Compact electrostatic beam optics for multi-element focused ion beams: simulation and experiments

Electrostatic beam optics for a multi-element focused ion beam (MEFIB) system comprising of a microwave multicusp plasma (ion) source is designed with the help of two widely known and commercially available beam simulation codes: AXCEL-INP and SIMION. The input parameters to the simulations are obtained from experiments carried out in the system. A single and a double Einzel lens system (ELS) with and without beam limiting apertures (S) have been investigated. For a 1 mm beam at the plasma electrode aperture, the rms emittance of the focused ion beam is found to reduce from ～0.9 mm mrad for single ELS to ～0.5 mm mrad for a double ELS, when S of 0.5 mm aperture size is employed. The emittance can be further improved to ～0.1 mm mrad by maintaining S at ground potential, leading to reduction in beam spot size (～10 μm). The double ELS design is optimized for different electrode geometrical parameters with tolerances of ±1 mm in electrode thickness, electrode aperture, inter electrode distance, and ±1° in electrode angle, providing a robust design. Experimental results obtained with the double ELS for the focused beam current and spot size, agree reasonably well with the simulations.     

Characterization of plasma parameters, first beam results, and status of electron cyclotron resonance source

Electron cyclotron resonance (ECR) plasma source at 50 keV, 30 mA proton current has been designed, fabricated, and assembled. Its plasma study has been done. Plasma chamber was excited with 350 W of microwave power at 2450 MHz, along with nitrogen and hydrogen gases. Microwave power was fed to the plasma chamber through waveguide. Plasma density and electron temperature were studied under various operating conditions, such as magnetic field, gas pressure, and transversal distance. Langmuir probe was used for plasma characterization using current-voltage variation. The nitrogen plasma density calculated was approximately 4.5 x 10(11) cm(-3), and electron temperatures of 3-10 eV (cold) and 45-85 eV (hot) were obtained. The total ion beam current of 2.5 mA was extracted, with two-electrode extraction geometry, at 15 keV beam energy. The optimization of the source is under progress to extract 30 mA proton beam current at 50 keV beam energy, using three-electrode extraction geometry. This source will be used as an injector to continuous wave radio frequency quadrupole, a part of 100 MeV proton linac. The required root-mean-square normalized beam emittance is less than 0.2pi mm mrad. This article presents the study of plasma parameters, first beam results, and status of ECR proton source.     

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.     

Note: emittance measurements of intense pulsed proton beam for different pulse length and repetition rate

The high intensity ion source (SILHI), in operation at CEA-Saclay, has been used to produce a 90 mA pulsed proton beam with pulse length and repetition rates suitable for the European Spallation Source (ESS) linac. Typical r-r(') rms normalized emittance values smaller than 0.2π mm mrad have been measured for operation in pulsed mode (0.01 < duty cycle < 0.15 and 1 ms < pulse duration < 10 ms) that are relevant for the design update of the Linac to be used at the ESS in Lund.     

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.     

Deuteron injector for Peking University Neutron Imaging Facility project

The deuteron injector developed for the PKUNIFTY (Peking University Neutron Imaging Facility) has been installed and commissioned at Peking University (PKU). The injector system must transfer 50 keV 50 mA of D(+) ion beam to the entrance of the 2 MeV radio frequency quadrupole (RFQ) with 10% duty factor (1 ms, 100 Hz). A compact 2.45 GHz permanent magnet electron cyclotron resonance (PMECR) ion source and a 1.36 m long low energy beam transport (LEBT) line using two solenoids was developed as the deuteron injector. A φ5 mm four-quadrant diaphragm was used to simulate the entrance of RFQ electrodes. The beam parameters are measured after this core with an emittance measurement unit (EMU) and a bending magnet for ion fraction analysis at the end of injector. During the commissioning, 77 mA of total deuteron beam was extracted from PMECR and 56 mA of pure D(+) beam that passed through the φ5 mm four-quadrant diaphragm was obtained at the position of RFQ entrance with the measured normalized rms emittance 0.12-0.16π?mm?mrad. Ion species analysis results show that the deuteron fraction is as high as 99.5%. All of the parameters satisfy PKUNIFTY's requirements. In this paper, we will describe the deuteron injector design and report the commissioning results as well as the initial operation.     

Optimization of a charge-state analyzer for electron cyclotron resonance ion source beams

A detailed experimental and simulation study of the extraction of a 24 keV He(+) beam from an ECR ion source and the subsequent beam transport through an analyzing magnet is presented. We find that such a slow ion beam is very sensitive to space-charge forces, but also that the neutralization of the beam's space charge by secondary electrons is virtually complete for beam currents up to at least 0.5 mA. The beam emittance directly behind the extraction system is 65 π mm mrad and is determined by the fact that the ion beam is extracted in the strong magnetic fringe field of the ion source. The relatively large emittance of the beam and its non-paraxiality lead, in combination with a relatively small magnet gap, to significant beam losses and a five-fold increase of the effective beam emittance during its transport through the analyzing magnet. The calculated beam profile and phase-space distributions in the image plane of the analyzing magnet agree well with measurements. The kinematic and magnet aberrations have been studied using the calculated second-order transfer map of the analyzing magnet, with which we can reproduce the phase-space distributions of the ion beam behind the analyzing magnet. Using the transfer map and trajectory calculations we have worked out an aberration compensation scheme based on the addition of compensating hexapole components to the main dipole field by modifying the shape of the poles. The simulations predict that by compensating the kinematic and geometric aberrations in this way and enlarging the pole gap the overall beam transport efficiency can be increased from 16% to 45%.     

Absolute beam brightness detector

In generally accepted emittance measurement, main attention is concentrated on emittance areas ?(x), ?(y) occupied by desired part of ion beam in transverse phase space and shape of these areas. The absolute beam phase density (brightness) as usually is not measured directly and the average beam brightness B is calculated from a beam intensity I and the transverse emittances. In the ion source and low energy beam transport (LEBT) optimization, it is important to preserve the beam brightness because some aberration of ion optic and beam instabilities can decrease the brightness of the central part of ion beam significantly. For these brightness measurements, it is convenient to use an absolute beam brightness detector with the brightness determination from one short considered in this article.     

Thermographic measurement of the emittance plot of a single positive ion beamlet

Measurements have been made of the emittance plot of a partly neutralized positive ion beamlet, using a slit, a polymer target, and an infrared camera. This thermographic approach is intrinsically linear and absolute (since the properties of the target are known and approximately independent of temperature). It is sufficiently sensitive that only one short pulse is required to capture the entire range of the angular coordinate y('). Ions and neutral atoms are both detected with equal sensitivity. The measurement is unaffected by secondary electrons emitted by the target and by electrons traveling with the positive ion beam, as both types of electron carry very little energy. No specialized electronics or beam deflection devices are required. Providing the region downstream of the mask is free of fields, the target can be several meters away from the mask, allowing good resolution in the y(') axis. The target can be as large as required.     

A plasma cathode for a radio-frequency gun

A plasma ferroelectric cathode is used to form electron beams with a high pulse charge and a high charge in an electron bunch in an rf electron gun of a 10-cm wavelength range. The design of the cathode is described, and the results of calculations of the densities of the cathode-emitted and the gun-outputted currents are presented. The operation of the cathode in the rf gun was studied experimentally: the electron energy, the pulse current, and the transverse emittance of the beam were measured. The electron beam obtained at the output of the single-resonator gun had a pulse current of up to 10 A, a pulse duration of 60 ns, and an electron energy of ?500 keV. The normalized beam emittance was 40 mm mrad.     

Stabilization of the phase-space portrait parameters of a hydrogen ion beam

This investigation carried out variations in the transverse phase-space portrait of an ion beam during a 200-μs pulse of a proton injector in the linear accelerator at the Institute for Nuclear Research. The parameters of the accelerated 400-keV beam at the entrance into the beam transport channel and the beam from the ion source were measured. Numerical simulation of ion-beam acceleration in the proton injector was performed. The noiseless mode of beam generation in the duoplasmatron was obtained. The generator of the duoplasmatron discharge current with a discharge current instability of ±1% or less was developed and put into operation. A compensated resistive-capacitive voltage divider was used in the accelerating tube. The beam emittance measurements showed that the variations in the phase-space portrait of the beam from the upgraded injector during the 200-μs pulse were absent within the measurement accuracy. The normalized emittance for 90% of the beam current was 0.15π × cm × mrad at an ion current of 65 mA.     

An alpha particle measurement system using an energetic neutral helium beam in ITER (invited)

An energetic helium neutral beam is involved in the beam neutralization measurement system of alpha particles confined in a DT fusion plasma. A full size strong-focusing He(+) ion source (2 A, the beam radius of 11.3 mm, the beam energy less than 20 keV). Present strong-focusing He(+) ion source shows an emittance diagram separated for each beamlet of multiple apertures without phase space mixing, despite the space charge of a beamlet is asymmetric and the beam flow is non-laminar. The emittance of beamlets in the peripheral region was larger than that of center. The heat load to the plasma electrode was studied to estimate the duty factor for the ITER application.     

Subcutoff microwave driven plasma ion sources for multielemental focused ion beam systems

A compact microwave driven plasma ion source for focused ion beam applications has been developed. Several gas species have been experimented including argon, krypton, and hydrogen. The plasma, confined by a minimum B multicusp magnetic field, has good radial and axial uniformity. The octupole multicusp configuration shows a superior performance in terms of plasma density (~1.3 x 10(11) cm(-3)) and electron temperature (7-15 eV) at a power density of 5-10 Wcm(2). Ion current densities ranging from a few hundreds to over 1000 mA/cm(2) have been obtained with different plasma electrode apertures. The ion source will be combined with electrostatic Einzel lenses and should be capable of producing multielemental focused ion beams for nanostructuring and implantations. The initial simulation results for the focused beams have been presented.     

Investigations on the structure of the extracted ion beam from an electron cyclotron resonance ion source

Using improved beam diagnostic tools, the structure of an ion beam extracted from an electron cyclotron resonance ion source (ECRIS) becomes visible. Especially viewing targets to display the beam profile and pepper pot devices for emittance measurements turned out to be very useful. On the contrary, diagnostic tools integrating over one space coordinate like wire harps for profile measurements or slit-slit devices, respectively slit-grid devices to measure the emittance might be applicable for beam transport investigations in a quadrupole channel, but are not very meaningful for investigations regarding the given ECRIS symmetry. Here we try to reproduce the experimentally found structure on the ion beam by simulation. For the simulation, a certain model has to be used to reproduce the experimental results. The model is also described in this paper.     

Ion energy distribution functions of low energy beams formed by wire extraction electrodes

The two-electrode extractor system made of 0.1 mm diameter tungsten wires separated by 0.7 mm has formed an argon ion beam with 50 V extraction potential. Energy spreads of the extracted beams were typically less than 2 eV when the beam current density was low. The beam intensity rapidly decreased as the distance between the extractor and the beam detector increased, indicating space charge limited transport of the beam. Problems associated with the emittance measurements are also discussed.     

Study of ion beam transport from the SECRAL electron cyclotron resonance ion source at the Institute of Modern Physics

Ion beam transport from the Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou (SECRAL) electron cyclotron resonance ion source was studied at the Institute of Modern Physics during 2010. Particle-in-cell simulations and experimental results have shown that both space charge and magnetic aberrations lead to a larger beam envelope and emittance growth. In the existing SECRAL extraction beam line, it has been shown that raising the solenoid lens magnetic field reduces aberrations in the subsequent dipole and results in lower emittance. Detailed beam emittance measurements are presented in this paper.     

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.     

The development of enabling technologies for producing active interrogation beams

A U.S./Russian collaboration of accelerator scientists was directed to the development of high averaged-current (～1?mA) and high-quality (emittance ～15?πmm?mrad; energy spread ～0.1%) 1.75 MeV proton beams to produce active interrogation beams that could be applied to counterterrorism. Several accelerator technologies were investigated. These included an electrostatic tandem accelerator of novel design, a compact cyclotron, and a storage ring with energy compensation and electron cooling. Production targets capable of withstanding the beam power levels were designed, fabricated, and tested. The cyclotron/storage-ring system was theoretically studied and computationally designed, and the electrostatic vacuum tandem accelerator at BINP was demonstrated for its potential in active interrogation of explosives and special nuclear materials.     

Intense source of spin-polarized electrons

Spin angular momentum conservation in chemiionization reactions involving optically oriented He(2(3)S) atoms in a flowing helium afterglow has been exploited to yield a source of spin-polarized electrons. Either transversely or longitudinally polarized electrons can be extracted. Polarized electron beam currents of approximately 2 muA have been realized at 40% polarization. The beam has an effective emittance of approximately 2 mrad/cm over the energy range 100-400 eV, an energy spread of less, similar0.15 eV, and the polarization is readily reversible. The source is relatively inexpensive and appears suitable for the majority of low-energy spin-dependent scattering experiments proposed to date.     

Intense beams from gases generated by a permanent magnet ECR ion source at PKU

An electron cyclotron resonance (ECR) ion source is designed for the production of high-current ion beams of various gaseous elements. At the Peking University (PKU), the primary study is focused on developing suitable permanent magnet ECR ion sources (PMECRs) for separated function radio frequency quadrupole (SFRFQ) accelerator and for Peking University Neutron Imaging Facility. Recently, other kinds of high-intensity ion beams are required for new acceleration structure demonstration, simulation of fusion reactor material irradiation, aviation bearing modification, and other applications. So we expanded the ion beam category from O(+), H(+), and D(+) to N(+), Ar(+), and He(+). Up to now, about 120 mA of H(+), 83 mA of D(+), 50 mA of O(+), 63 mA of N(+), 70 mA of Ar(+), and 65 mA of He(+) extracted at 50 kV through a φ 6 mm aperture were produced by the PMECRs at PKU. Their rms emittances are less than 0.2 π?mm?mrad. Tungsten samples were irradiated by H(+) or He(+) beam extracted from this ion source and H∕He holes and bubbles have been observed on the samples. A method to produce a high intensity H∕He mixed beam to study synergistic effect is developed for nuclear material irradiation. To design a He(+) beam injector for coupled radio frequency quadruple and SFRFQ cavity, He(+) beam transmission experiments were carried out on PKU low energy beam transport test bench and the transmission was less than 50%. It indicated that some electrode modifications must be done to decrease the divergence of He(+) beam.