Microwave Ion Source Research Articles

IBSIMU: A three-dimensional simulation software for charged particle optics

A general-purpose three-dimensional (3D) simulation code IBSIMU for charged particle optics with space charge is under development at JYFL. The code was originally developed for designing a slit-beam plasma extraction and nanosecond scale chopping for pulsed neutron generator, but has been developed further and has been used for many applications. The code features a nonlinear FDM Poisson's equation solver based on fast stabilized biconjugate gradient method with ILU0 preconditioner for solving electrostatic fields. A generally accepted nonlinear plasma model is used for plasma extraction. Magnetic fields can be imported to the simulations from other programs. The particle trajectories are solved using adaptive Runge-Kutta method. Steady-state and time-dependent problems can be modeled in cylindrical coordinates, two-dimensional (slit) geometry, or full 3D. The code is used via C++ programming language for versatility but it features an interactive easy-to-use postprocessing tool for diagnosing fields and particle trajectories. The open source distribution and public documentation make the code well suited for scientific use. IBSIMU has been used for modeling the 14 GHz ECR ion source extraction and for designing a four-electrode extraction for a 2.45 GHz microwave ion source at Jyväskylä. A grid extraction has also been designed for producing large uniform beam for creating conditions similar to solar wind. The code has also been used to design a H(-) extraction with electron dumping for the Cyclotron Institute of Texas A&M University.

Ultracompact/ultralow power electron cyclotron resonance ion source for multipurpose applications

In order to drastically reduce the power consumption of a microwave ion source, we have studied some specific discharge cavity geometries in order to reduce the operating point below 1 W of microwave power (at 2.45 GHz). We show that it is possible to drive an electron cyclotron resonance ion source with a transmitter technology similar to those used for cellular phones. By the reduction in the size and of the required microwave power, we have developed a new type of ultralow cost ion sources. This microwave discharge system (called COMIC, for COmpact MIcrowave and Coaxial) can be used as a source of light, plasma or ions. We will show geometries of conductive cavities where it is possible, in a 20 mm diameter chamber, to reduce the ignition of the plasma below 100 mW and define typical operating points around 5 W. Inside a simple vacuum chamber it is easy to place the source and its extraction system anywhere and fully under vacuum. In that case, current densities from 0.1 to 10 mA/cm(2) (Ar, extraction 4 mm, 1 mAe, 20 kV) have been observed. Preliminary measurements and calculations show the possibility, with a two electrodes system, to extract beams within a low emittance. The first application for these ion sources is the ion injection for charge breeding, surface analyzing system and surface treatment. For this purpose, a very small extraction hole is used (typically 3/10 mm for a 3 microA extracted current with 2 W of HF power). Mass spectrum and emittance measurements will be presented. In these conditions, values down to 1 pi mm mrad at 15 kV (1sigma) are observed, thus very close to the ones currently observed for a surface ionization source. A major interest of this approach is the possibility to connect together several COMIC devices. We will introduce some new on-going developments such as sources for high voltage implantation platforms, fully quartz radioactive ion source at ISOLDE or large plasma generators for plasma immersion, broad or ribbon beams generation.

A Continuous-Flow Gas Chromatography 14C Accelerator Mass Spectrometry System

Gas-accepting ion sources for radiocarbon accelerator mass spectrometry (AMS) have permitted the direct analysis of CO2 gas, eliminating the need to graphitize samples. As a result, a variety of analytical instruments can be interfaced to an AMS system, processing time is decreased, and smaller samples can be analyzed (albeit with lower precision). We have coupled a gas chromatograph to a compact 14C AMS system fitted with a microwave ion source for real-time compound-specific 14C analysis. As an initial test of the system, we have analyzed a sample of fatty acid methyl esters and biodiesel. Peak shape and memory was better then existing systems fitted with a hybrid ion source while precision was comparable. 14C/12C ratios of individual components at natural abundance levels were consistent with those determined by conventional methods. Continuing refinements to the ion source are expected to improve the performance and scope of the instrument.

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/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A high-current microwave ion source with permanent magnet and its beam emittance measurement

The progress of a 2.45 GHz high-current microwave ion source with permanent magnet for T(d,n)4He reaction neutron generator is reported in this paper. At 600 W microwave power and 22 kV extraction voltage, 90 mA peak hydrogen ion beam is extracted from a single aperture of 6 mm diameter. The beam emittance is measured using a simplified pepper-pot method. The (x,x(')) emittance and the (y,y(')) emittance for 14 keV hydrogen ion beam are 55.3pi and 58.2pi mm mrad, respectively. The normalized emittances are 0.302pi and 0.317pi mm mrad, respectively.

Development of an all-permanent-magnet microwave ion source equipped with multicusp magnetic fields for high current proton beam production

An all-permanent-magnet (APM) microwave hydrogen ion source was developed to reduce the size and to simplify structure of a conventional solenoid coil microwave ion source developed for reliability improvement of high current proton linac application systems. The difficulty in developing the APM source was sensitive dependence of the source performance on axial magnetic field in the microwave discharge chamber. It was difficult to produce high current proton beam stably without precise tuning of the magnetic field using solenoid coils. We lowered the sensitivity using multicusp magnetic fields for plasma confinement at the discharge chamber sidewall of the source. This enabled stable high current proton beam production with the APM microwave ion source with no tuning coil. The water cooling and the power supply for the coils are not necessary for the APM source, which leads to better reliability and system simplification. The outer diameter of the APM source was around 300 mm, which was 20% lower than the coil source. The APM source produced a maximum hydrogen ion beam current of 65 mA (high current density of 330 mA/cm(2), proton ratio of 87%, and beam energy of 30 keV) with a 5 mm diameter extraction aperture, pulse width of 400 micros, and 20 Hz repetition rate at 1.3 kW microwave power. This performance is almost the same as the best performances of the conventional coil sources. The extracted ion beams were focused with electrostatic five-grid lens to match beam to acceptance of radio-frequency quadrupole linacs. The maximum focused beam current through the orifice (5 mm radius) and the lens was 36 mA and the 90% focused beam half-width was 1-2 mm.

Off-line ion source terminal for ISAC at TRIUMF

The off-line ion source (OLIS) terminal consists of a microwave cusp ion source, either a surface ion source or a hybrid surface-arc discharge ion source and an electrostatic switch that allows selecting any one of the sources without mechanical intervention. These sources provide variety of beams to ISAC experiments, for commissioning the accelerators, for setting up the radioactive experiments, and for tuning the beam lines. The microwave ion source has been operational since 1995 and provides singly and doubly charged beams from various stable isotopes for many ISAC experiments at high and low energy areas. Originally its prime goal was to provide beams from gaseous elements, but later two ovens and a sputtering system were added in order to provide beams from liquids and from solids. The surface ion source installed in 2002 can provide low energy spread beams from alkali and semialkali elements. It also has three separate ovens and an ionizer. Therefore, it can provide three different temperature regions simultaneously to provide different beams to ISAC. It is mainly used for laser spectroscopy experiments and other experiments, which require a finite beam quality. A hybrid surface-arc discharge ion source was also developed and installed in order to meet specific demands from experiments. This source terminal is now automated for start up and for mass selection. It is capable of providing stable beams for months without maintenance and it is also capable of providing negative ion beams if required. To date, over 40 different isotopes including many rear isotopes were delivered to various experiments from the OLIS source terminal. Performances of the ion sources and some of the results are discussed.

Development of low energy ion beam system for space charge compensation experiments

A low energy ion beam system for space charge compensation (SCC) experiments was developed and evaluated. This system was designed for observation of SCC of a positive ion beam with an electron beam. The system consisted of the ion source chamber and the SCC experiment chamber. The ion source chamber was equipped with the compact microwave ion source for low voltage extraction. Ion current at initial position of the analysis chamber was 84 microA at extraction voltage of 500 V, and satisfied a condition to observe the SCC effect clearly. In order to evaluate the SCC, we measured the arrival ion current by supplying thermionic electrons, which were extracted from a tungsten filament driven by ac voltage. As the electron supply, the arrival ion current increased from 40 to 68 microA at the potential of filament of +3 eV which produced the thermionic electron with extremely low energy extracted by space charge of the ion beam.

The synchrotron and its related technology for ion beam therapy

Hitachi has developed several new technologies for the synchrotron and its related system to realize reliable and flexible operation of a proton therapy system. Especially important among them are a non-resonant RF acceleration cavity using FINEMET core with multiple power feeding and radio frequency driven beam extraction technique (RF-DE) for a synchrotron. Various treatment operations such as variable acceleration energy or respiration gating became possible and simple due to the above technique. For beam transport, a beam steering method for the beam, using transfer matrix realizes quick and precise correction of the beam orbit. A compact microwave ion source has also been developed for the injector to obtain further higher reliability and availability. Most of these technologies are also effective to enhance the reliability and flexibility of other ion beam therapy systems.

Sub-Milli-Newton Class Miniature Microwave Ion Thruster

A miniaturized microwave ion source with a 1.6-cm beam diameter grid system was designed and then evaluated experimentally. Based on the HAYABUSA μ10 neutralizer, we fabricated a small 18-nun-diam discharge chamber, into which 4.2 GHz microwaves were launched through an L-shaped antenna that was located in a magnetic field created by permanent magnets and iron yokes. Ion beams were emitted from the small discharge chamber when operated with a grid system whose respective hole diameters of the screen grid and acceleration grid were 0.72 and 0.43 mm, and the total number of grid holes was 211. For a beam voltage of 1500 V and a microwave input power of 10 W, the typical thruster performance was thrust of 0.34 mN, a thrust/power ratio of 16 mN/kW, propellant utilization efficiency of 68%, and a specific impulse of 3200 s. If we were able to further reduce the ion production cost (circa 3000 W/A in the current experiment), this thruster would be a candidate for main propulsion of a small satellite or precise attitude control of various sizes of satellites.

Electromagnetic field modeling and ion optics calculations for a continuous-flow AMS system

A continuous-flow 14C AMS (CFAMS) system is under construction at the NOSAMS facility. This system is based on a NEC Model 1.5SDH-1 0.5MV Pelletron accelerator and will utilize a combination of a microwave ion source (MIS) and a charge exchange canal (CXC) to produce negative carbon ions from a continuously flowing stream of CO2 gas. For high-efficiency transmission of the large emittance, large energy-spread beam from the ion source unit, a large-acceptance and energy-achromatic injector consisting of a 45° electrostatic spherical analyzer (ESA) and a 90° double-focusing magnet has been designed. The 45° ESA is rotatable to accommodate a 134-sample MC-SNICS as a second ion source. The high-energy achromat (90° double focusing magnet and 90° ESA) has also been customized for large acceptance. Electromagnetic field modeling and ion optics calculations of the beamline were done with Infolytica MagNet, ElecNet, and Trajectory Evaluator. PBGUNS and SIMION were used for the modeling of ion source unit.

Experimental Simulation of Spacecraft Charging by Artificial Ion Beam Emission

Charging of a small electrically floated body, equipped with an unneutralized microwave ion source, was experimentally studied in a vacuum chamber. When a xenon ion beam of 16 mm in diameter was released from a cubic floating body, 10 cm on each side, the body was negatively charged at a rate of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /C, where I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> is the ion-beam current and C is the capacitance between the floated body and the ground. For large I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /C parameters, the body potential changes faster than the xenon ions can react. This simulation results in space-charge-induced potential oscillations, which are detected in experimental measurements and observed in numerical simulations. The time scale of the oscillation relaxation time was found to be at least several times, corresponding to the ion movements around the body. As expected, the steady-state potential of the floated body was relatively negative to the vacuum-facility ground at a value equal to the magnitude of the ion-beam voltage

Microwave proton sources for the IUCF LENS project

The Indiana University Cyclotron Facility (IUCF) has constructed and placed in operation a Low Energy Neutron Source using a 10mA, 7MeV proton beam incident on a beryllium target. A microwave ion source used for the IUCF cyclotrons was modified to work in this application. In parallel, a 150mA, 75keV injector, based on the same ion source technology, is under construction. This higher current injector will be used to inject beam into a new 100mA peak current linac that will be installed in 2006. The simulations of the 75keV injector are reported.

Minipermanent magnet high-current microwave ion source

A 2.45GHz high-current microwave ion source with permanent magnet was developed at Peking University. The source body is about 10cm in diameter and 10cm in height, and its weight is less than 5kg. It can operate in both pulse and continuous wave (cw) modes. In pulse mode, more than 100mA hydrogen ion current was extracted from a 5mm ion extraction aperture, when the microwave power is 500W and the extraction voltage is 45kV, which corresponds to a beam density of 500mA∕cm2. In cw mode, up to 60mA of hydrogen ion current can be extracted, and the beam density is 300mA∕cm2 at the same conditions. The proton ratio is about 80% and the normalized root-mean-square emittance is less than 0.1πmmmrad in both modes.

Development of an electrostatic beam focus lens equipped with electron suppression grids for a microwave ion source to inject high-current ion beams into a linac

An electrostatic beam focus lens equipped with electron suppression grids (SG) was developed to focus and inject high-current ion beams into a linac from a microwave ion source developed for improving the availability of linac application systems. The SG suppresses electron acceleration by the lens electric field. This contributes to the confinement of electrons in beam plasma in the drift space between the ion source and lens. The confined electrons neutralize ion space charge and prevent the divergence of ion beams by space charge force, which enables an efficient transportation of high-current ion beams through the lens from the microwave ion source with small beam loss. High-current hydrogen beams of around 40 mA with 30 keV beam energy, 400 μs pulse width and 90% proton ratio were focused successfully using the developed lens with high transport efficiency of about 70%; without the SG this efficiency was less than 30%. The 90% beam half-width of the focused beam was around 1 mm, which would be acceptable for injection into a radio-frequency quadrupole (RFQ) linac. The microwave ion source system including a low-energy beam transport can be compact to less than 1 m in length with the developed electrostatic lens in comparison with a solenoid coil lens system which is more than 2 m length. Long-time operation stability (16 h×3 days with 0.8% duty: a condition for a typical application; 110 h with 50% duty: 62.5 times of the condition corresponding to 6875 h operation) was confirmed with a focused beam current of 35 mA and change in the current was less than 2%.

Off-resonance microwave ion source for high-current molecular ion-beams

As the integration of semiconductor devices proceeds, each device size shrinks, resulting in requiring shallower p–n junction depth. Thus, molecular ion BF2+ is often utilized in place of the atomic ion B+for p-region formation. Microwave ion source is tuned to provide very high-current beams of such molecular ions. In this study we vary the source-magnetic field for a wide range from less than a tenth of the electron cyclotron resonance (ECR) magnetic field (87.5mT for 2.45GHz) to 1.3 times the ECR field. The result shows that the highest current for BF2+ ions is obtained at the magnetic field of 10mT, although the highest B+ current is obtained at over the ECR field. The plasma generated at this condition is considered to be of low electron temperature and high electron density. The ion source is mounted on a commercial implanter beam line. The BF2+ ion current from the off-resonance microwave source is resultantly more than twice that of conventional Bernas source.

Plasma Characterization of a 10-cm Diameter Microwave Discharge Ion Thruster

Plasma characterization was conducted for an electron-cyclotron-resonance (ECR) type ion thruster. For a 10-cm diameter microwave discharge ion source consisting of two samarium cobalt magnet rings surrounding a centered waveguide for launching microwaves, plasma profiles were found to have severely non-uniform distributions, with localized plasma found near the magnet rings. This localized plasma is mainly produced in the magnetic flux tubes between the two ring magnets, where electrons gain microwave energy as they pass the ECR line during the bouncing movement between magnetic mirrors. To obtain a low-cost microwave ion source, this type of ionization mechanism can be exploited. When introducing microwaves through a low magnetic field boundary, however, it is impossible to eliminate the accessibility difficulty related to the cutoff density, which results in a plasma below the cutoff density. Because of the accessibility difficulty, in this work, only a relatively small ion beam current density of 1.8 mA/cm2 was achieved.

Simple structure holey-plate ion source

A high-efficiency, lightweight, and low-cost microwave ion source, created and sustained by evanescent waves emitted from a parallel-plate type source with a holey-plate has been studied. This source is called a holey-plate (HP) ion source. The microwaves at 2.45 GHz propagate inside the parallel-plate structure and then are converted into an evanescent mode through the use of a HP with 3-mm-diam holes. The plate dimensions are 100 mm×140 mm and are separated by 10 mm. In these experiments, the entire unit of the parallel plate is placed in a vacuum chamber. Dense plasma is produced only on the HP. A 78 mm×108 mm argon ion beam is extracted from the HP using a single grid with 3-mm-diam holes. The ion current densities are in excess of 1.2 mA/cm2 at an extraction voltage of 500 V.

A high intensity microwave ion source for high current RFQ

A high intensity microwave ion source for high current RFQ has been developed at China Institute of Atomic Energy (CIAE), China, for the China Demonstration Prototype of Accelerator Driven Sub-critical System (CDPADS) project. The aim of the source is to produce up to 50 mA cw proton beams at 75 keV at the RFQ match point. A more uniform magnetic field of the source is produced by electrical solenoids with the aid of permanent magnets. A hydrogen beam of more than 60 mA at 75 keV can be routinely extracted from a single 6.5-mm-diam aperture. To measure reliability/availability of the source, a continuous 120 h run has been performed. This article will report the main features and the current status of the source.

Development of a high-current microwave ion source for proton linac application systems

A microwave hydrogen ion source was developed to improve reliability, and to increase operation time of proton linac application systems. The ion source needs no filament in the discharge chamber, which leads to better reliability and less maintenance time. The developed source produced a maximum hydrogen ion beam current of 70 mA (high current density of 360 mA/cm2, beam energy of 30 keV) with a 5 mm diam extraction aperture and 1.2 kW microwave power. The proton fraction was increased with an increase in rf power and reached around 90% at 1 kW. Measured 90% beam normalized emittance was 0.4 π mm mrad. Rise times of rf power and beam current to 90% of the final values were about 30 and 35 μs, respectively, at a pulse operation mode with 400 μs pulse width and 100 Hz repetition rate. The dynamic range of beam currents was enlarged (3–63 mA) in the pulse mode with a modified rf wave form to assist ignition of microwave discharge. These performance parameters will be desirable for pulse operation accelerator applications like proton therapy systems. A long time operation stability (150 h) was confirmed with a beam current of 51 mA; change in the current was 2%.