion source and target cannot be water-cooled and the power deposited on the target must be low. By increasing the atomic ion fraction, the ion beam can be used more efficiently to generate neutrons, resulting in a lower beam power requirement and an increased lifetime of the alpha detector inside the acceleration column. The RF ion source produces very high atomic ion fractions (> 90%) compared to traditional Penning ion sources that produce less than 10% of atomic ions. Experimental measurements of the ion source plasma parameters including ion current density, atomic ion fraction, ignition and operating pressures, will be presented along with a discussion on the ion optics and engineering challenges.

Ion source electrode biasing technique for microsecond beam pulse rise times

Heavy ion fusion (HIF) induction accelerators require ion sources that can deliver intense heavy ion beams with low emittance. The typical pulse length is 20 μs with a rise time less than 1 μs and a repetition rate of 10 Hz. So far, the surface ionization sources have been used in most HIF induction linac designs. However, there are other ions of interest to HIF (e.g., Hg, Xe, Rb, Ar, and Ne) which cannot be produced by the surface ionization sources, but rather by volume ion sources. In this paper, we describe an experiment that uses a multicusp source with a magnetic filter to produce beam pulses that have a rise time in the order of 1 μs. By applying a positive biasing pulse on the plasma electrode with respect to the source body, the positive plasma ions can be temporarily repelled from the neighborhood of the extraction aperture, leading to a suppression of the ion beam. As the bias is removed, positive ions flow to the extraction region, enabling a fast-rising beam pulse. The beam current pulses sho...

Initial Testing of a Compact Portable Microwave-Driven Neutron Generator

A portable, moderate yield D-D neutron generator based on permanent magnet microwave ion source is being developed at Lawrence Berkeley National Laboratory for applications such as short range SNM detection in suitcases and small parcels. Microwave power (2.45 GHz) can be coupled to a pyrex tube through a standard wave guide and generate plasma. In this source configuration, the wave guide serves as a secondary containment for sealed tube design. Hydrogen plasma has been successfully ignited in the pyrex tube at a microwave power of 200 W. The 2.45 GHz microwave signal can also be directly coupled to the wave guide through a standard coaxial cable with a N-type connection and generate plasma. Preliminary results show that over 60% of atomic hydrogen ions were generated at a microwave power of 300 W. Higher atomic fraction is expected with insertion of boron nitride liner. The current density at power of 200 W (with an extraction aperture of 2 mm in diameter, and gas flow at 0.2 sccm) was approximately 22 mA/cm2.

High current injectors for heavy ion driven inertial fusion

Conceptual heavy ion driven inertial fusion (HIF) drivers typically have an array of up to 100 parallel beams each supplying a beam current of ≈0.25 A. According to space-charge limitations in beam extraction and in the low energy beam transport section, there are two options in building injectors for HIF drivers. The traditional way is to use low current density, large aperture, contact ionization sources. The major disadvantage of this approach is the very large size of the injector and matching section. The other option is to use high current density, multiple beamlet ion sources. From various scaling rules, it is found that the multiple beamlet approach is the more attractive one because it can be smaller, and more efficient, although the requirements on the ion source are more demanding.

Developing high brightness beams for heavy ion driven inertial fusion

Heavy ion fusion (HIF) drivers require large currents and bright beams. In this paper we review the two different approaches for building HIF injectors and the corresponding ion source requirements. The traditional approach uses large aperture, low current density ion sources, resulting in a very large injector system. A more recent conceptual approach merges high current density mini-beamlets into a large current beam in order to significantly reduce the size of the injector. Experiments are being prepared to demonstrate the feasibility of this new approach.

A high power long pulse RF-driven H/sup -/ source

We have tested the radio-frequency driven H/sup -/ source and have shown that the H/sup -/ production efficiency and the beam emittance are similar to those obtained from the filament discharge. Typically the numbers are 2.8 mA/cm/sup 2kW and 0.017 /spl pi/-mrad-cm (which corresponds to 1.9 eV) respectively. So far we have operated RF pulses of /spl ap/10 kW for /spl ap/50 ms with a porcelain-coated antenna and /spl ap/15 kW for /spl ap/1 s with additional layers of quartz sleeving. It is necessary to develop better antenna coating material that can withstand the intense plasma heating and sputtering in order to operate at higher power with longer pulse length.<<ETX>>

Initial Evaluation of a Pulsed White Spectrum Neutron Generator for Explosive Detection

Successful explosive material detection in luggage and similar sized containers is a critical issue in securing the safety of all airline passengers. Tensor Technology Inc. has recently developed a methodology that will detect explosive compounds with pulsed fast neutron transmission spectroscopy. In this scheme, tritium beams will be used to generate neutrons with a broad energy spectrum as governed by the T(t,2n)4He fission reaction that produces 0-9 MeV neutrons. Lawrence Berkeley National Laboratory, in collaboration with Tensor Technology Inc., has designed and fabricated a pulsed white-spectrum neutron source for this application. The specifications of the neutron source are demanding and stringent due to the requirements of high yield and fast pulsing neutron emission, and sealed tube, tritium operation. In a unique co-axial geometry, the ion source uses ten parallel rf induction antennas to externally couple power into a toroidal discharge chamber. There are 20 ion beam extraction slits and three concentric electrode rings to shape and accelerate the ion beam into a titanium cone target. Fast neutron pulses are created by using a set of parallel-plate deflectors switching between plusmn750 V and deflecting the ion beams across a narrow slit. The generator is expected to achieve 5 ns neutron pulses at tritium ion beam energies between 80-120 kV. First experiments demonstrated ion source operation and successful beam pulsing.