Wave frequency dependence of H- ion production and extraction in a transformer coupled plasma H- ion source at SNU

The effect of rf wave frequencies on the production of H(-) ion is investigated in a transformer coupled plasma H(-) ion source at Seoul National University. A Langmuir probe is installed to measure the plasma density and temperature, and these plasma parameters are correlated to the extracted H(-) beam currents at various frequencies. The Langmuir probe is also used to measure the density of H(-) ions at the ion source by generating photodetachment with an Nd:YAG laser. The extracted H(-) currents decrease to a minimum value until 13 MHz and then, increase as the driving frequency increases from 13 MHz while the relative H(-) population measured by photodetachment monotonically decreases as the driving rf frequency increases from 11 MHz to 15 MHz. A potential well formed at the extraction region at high frequencies of more than 13 MHz is considered responsible for the increased H(-) beam extraction even with a lower photodetachment signal. The variation in the driving rf frequency not only affects the density and temperature of the plasma but also modifies the plasma potential with the existence of a filtering magnetic field and consequently, influences the extracted H(-) current through the extraction as well as formation of H(-) ions.     

Investigation of ISIS and Brookhaven National Laboratory ion source electrodes after extended operation

Linac4 accelerator of Centre Europe?en de Recherches Nucle?aires is under construction and a RF-driven H(-) ion source is being developed. The beam current requirement for Linac4 is very challenging: 80 mA must be provided. Cesiated plasma discharge ion sources such as Penning or magnetron sources are also potential candidates. Accelerator ion sources must achieve typical reliability figures of 95% and above. Investigating and understanding the underlying mechanisms involved with source failure or ageing is critical when selecting the ion source technology. Plasma discharge driven surface ion sources rely on molybdenum cathodes. Deformation of the cathode surfaces is visible after extended operation periods. A metallurgical investigation of an ISIS ion source is presented. The origin of the deformation is twofold: Molybdenum sputtering by cesium ions digs few tenths of mm cavities while a growth of molybdenum is observed in the immediate vicinity. The molybdenum growth under hydrogen atmosphere is hard and loosely bound to the bulk. It is, therefore, likely to peel off and be transported within the plasma volume. The observation of the cathode, anode, and extraction electrodes of the magnetron source operated at BNL for two years are presented. A beam simulation of H(-), electrons, and Cs(-) ions was performed with the IBSimu code package to qualitatively explain the observations. This paper describes the operation conditions of the ion sources and discusses the metallurgical analysis and beam simulation results.     

Development of a compact filament-discharge multi-cusp H- ion source

A 14 MeV medical cyclotron with the external ion source has been designed and is being constructed at China Institute of Atomic Energy. The H(-) ion will be accelerated by this machine and the proton beam will be extracted by carbon strippers in dual opposite direction. The compact multi-cusp H(-) ion source has been developed for the cyclotron. The 79.5 mm long ion source is 48 mm in diameter, which is consisting of a special shape filament, ten columns of permanent magnets providing a multi-cusp field, and a three-electrode extraction system. So far, the 3 mA∕25 keV H(-) beam with an emittance of 0.3 π mm mrad has been obtained from the ion source. The paper gives the design details and the beam test results. Further experimental study is under way and an extracted beam of 5 mA is expected.     

A new extraction system for the Linac4 H- ion source

As part of the CERN accelerator complex upgrade, a new linear accelerator for H(-) (Linac4) is under construction. The ion source design is based on the non-caesiated DESY RF-driven ion source, with the goal of producing an H(-) beam of 80 mA beam current, 45 keV beam energy, 0.4 ms pulse length, and 2 Hz repetition rate. The source has been successfully commissioned for an extraction voltage of 35 kV, corresponding to the one used at DESY. Increasing the extraction voltage to 45 kV has resulted in frequent high voltage breakdowns in the extraction region caused by evaporating material from the electron dump, triggering a new design of the extraction and electron dumping system. Results of the ion source commissioning at 35 kV are presented as well as simulations of a new pulsed extraction system for beam extraction at 45 kV.     

Optimizing the front end test stand high performance H- ion source at RAL

The aim of the front end test stand project is to demonstrate that chopped low energy H(-) beams of high quality can be produced. The beam line currently consists of the ion source, a 3 solenoid low energy beam transport and a suite of diagnostics. A brief status report of the radio frequency quadrupole is given. This paper details the work to optimize the ion source performance. A new high power pulsed discharge power supply with greater reliability has been developed to allow long term, stable operation at 50 Hz with a 60 A, 2.2 ms discharge pulse and up to 100 A at 1.2 ms. The existing extraction power supply has been modified to operate up to 22 kV. Results from optical spectroscopy measurements and their application to source optimization are summarized. Source emittances and beam currents of 60 mA are reported.     

Angular distribution of plasma in the vacuum arc ion source

This paper presents measurements of the angular distribution of the plasma components and different charge states of metal ions generated by a MEVVA-type ion source and measured by a time-of-flight mass-spectrometer. The experiments were performed for different cathode materials (Al, Cu, and Ti) and for different parameters of the vacuum arc discharge. The results are compared with prior results reported by other authors. The influence of different discharge parameters on the angular distribution in a vacuum arc source is discussed.     

Laser measurement of H- ions in a field-effect-transistor based radio frequency ion source

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.     

Shunting arc plasma source for pure carbon ion beam

A plasma source is developed using a coaxial shunting arc plasma gun to extract a pure carbon ion beam. The pure carbon ion beam is a new type of deposition system for diamond and other carbon materials. Our plasma device generates pure carbon plasma from solid-state carbon material without using a hydrocarbon gas such as methane gas, and the plasma does not contain any hydrogen. The ion saturation current of the discharge measured by a double probe is about 0.2 mA∕mm(2) at the peak of the pulse.     

Simulation of H- ion source extraction systems for the Spallation Neutron Source with Ion Beam Simulator

A three-dimensional ion optical code IBSimu, which is being developed at the University of Jyva?skyla?, features positive and negative ion plasma extraction models and self-consistent space charge calculation. The code has been utilized for modeling the existing extraction system of the H(-) ion source of the Spallation Neutron Source. Simulation results are in good agreement with experimental data. A high-current extraction system with downstream electron dumping at intermediate energy has been designed. According to the simulations it provides lower emittance compared to the baseline system at H(-) currents exceeding 40 mA. A magnetic low energy beam transport section consisting of two solenoids has been designed to transport the beam from the alternative electrostatic extraction systems to the radio frequency quadrupole.     

Experimental development on the 18 mA, H- multi-cusp ion source at China Institute of Atomic Energy

The ion source is one of the key devices for the high-intensity cyclotron, which exerts influence on the beam intensity and applications of the machine. The H(-) multi-cusp ion source developed at China Institute of Atomic Energy has been used to perform experimental study on beam intensity and emittance versus the bias voltage, arc power, lens voltage, and pressure of the ion source. Up to now, 18 mA H(-) ion beam with emittance of 0.93 πmm mrad (four times RMS normalized emittance) was obtained from this ion source through the in-depth study and optimization on some essential factors affecting the beam intensity and quality. The paper will present the experimental study on the ion source as well as the beam test results.     

Research and development of H- ion source and low energy beam transport for a kaon-neutrino factory

A baseline H(-) ion source and low energy beam transport (LEBT) system have been identified for Project X. The filament-discharge H(-) ion source has been fabricated by D-Pace, Inc. and is now in operation at LBNL. The source is capable of delivering over 10 mA of H(-) beam in cw operation with normalized 4 rms emittances less than 0.7 π mm mrad. A two-solenoid magnetic lens LEBT system has been design. The design has been validated with simulations of beam transport for 5 mA 30 keV H(-) beams using various simulation codes.     

Development of a radio-frequency ion beam source for fast-ion studies on the large plasma device

A helium ion beam source (23 kV/2.0 A) has been constructed for studying fast-ion physics in the cylindrical magnetized plasma of the large plasma device (LAPD). An inductive RF source produces a 10(19) m(-3) density plasma in a ceramic dome. A multi-aperture, rectangular (8 cm × 8 cm) three-grid system extracts the ion beam from the RF plasma. The ion beam is injected at a variety of pitch angles with Alfve?nic speeds in the LAPD. The beam current is intense enough to excite magnetic perturbations in the ambient plasma. Measurements of the ion beam profile were made to achieve an optimum beam performance and a reliable source operation was demonstrated on the LAPD.     

Brightness enhancement of plasma ion source by utilizing anode spot for nano applications

Anode spots are known as additional discharges on positively biased electrode immersed in plasmas. The anode spot plasma ion source (ASPIS) has been investigated as a high brightness ion source for nano applications such as focused ion beam (FIB) and nano medium energy ion scattering (nano-MEIS). The generation of anode spot is found to enhance brightness of ion beam since the anode spot increases plasma density near the extraction aperture. Brightness of the ASPIS has been estimated from measurement of emittance for total ion beam extracted through sub-mm aperture. The ASPIS is installed to the FIB system. Currents and diameters of the focused beams with∕without anode spot are measured and compared. As the anode spot is turned on, the enhancement of beam current is observed at fixed diameter of the focused ion beam. Consequently, the brightness of the focused ion beam is enhanced as well. For argon ion beam, the maximum normalized brightness of 12,300 A∕m(2) SrV is acquired. The ASPIS is applied to nano-MEIS as well. The ASPIS is found to increase the beam current density and the power efficiency of the ion source for nano-MEIS. From the present study, it is shown that the ASPIS can enhance the performance of devices for nano applications.     

Beam current enhancement of microwave plasma ion source utilizing double-port rectangular cavity resonator

Microwave plasma ion source with rectangular cavity resonator has been examined to improve ion beam current by changing wave launcher type from single-port to double-port. The cavity resonators with double-port and single-port wave launchers are designed to get resonance effect at TE-103 mode and TE-102 mode, respectively. In order to confirm that the cavities are acting as resonator, the microwave power for breakdown is measured and compared with the E-field strength estimated from the HFSS (High Frequency Structure Simulator) simulation. Langmuir probe measurements show that double-port cavity enhances central density of plasma ion source by modifying non-uniform plasma density profile of the single-port cavity. Correspondingly, beam current from the plasma ion source utilizing the double-port resonator is measured to be higher than that utilizing single-port resonator. Moreover, the enhancement in plasma density and ion beam current utilizing the double-port resonator is more pronounced as higher microwave power applied to the plasma ion source. Therefore, the rectangular cavity resonator utilizing the double-port is expected to enhance the performance of plasma ion source in terms of ion beam extraction.     

Development of a compact permanent magnet helicon plasma source for ion beam bioengineering

A compact helicon plasma source was developed as a millimeter-sized ion source for ion beam bioengineering. By employing a stacked arrangement of annular-shaped permanent magnets, a uniform axial magnetic flux density up to 2.8 kG was obtained. A cost effective 118 MHz RF generator was built for adjusting forward output power from 0 to 40 W. The load impedance and matching network were then analyzed. A single loop antenna and circuit matching elements were placed on a compact printed circuit board for 50 Ω impedance matching. A plasma density up to 1.1 × 10(12) cm(-3) in the 10 mm diameter tube under the magnetic flux density was achieved with 35 W applied RF power.     

H- beam extraction from a cesium seeded field effect transistor based radio frequency negative hydrogen ion source

H(-) beam was successfully extracted from a cesium seeded ion source operated using a field effect transistor inverter power supply as a radio frequency (RF) wave source. High density hydrogen plasma more than 10(19) m(-3) was obtained using an external type antenna with RF frequency of lower than 0.5 MHz. The source was isolated by an isolation transformer and H(-) ion beam was extracted from a single aperture. Acceleration current and extraction current increased with the increase of extraction voltage. Addition of a small amount of cesium vapor into the source enhanced the currents.     

Effect of fast positive ions incident on caesiated plasma grid of negative ion source

This paper describes the effect on negative ion formation on a caesiated surface of the backscattering of positive ions approaching it with energy of a few tens of eV. For a positive ion energy of 45 eV, the surface produced negative ion current density due to these fast positive ions is 12 times larger than that due to thermal atoms, thus dominating the negative ion surface production instead of the thermal atoms, as considered until now.     

Analysis of the H- ion emissive surface in the extraction region of negative ion sources

To understand the plasma characteristics in the extraction region of negative H(-) sources is very important for the optimization of H(-) extraction from the sources. The profile of plasma density and electrostatic potential in the extraction region with and without extraction grid voltage are analyzed with a 2D particle in cell modeling of the NIFS-RD H(-) sources. The simulation results make clear the physical process forming a double ion plasma layer (which consists only of positive H(+) and negative H(-) ions) recently observed in the Cs-seeded experiments of the NIFS-R&D source in the vicinity of the extraction hole and the plasma grid. The results also give a useful insight into the formation mechanism of the plasma meniscus and the H(-) extraction process for such double ion plasma.     

Characteristics of the Berkeley multicusp ion source

The performance of a cubical permanent magnet generated line-cusp ion source has been investigated for use with neutral beam injectors. This source has been operated with discharge currents greater than 500 A and ion current densities higher than 400 mA/cm2 at the extraction grid. The uniformity of the density profile across the extraction area is found to be dependent on the gas pressure. By using a fast Langmuir probe sweeping circuit, the electron temperature and the plasma density and potential have been analyzed for different discharge powers and gas pressures. The heat load on the plasma grid when it is electrically floating or connected to the negative cathode has been compared calorimetrically. The use of lanthanum hexaboride and impregnated oxide cathodes have been investigated for the purpose of long pulse operation. The phenomenon of mode flipping is found to occur quite frequently during a discharge with these magnetic-field-free cathodes. Species composition as a function of discharge power and chamber length is measured by a mass spectrometer.