We describe the development of a Cyclotron Mass Spectrometer (CMS) for the detection of trace amounts of rare isotopes. A compact low energy cyclotron optimized for high mass resolution has been designed and fabricated. The CMS has high sensitivity and is designed to measure 14C at abundances of 10-12. A novel feature of the CMS is the use of permanent magnets to energize the 30 cm diameter iron poles of the cyclotron. Axial injection is used, employing a spiral inflector. Preliminary measurements of the magnetic field show that it has a uniformity on the order of 7 parts in 104 over most of the acceleration region.

Multicusp ion sources are capable of producing positive and negative ions with good beam quality and low energy spread. The ion energy spread of multicusp sources has been measured by three different techniques. The axial ion energy spread has been reduced by introducing a magnetic filter inside the multicusp source chamber which adjusts the plasma potential distribution. The axial energy spread is further reduced by optimizing the source configuration. Values as low as 0.8 eV have been achieved.

A multicusp ion source has been designed for use in ion projection lithography. Longitudinal energy spreads of the extracted positive hydrogen ion beam have been studied using a retarding field energy analyzer. It has been found that the filament-discharge multicusp ion source can deliver a beam with an energy spread less than 3 eV which is required for the ALG-1000 machine. The multicusp ion source can also deliver the current required for the application.

Axial energy spread and useful beam current of positive ion beams have been carried out using a radio frequency (rf)-driven multicusp ion source. Operating the source with a 13.56 MHz induction discharge, the axial energy spread is found to be approximately 3.2 eV. The extractable beam current of the rf-driven source is found to be comparable to that of filament-discharge sources. With a 0.6 mm diameter extraction aperture, a positive hydrogen ion beam current density of 80 mA/cm2 can be obtained at a rf input power of 2.5 kW. The expected source lifetime is much longer than that of filament discharges.

The feasibility of laser-induced photoemission as a driving mechanism for short plasma pulse production is currently being investigated with a pulsed excimer laser at 248 nm. Low work function materials such as LaB6 and barium are used as cathode materials and the resulting plasma characteristics are being examined. Results from early measurements of the barium photocathode show a strong dependence of the photoemitted current on the source pressure and cathode voltage. Additionally, the temporal behavior of the emitted electron pulse is found to consist of two components: a short 50 ns burst corresponding to laser-induced photoemission, and a larger, longer 100 ns pulse increasing from the tail end of the photoemitted electron pulse.

For the production of future microelectronics devices, various alternate methods are currently being considered to replace the presently used method of lithography with ion beam lithography. One of these methods is Ion Projection Lithography (IPL), which aims at the possibility of projecting sub-0.25 /spl mu/m patterns of a stencil mask onto a wafer substrate. In order to keep the chromatic aberrations below 25 nm, an ion source which delivers a beam with energy spread of less than 3 eV is desired. For this application, multicusp ion sources are being considered. We measure the longitudinal energy spread of the plasma ions by using a two-grid electrostatic energy analyzer. The energy spread of the extracted beam is measured by a high-voltage retarding-field energy analyzer. In order to obtain the transverse ion temperature, a parallel-plate scanner is being set up to study the beam emittance. In this paper, comparisons are made for different ion source configurations.

Radio-frequency (rf)-driven multicusp ion sources have been developed extensively at Lawrence Berkeley National Laboratory (LBNL) for many applications, each requiring specific source designs. These uses have ranged from large ion sources for neutral-beam injectors-several tens of centimeters in size-to small sources for oil-well logging neutron tubes-a few centimeters in diameter. The advantages associated with internal antenna, rf-driven ion sources include reliability, long component life, ease of operation, and the ability to generate plasmas free of the impurities commonly found in hot-filament discharge sources. We have investigated and characterized the performance of rf-driven sources with respect to the rf operating frequency and ion source size for hydrogen ion species and current density. Furthermore, we have included in this study the aspects of proper coupling of the rf generator to the antenna through an impedance matching network. Finally, critical issues pertaining to general rf operation including beam extraction, rf Shielding, and cooling of transformer cores are discussed.

A compact filament-driven multicusp ion source has been studied using both hydrogen and helium. Three aspects of the source have been investigated: hydrogen ion species, axial energy spread and extractable current. An atomic ion fraction (H+) of approximately 30% could be obtained with a discharge power of 80 V and 3 A. A magnetic analyzer was used to determine the axial energy spread of the extracted (i.e. accelerated) ion beam species, and an electrostatic energy analyzer was used to determine the energy spread of the ions at the source exit. The energy spread of the extracted beam for the individual species of positive hydrogen ions (H+, H2+, H3+) and that for the negative hydrogen ions (H−) was measured as well. Energy spreads as low as 2.3 eV were obtained for H+, 2 eV for H2+, 1.7 eV for H3+, and 1 eV for H−. The axial energy spread in the source exit without extraction for hydrogen and helium was measured to be approximately 1 eV for both cases. The source can generate a hydrogen beam current density of approximately 12 mA/cm2.

The use of multicusp sources to generate ion beams with narrow energy spread has been investigated. It is found that the presence of a magnetic filter can reduce the longitudinal energy spread significantly. This is achieved by creating a uniform plasma potential distribution in the discharge chamber region, eliminating ion production in the extraction chamber and in the sheath of the exit aperture and by minimizing the probability of charge exchange processes in the extraction chamber. An energy spread as low as 1 eV has been measured.

Multicusp ion sources are capable of generating ion beams with low axial energy spread as required by the ion projection lithography (IPL). Longitudinal ion energy spread has been studied in two different types of plasma discharge: the filament discharge ion source characterized by its low axial energy spread, and the rf-driven ion source characterized by its long source lifetime. For He+ ions, longitudinal ion energy spreads of 1–2 eV were measured for a filament discharge multicusp ion source which is within the IPL source requirements. Ion beams with larger axial energy spread (∼7 eV) were observed in the rf-driven source. A double-chamber ion source has been designed which combines the advantages of low axial energy spread of the filament discharge ion source with the long lifetime of the rf-driven source. The energy spread of the double chamber source is approximately 2 eV.

The authors describe the development of a new instrument for the detection of trace amounts of rare isotopes, a Cyclotron Mass Spectrometer (CMS). A compact low energy cyclotron optimized for high mass resolution has been designed and has been fabricated. The instrument has high sensitivity and is designed to measure carbon-14 at abundances of < 10{sup {minus}12}. A novel feature of the instrument is the use of permanent magnets to energize the iron poles of the cyclotron. The instrument uses axial injection, employing a spiral inflector. The instrument has been assembled and preliminary measurements of the magnetic field show that it has a uniformity on the order of 2 parts in 10{sup 4}.

We describe the development of a new instrument for the detection of trace amounts of rare isotopes, a Cyclotron Mass Spectrometer (CMS). A compact low energy cyclotron optimized for high mass resolution has been designed and is under construction. The instrument has high sensitivity and is designed to measure carbon-14 at abundances of <10/sup -12/. A novel feature of the instrument is the use of permanent magnets to excite the iron poles of the cyclotron, giving a field uniformity on the order of 1 part in 10/sup 4/. The instrument uses axial injection, employing a spiral inflector. The instrument is nearing completion, with most major components completed.<>

A technique for the detection of trace amounts of rare isotopes, Cyclotron mass Spectrometry (CMS), is described. This technique uses the relationships between particle mass, charge, magnetic field strength and cyclotron orbital frequency to provide high mass resolution. The instrument also has high sensitivity and is capable of measuring isotopes with abundances of < 10{sub {minus} 12}. Improvements now being implemented will lead to further increases in the sensitivity and enhance operating parameters such as cost, portability, and sample throughput.

Author(s): Young, A.T.; Bertsche, K.J.; Clark, D.J.; Halbach, K.; Kunkel, W.B.; Leung, K.N.; Li, C.Y.

The design of a compact, low energy cyclotron used as a mass spectrometer is presented. The instrument is designed for high resolution, high sensitivity detection of trace. It features the use of permanent magnets to excite the soft iron pole pieces which provide the magnetic field of the cyclotron. Tuning magnets are used to enable the field to be varied. This significantly improves the operational requirements of the instrument when compared to one which uses electromagnets. The cyclotron will use a spiral reflector for axial injection.

A D− surface-conversion source using a solid barium converter is designed for steady-state operation to produce 200 mA of D−. A similar ion source of twice the size as the one discussed here will meet the requirements set by the present US-ITER neutral beam injector design. Among the possible types of ion sources being considered for the US-ITER neutral beam design, the barium converter surface-conversion source is the only kind that does not use cesium in the discharge. This absence of cesium will minimize the number of accelerator breakdowns.

The populations of ground electronic state atomic hydrogen and ground electronic state, vibrationally–rotationally excited hydrogen molecule in a negative hydrogen ion source discharge have been measured using vacuum ultraviolet (VUV) laser absorption spectroscopy. These populations have been measured under a variety of discharge conditions in two different regions of a tandem chamber ion source. Preliminary results of the measurements in the driver region and filter region are given. It is observed that the atomic hydrogen density decreases as one goes from the driver to the filter region. This indicates that the surfaces directly adjacent to the filter region are net sinks for hydrogen atoms. In contrast, the molecular vibrational population distribution shows only a small difference between the two regions, indicating that these surfaces are not net sinks for the excited molecules.