J. Reijonen

rf ion source development for neutron generation and for material modification

rf driven multicusp ion sources have been successfully used in various different applications. Lately the Plasma and Ion Source Technology Group at Lawrence Berkeley National Laboratory has been developing a rf ion source for neutron production and a high current density cw-operated ion source for SIMOX (Separation by Implantation of Oxygen)-application. The group has developed a small ion source, which consists of a quartz plasma chamber, an external rf-antenna, an extraction electrode, and a target assembly, all in a tube that is approximately 25 cm in length and 5 cm in diameter. Another neutron generator currently under development is a multiaperture, high power generator. The neutron generator currently operates at 1% duty cycle, 80 kV, and 150 mA of deuterium beam. The neutron yield measured from the generator is 1.6×107 n/s. For oxygen implantation, the group has been developing a source which could provide a high percentage of O+ at high current density using cw operation. A dual-antenna has been ...

Fast ion beam chopping system for neutron generators

Fast deuterium (D+) and tritium (T+) ion beam pulses are needed in some neutron-based imaging systems. A compact, integrated fast ion beam extraction and chopping system has been developed and tested at the Lawrence Berkeley National Laboratory for these applications, and beam pulses with 15ns full width at half maximum have been achieved. Computer simulations together with experimental tests indicate that even faster pulses are achievable by shortening the chopper voltage rise time. This chopper arrangement will be implemented in a coaxial neutron generator, in which a small point-like neutron source is created by multiple 120keV D+ ion beams hitting a titanium target at the center of the source.

Ion-source and low-energy beam-transport issues with the front-end systems for the spallation neutron source

The front-end systems (FES) of the spallation neutron source project are being built by Berkeley Lab and will deliver a pulsed 40 mA H− ion beam at 2.5 MeV energy to the subsequent drift-tube linac. The FES accelerator components comprise a rf driven, volume-production, cesium-enhanced, multicusp ion source; an electrostatic low-energy beam transport (LEBT) that includes provisions for transverse focusing, steering, and beam chopping; a radio-frequency quadrupole accelerator; and a medium-energy beam transport line. The challenges for ion source and LEBT design are the generation of a plasma suitable for creating the required high H− ion density, lifetime of the rf antenna at 6% duty factor, removal of the parasitic electron population from the extracted negative ions, and emittance conservation. The article discusses these issues in detail and highlights key experimental results obtained so far.

Development of advanced neutron/gamma generators for imaging and active interrogation applications

We report here on the development of neutron and photon sources for use in imaging and active interrogation applications, where there is a growing urgency for more advanced interrogation tools. These devices include high yield D-D, D-T and T-T fusion reaction based neutron generators and also low energy nuclear reaction based high-energy gamma generators. One common feature in these various devices is the use of a high-efficiency, RF-induction discharge ion source. This discharge method provides high plasma density for high output current, high atomic species from molecular gases for high efficiency neutron or gamma generation and long lifetime. Predictable discharge characteristics of these plasma generators allow accurate modeling for both the beam dynamics and for the heat loads at the target spot. Current status of the neutron and gamma generator development with experimental data will be presented.

Experiments with planar inductive ion source meant for creation of H+ beams

In this article the effects of different engineering parameters of rf-driven ion sources with an external spiral antenna and a quartz rf window are studied. This article consists of three main topics: the effect of source geometry on the operation gas pressure, the effect of source materials and magnetic confinement on extracted current density and ion species, and the effect of different antenna geometries on the extracted current density. The effect of source geometry was studied using three cylindrical plasma chambers with different inner diameters. The chamber materials were studied using two materials, aluminum (Al) and alumina (Al(2)O(3)). The removable 14 magnet multicusp confinement arrangement enabled us to compare the effects of the two wall materials with and without the magnetic confinement. The highest measured proton fractions were measured using Al(2)O(3) plasma chamber and no multicusp confinement. For the compared ion sources the source with multicusp confinement and Al(2)O(3) plasma chamber yields the highest current densities. Multicusp confinement increased the maximum extracted current by up to a factor of 2. Plasma production with different antenna geometries were also studied. The highest current density was achieved using 4.5 loop solenoid antenna with 6.0 cm diameter. A slightly lower current density with lower pressure was achieved using a tightly wound 3 loop spiral antenna with 3.3 cm inner diameter and 6 cm outer diameter.

Fast Slit-beam extraction and chopping for neutron generator

High-intensity fast white neutron pulses are needed for pulsed fast neutron transmission spectroscopy (PFNTS). A compact tritium–tritium fusion reaction neutron generator with an integrated ion beam chopping system has been designed, simulated, and tested for PFNTS. The design consists of a toroidal plasma chamber with 20 extraction slits, concentric cylindrical electrodes, chopper plates, and a central titanium-coated beam target. The total ion beam current is 1A. The beam chopping is done at 30keV energy with a parallel-plate deflector integrated with an Einzel lens. Beam pulses with 5ns width can be achieved with a 15ns rise/fall time ±1500V sweep on the chopper plates. The neutrons are produced at 120keV energy. A three-dimensional simulation code based on Vlasov iteration was developed for simulating the ion optics of this system. The results with this code were found to be consistent with other simulation codes. So far we have measured 50ns ion beam pulses from the system.

Plasma ignition schemes for the Spallation Neutron Source radio-frequency driven H- source

The H− ion source for the Spallation Neutron Source (SNS) is a cesiated, radio-frequency driven multicusp volume source which operates at a duty cycle of 6%. In pulsed rf driven plasma sources, ignition of the plasma affects the stability of source operation and the antenna lifetime. We report on ignition schemes, based on secondary electron generation by UV light, a hot filament, a low power rf plasma (cw, 13.56 MHz), as well as source operation solely with the high power 2 MHz rf. We find that the dual frequency, single antenna scheme is most attractive for the operating conditions of the SNS H− source.

Microwave Ion source for low charge state Ion production

Microwave Ion Source for Low Charge State Ion Production * J. Reijonen † , M. Eardley, R. Gough, K. Leung, R. Thomae Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 5-119, Berkeley, CA 94720, USA Abstract. The Plasma and Ion Source Technology Group at LBNL have developed a microwave ion source. The source consists of a stainless-steel plasma chamber, a permanent-magnet dipole structure and a coaxial microwave feed. Measurements were carried out to characterize the plasma and the ion beam produced in the ion source. These measurements included current density, charge state distribution, gas efficiency and accelerated beam emittance measurements. Using a computer controlled data acquisition system a new method of determining the saturation ion current was developed. Current density of 3-6 mA/cm 2 was measured with the source operating at the over dense mode. The highest measured charge-states were Ar 5+ , O 3+ and Xe 7+ . Gas efficiency was measured using a calibrated argon leak. Depending on the source pressure and discharge power, more than 20% total gas efficiency was achieved. The emittance of the ion beam was measured by using a pepper-pot device. Certain spread was noticed in the beam emittance in the perpendicular direction to the source dipole field. For the parallel direction to the magnetic field, the normalized rr’ emittance of 0.03 π-mm-mrad at 13 kV of acceleration voltage and beam exit aperture of 3-mm-in-diameter was measured. This compares relatively well with the simulated value of 4rms, normalized emittance value of 0.024 π-mm- mrad. INTRODUCTION Single to low charge state ion beams with good beam quality and ion sources with good gas efficiency are needed in various applications, ranging from radio- active ion beam production to high energy ion implanters. A simple permanent-magnet microwave ion source has been developed to fulfill the various requirements. 1. SOURCE DESIGN The plasma chamber of the microwave ion source was made from stainless-steel. Three vacuum feed- through ports were designed: one to the opposite side of the extraction and two facing each other in the middle of the magnet-rings. The two facing ports were designed to be different in length. This enabled one to test different lengths of the coaxial antenna inside the source. The other two ports were used for gas feed and for a window port. The magnets were attached around the source using an iron yoke. They were floating around the source chamber so that the cooling air was flowing between the source body and the magnets. This arrangement maintained the temperature of the magnets reasonably low. The magnet rings were constructed from a SmCo magnet blocks measuring 1.3 cm, 2.5 cm and 3.8 cm in width, height and depth respectively. To have a ring configuration, using bar magnets, the bar ends close to the feed-trough port were touching each other, but the other ends of the magnet bars were separated. To achieve a fairly uniform field, the magnet poles of the opposite sides were rotated slightly so that the magnets in the poles did not face each other. In figure 1 a schematic drawing of the micro-wave ion source is shown. Initially the magnetic field (B dip (d)) using a yoke, as a function of the distance between the poles was approximately given by This Work was supported by Department of Energy under Contract No. DE-AC03-76SF00098 email: jreijonen@lbl.gov

Improvement of the lifetime of radio frequency antenna for plasma generation

At Lawrence Berkeley National Laboratory different antenna protection schemes have been investigated for the radio frequency-driven multicusp ion source. It was found that the antenna lifetime can be greatly enhanced by an additional shielding, which consists of porcelain, quartz or boron nitride. Different antenna configurations and their influence on the plasma generation will be discussed. Antenna life time greater than 500 hours continuous wave operation has been demonstrated in hydrogen plasma using a novel quartz antenna design.

The use of prompt gamma activation analysis (PGAA) for the analyses and characterization of materials: Photochromic materials

Neutron-induced prompt gamma activation analysis (PGAA) has been used to analyze model photochromic host materials that contain low concentrations of elemental dopants. Analyses are also presented for typical low concentration compounds used as dopants in inorganic photochromic materials. Elemental analytical results are given and discussed, along with sensitivity values for the appropriate elements. The significance of several of the trace metals ions found to be present in the materials is discussed.