Microwave Discharge Ion Source Research Articles

Characterization of the versatile ion source and possible applications as injector for future projects

The versatile ion source (VIS) is an off-resonance microwave discharge ion source which generates a slightly overdense plasma (n(e) ≈ 10(17) cm(-3)) operating at 2.45 GHz and producing more than 50 mA of proton beams. A detailed characterization of the source, by operating between 60 and 75 kV, in terms of emittance, current extracted and proton fraction is reported below. Moreover, passive techniques (alumina coating of the plasma chamber walls, BN disks at the injection and extraction endplates) have been used to improve the performance of the source, increasing the electron density for a more efficient ionization. The know-how achieved with the VIS source may be useful for the different project, particularly for the European spallation source.

Observations of resonant modes formation in microwave generated magnetized plasmas

Ion sources have a significant number of applications in accelerator facilities and in industrial applications. In particular, the electron cyclotron resonance ion sources (ECRIS) are nowadays the most effective devices that can feed particle accelerators in a continuous and reliable way, providing high current beams of low and medium charge state ions and lower, but still remarkable, beam current for highly charged ions. In recent years several experiments have shown that the current, the charge states and even the beam shape change by slightly varying the microwave frequency (the so-called frequency tuning effect – FTE). The theoretical explanation of these results is based on the difference in the electromagnetic field pattern over the resonance surface, i.e. that region where the electrons resonantly interact with the incoming wave. In order to be consistent with the experiments, this model requires that standing waves are formed also in presence of a dense plasma. The proof was sought by means of a series of measurements performed with a network analyzer and with a plasma reactor operating at 2.45 GHz, according to the principles of the microwave discharge ion sources (MDIS). The measurements have been carried out with the aim to achieve the electromagnetic characterization of the plasma chamber in terms of possible excited resonant modes with and without plasma, and they reported that resonant modes are excited inside the cavity even in presence of a dense plasma. It was observed that the plasma dynamics strongly depends on the structure of the standing waves that are generated. The measurement of the eigen-frequencies' shifts were carried out for several values of pressure and RF power, thus linking the shift with the plasma density measured by a Langmuir probe. The changes in plasma shape, density and electron temperature have been also monitored for different operating conditions. A strong variation of plasma properties has been observed as a consequence of the introduction of the Langmuir probe inside the resonant cavity, thus demonstrating that the standing wave can be strongly perturbed even by means of relatively small metallic electrodes. The measurements reported hereinafter are relevant also for ECRIS, because they confirm the validity of the theoretical model that describes the frequency tuning.

Thomson-Scattering Diagnostics of Plasmas Produced in Miniature Microwave Discharge Ion Engine

T HE adoption of small satellites, with their flexibility, short development time, and low cost, has been a breakthrough in space applications [1,2]. Until recently, however, size restrictions have limited the capacity of the available propulsion systems. Hence, the demand for millinewton-class miniature propulsion systems is expected to grow in the future with miniature microwave discharge ion engines as ideal candidates for use on these satellites [3–5]. Conventional ion engines produce high thrust efficiency (60–70%) with specific impulses above 3000 s [6–8]. A 30 W miniature ion engine has been developed for deorbiting 100-kg-class satellites [5]. To miniaturize the engine while maintaining its superiority in performance, a microwave discharge ion source was used. The results demonstrate good performance of the 16-mm-diam version of the source, including a thrust efficiency of 0.51. This is competitive with the miniature ion engine developed at NASA, for which the thrust efficiency is 0.56 [9], and has been the best performance at its size [9]. The thrust density of our engine was 5:6 N=m, which is several times higher than that of conventional ion engines (1:1–1:6 N=m) [1–3]. This higher thrust density is a significant advantage for small satellites, since it means the volume of the engine can be dramatically reduced. Understanding the reason for this superiority is essential in developing this engine. It has not yet, however, been thoroughly investigated, since it is difficult to measure the plasma properties without disturbing the plasma as a result of intrusive diagnostic methods, such as an electrostatic probe. The aim of this study is to measure plasma properties by laser Thomson scattering (LTS) to understand the physics inside this engine. LTS is nonintrusive, in that no physical object need be placed in the plasma [10–12]. Application of thismethod to the plasma produced in theminiature microwave ion engine faces the following difficulties. First, the ne is estimated to be less than 10 m . This results in a weak Thomsonscattering signal. Second, the effect of stray laser light becomes very strong. To overcome these difficulties, we used the photon-counting methodwith a doublemonochromator. These effortsmade it possible to detect LTS signals.

Optimization of ECR ion sources for high charge state beam generation

The most important developments in the field of ion sources have been based on the generation of warm plasmas by means of microwaves at different frequencies.Different type of sources as electron cyclotron resonance ion sources (ECRIS) and microwave discharge ion sources have been designed and built at INFN-LNS, at operating frequencies between 2.45 and 28GHz, with different purposes ranging from high efficiency ionization or high current production of light ions to high charge state generation of heavy ion beams.The production of intense beams of highly charged ions (HCIs) has been a major task in the last few years.Different solutions have been proposed to improve the production rate. In particular the microwave coupling to plasma can be significantly improved with beneficial effects on ion beam current.Experimental activities in the field of microwave coupling to plasmas at different frequencies (14, 18 and 28GHz) along with theoretical modelling of the plasma formation and confinement have been undertaken to improve production of HCI.The description of the different methods and tasks to improve the performances of such sources will be carried out, along with a scenario of the existing and forthcoming ECRIS for HCI production.

マイクロ波放電式イオンエンジンの高比推力化に関する基礎研究

An ion engine with 10,000sec-class high specific impulse is expected as primary propulsion for interplanetary space mission with extremely high delta-V, and needs high voltage for ion acceleration over 10kV. The microwave discharge ion source without solid electrodes can supply sophisticated technologies and simple composition on electrical isolation for such a high voltage. New electro-static grid made of carbon-carbon composite material was fabricated based on the numerical simulation of “igx” code. The “μ10HIsp” ion engine combining the ECR ion source, microwave discharge neutralizer, DC blocks, propellant isolators, carbon-carbon composite grid and so on generated successfully a plasma beam with 10,000sec specific impulse using 15kV acceleration voltage.

Enhancement of ion current from the TRIPS source by means of different electron donors

A series of measurements were carried out with the TRasco Intense Proton Source (TRIPS) to determine the effectiveness of different materials as electron donors. It is well known that the use of boron nitride (BN) disks inside the plasma chamber increases the current extracted from microwave discharge ion sources, generating additional electrons. In the past, one of the two disks was replaced by a 40μm Al2O3 coating over the extraction electrode, which gave some increase of current, but after less than 200h was heavily damaged. The tests here reported concern three different options: (a) thicker Al2O3 layer (100μm) deposited over the extraction electrode; a 1-mm-thick aluminium foil over which an alumina layer is deposited, inserted in the plasma chamber; a 5-mm-thick Al2O3 tube embedded in the plasma chamber of the TRIPS source (the outer diameter of the tube being slightly smaller than the inner diameter of the chamber). The tests were carried out in the same conditions as for magnetic field topology and only rf power and gas input were variable. Special attention was paid to the proton fraction. In fact, a higher proton fraction can be considered as a signature of the higher availability of electrons in the plasma. With the thick alumina tube not only was a better current and proton fraction observed but also a lower beam ripple and better stability.

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.

Innovative ion sources for accelerators: the benefits of the plasma technology

The most important developments in the field of ion sources at INFN-LNS, Catania, have been based on the generation of warm plasmas by means of microwaves at different frequencies ranging from 2.45 GHz to 28 GHz. Different Electron Cyclotron Resonance Ion Sources (ECRIS) have been designed and built, in particular SERSE and CAESAR are working for the Supereonducting Cyclotron at LNS, at the operating frequency of 14, 18 and 28 GHz. The design of an innovative source to be operated at 28 or 35 GHz, named GyroSERSE, has been completed, with the aim to provide mA beams of highly charged heavy ions. This kind of design is particularly suited for the beam injectors of accelerator facilities to be built in the next decade. Three different 2.45 GHz microwave discharge ion sources (MDIS) have also been built for different purposes: to obtain high efficiency ionization for RIB production or to produce high current proton beam for Accelerator Driven System (ADS). A fourth compact one, with permanent magnets, is under construction. The description of these equipments will be carried out, along with the results of the hybrid ion source, consisting of a laser-generated plasma source coupled to the SERSE high performance ECRIS.

Off-line efficiency measurements of the microwave discharge source (MIDAS) at the Laboratorio Nazionale del Sud

The aim of the EXCYT project at the Laboratorio Nazionale del Sud (LNS) is the production of radioactive beams, and for this we need to carry out the ionization of recoils produced in a target with as high an efficiency as possible. In order to obtain efficient positive ionization the microwave discharge ion source (MIDAS) has been designed to be followed by a charge exchange canal. A prototype of this source has been designed, built, and tested at the LNS and a description of this source is presented in this article along with the preliminary results and a design of the final version.

Study of microwave coupling in electron cyclotron resonance ion sources and microwave ion sources

At the “Laboratorio Nazionale del Sud” two different ion sources are under development: SERSE (a superconducting electron cyclotron resonance ion source to be coupled to the superconducting cyclotron) and MIDAS (a microwave discharge ion source for recoils ionization). In most cases, for these kinds of sources, the coupling system remains empirical, and the exact description of the waves absorbed from the plasma is poorly known. In the following paper an analysis of power transfer characteristics between an arbitrary power source and load impedances is presented. Particular attention is paid to the matching systems. Herein is presented a 2.45 Ghz microwave line for the MIDAS ion source and a proposal for a matching system that can be used in SERSE when it will be driven by three frequencies: 10, 14, and 18 Ghz.

MIDAS, a high efficiency microwave discharge ion source for the EXCYT facility

A microwave discharge ion source has been designed in order to obtain high efficiencies for positive ionization of the recoils to be produced at the exotics at the cyclotron Tandem facility. After a charge exchange process the negative ions will be injected into the 15-MV Tandem, already working at the Laboratorio Nazionale del Sud. The short ionization time and the fast wall recycling make this source very well suited for the purpose of high efficiency ionization of the recoils. The operational principle and the design are described in the following.

An advanced high-current low-emittance dc microwave proton source

The high-current low-emittance microwave proton source introduced two years ago act the Chalk River Laboratories has been extensively redesigned. The 2.45 GHz microwave line was modified to incorporate a 50 kV dc break and a rectangular-to-ridged waveguide transition. The layered microwave window has been replaced by a single plate of aluminum nitride. The solenoids were electrically isolated from the plasma chamber. Only the plasma chamber remains at the extraction potential. The total beam current and the proton beam current have been measured as a function of the microwave power, the hydrogen mass flow and the solenoid positions and currents. More than 95 mA dc of hydrogen ions with a proton fraction in excess of 85% were extracted from a single 2.5 mm radius aperture with only 500 W of microwave power and 2.0 sccm (3 μg/s) of hydrogen mass flow. The perveance at minimum divergence and the minimum emittance have been determined for various extraction geometries. The variation of the perveance with the aspect ratio of the extraction system was found to be virtually identical for microwave and arc discharge ion sources. The normalized rms emittance was less than 0.13 p mm mrad whatever the extraction geometry.

Sodium ion production in a simple microwave discharge ion source

Microwave discharge ion sources have commonly been used to produce ions from gases. The authors report the extension of such a source to the production of ions from a non-gaseous material without the complication of a magnetic field. The source is simple in design and easily constructed. Discharges which were initiated in argon produced Na+ ions from both sodium iodide and pure sodium. The absolute and relative rates of production of Na+ ions were found to vary greatly with the discharge pressure and incident microwave power. Operating conditions ranging from 0.016 to 0.064 mbar (1.6-6.4 Pa) and from 20 to 60 W were tested. The percentage of all the ions which were Na+ ions in discharges with sodium iodide was as high as 17% while with pure sodium it was as high as 89%.

ChemInform Abstract: Flowing Afterglow Negative Ion Photoelectron Spectroscopy of Dirhenium: Evidence for Multiple Bonding in Re2 and Re2‐ .

AbstractReg and Reg′ are observed for the first time by negative ion photoelectron Spectroscopy following dissociation/ionization of Reg(CO)", in a flowing afterglow apparatus incorporating a microwave discharge ion source.

RIKEN 200 KV high current implanter for metal surface modification

A high current, metal ion implanter was constructed in order to aid the formation of a new metastable surface alloy. This implanter, called a RIKEN 200 kV high current implanter, is a modified Lintott high current machine (Series III), which has the advantages of having its own microwave ion source and an extra target chamber. The microwave discharge ion source without a hot-filament has a comparatively long lifetime because the chloride ions and radicals in a plasma during discharge of metal chlorides might prevent metal to deposit on the inner walls of the discharge chamber by bombarding and chemically cleaning them. An extra target chamber for metal modification is able to control the surface composition by utilizing the sputtering effect of the ion beam during ion implantation. The use of this ion source and the extra target chamber is suggested to be suitable for the production of metallic ions and for the implantation into metals. The case study will be introduced for Ti implantation into Fe.