Negative Ion Accelerator Research Articles

Theoretical Study of the Electrostatic Lens Aberrations of a Negative Ion Accelerator for a Neutral Beam Injector

Aberrations due to the electrostatic lenses of a negative ion accelerator for a neutral beam injector and the space charge effect are theoretically investigated. A multi-stage extractor/accelerator is modeled and the aberration coefficients are numerically calculated using the eikonal method, which is conventionally used in electron optics. The aberrations are compared with the radii of a beam core with good beam divergence and a beam halo with poor beam divergence. H- beamlet profile measurements give the 1/e radii of the beam core and beam halo of 5.8 mm (beam divergence angle: 6 mrad) and 11.5 mm (beam divergence angle: 12 mrad), respectively. When the beam divergence angle of the beam core is 5 mrad and the beam energy is 406 keV, the aberrations due to the electrostatic lenses are less than a few millimeters, thus are less than the radii of the beam core and beam halo. The geometrical aberrations due to the space charge effect (negative ion current density: 10 mA/cm2 ), however, are estimated to be much larger than the radius of the beam halo. Although the aperture radii of the grids are not taken into account in this estimation, the results indicate that the space charge effect is an important factor in the aberration or beam halo in a negative ion accelerator.

Power Loading of Electrons Ejected From the JT-60 Negative Ion Source

The power loading of electrons ejected from the negative ion accelerator to the beamline was first measured in the negative-ion-based neutral beam injector on JT-60U. At 0.3 Pa of the operating pressure in the arc chamber, the heat flux and the total power load for the single segment were ~8 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 27 kW for the D ion beam of 300 keV and 3.4 A, respectively. The normalized total power loading on the electron dump was no more than 2.6% of the electric power in the acceleration power supply. About 70% of the total power is originated by the electrons stripped from D ions due to collisions with residual gas molecules in the accelerator. The calculation of the stripped electron trajectories shows that the electrons stripped in the second acceleration gap are the main origin of the power loading in the beamline.

Real-time imaging of surface evolution driven by variable-energy ion irradiation

We describe the design of a tandem instrument combining a low-energy electron microscope (LEEM) and a negative ion accelerator. This instrument provides video rate imaging of surface microtopography and the dynamics of its evolution during irradiation by energetic ions, at temperatures up to 1700 K. The negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range 0–5 keV, and with current densities up to 30 μA/cm 2 (∼2×10 14 ions/cm 2 s or ∼0.2 ML/s). The LEEM operates at a base pressure in the 10 −9 Pa range. We describe the design and operating principles of the instrument and present examples of Pt(1 1 1) and Si(0 0 1) self-ion irradiation experiments.

Thermo-mechanical design of the SINGAP accelerator grids for ITER NB injectors

The SINGle Aperture–SINgle GAP (SINGAP) accelerator for ITER neutral beam injector foresees four grids for the extraction and acceleration of negative ions, instead of the seven grids of the Multi-Aperture Multi-Grid (MAMuG) reference configuration. The grids have to fulfil specific requirements coming from ion extraction, beam optics and thermo-mechanical issues. This paper focuses on the thermo-hydraulic and thermo-mechanical design of the grids carried out by Consorzio RFX for the design of the first ITER NB injector and the ITER NB Test Facility. The cooling circuit design (position and shape of the channels) and the cooling parameters (water coolant temperatures, pressure and velocity) were optimized with sensitivity analyses in order to satisfy the grid functional requirements (temperatures, stresses, in plane and out of plane deformations). The design required a complete modelling of the grids and their support frames by means of 3D FE and CAD models.

1 MeV, ampere class accelerator R&D for ITER

The present objective of the 1 MeV electrostatic accelerator R&D for the ITER NB system is the acceleration of ampere class negative ion beams at the required current density (200 A m−2) up to the beam energy of 1 MeV. To produce such high current density H− ions, the KAMABOKO negative ion source was operated under high power arc discharge (≤40 kW) and with Caesium seeding for enhancement of the negative ion surface production. The H− ion beams of 146 A m−2 (total ion current: 0.2 A) have been obtained stably at the beam energy of 836 keV (pulse length: ≥0.2 s). The pulse length was limited due to the power density of the beams, which is more than twice as high as those of the existing negative ion based NB systems (JT-60U and LHD). A preliminary measurement of beam optics showed a beamlet divergence of ≈5 mrad even in the beams of such high power density. The Bremsstrahlung generation as a consequence of electron acceleration was estimated. A discussion then follows for photoelectron production and possible breakdowns in insulators for the acceleration of higher current negative ions.

Experimental results from the Cadarache 1 MV test bed with SINGAP accelerators

A prototype negative ion accelerator, based on a pre-accelerator normally used for positive ion acceleration, was used on the Cadarache MV negative ion beam facility up to 2004. Even though this accelerator demonstrated the feasibility of SINgle GAP, SINGle APerture (SINGAP) accelerators it was not possible to produce beams with the optical quality required for ITER. A new ‘ITER-like’ accelerator, which is a scaled down version of the ITER SINGAP accelerator, has been built and installed on the Cadarache 1 MV test bed. The objective of the ‘ITER-like’ is to demonstrate reliable D− beam acceleration as close as possible to 1 MeV with a current density j− ≈ 200 A m−2 with the beam optics required for ITER, i.e. a beamlet divergence of ⩽7 mrad and beamlet steering within ±2 mrad of that specified. High voltage hold-off tests have been performed and 940 kV has been held without breakdowns. Beams up to 850 keV (D−, 15 A m−2) were obtained after four weeks of experiments and the highest current density that has been obtained so far is 150 A m−2 (D−, 580 keV, 23 mA for one beamlet). The required beam optics for ITER has been obtained at near perveance match (at 120 A m−2 D−, 727 keV, 18.5 mA for one beamlet), but a larger than expected halo has been measured.

Caesium and tungsten behaviour in the filamented arc driven Kamaboko-III negative ion source

The ITER neutral beam injection is based on the acceleration and neutralization of negative deuterium ions. The target performance for the ITER beam source is to accelerate to 1 MeV a 40 A D− beam, with a current density of 200 A m−2, with pulse lengths of ⩾1000 s. It was found that in long pulse operation the negative ion yield from the filamented Kamaboko III ion source (a model of ITER ion source) degrades in comparison with short pulse operation, <5 s. This could be linked to the behaviour of caesium (Cs), which is added to the source to increase its negative ion yield and tungsten (W) evaporated from filaments. Cs and W are co-adsorbed on the source walls and the plasma grid and the composition of this coating can vary during long pulse operation. The possible consequences of this changing surface on the negative ion production will be discussed.Tungsten filaments have a limited lifetime in the ion source and changing filaments and refilling of the Cs oven are the only scheduled maintenance events for the ITER injectors. These are complicated operations as the ITER injectors will be highly activated and all maintenance has to be carried out remotely. Therefore, increasing the filament lifetime and decreasing the Cs consumption are highly desirable. This paper presents results of relative measurements (including spectroscopic and chemical) of the W content of the Kamaboko-III source and reports relevant calculations on Cs consumption and W evaporation.

Some lessons from long pulse operation of negative ion sources and accelerators

Operation of the neutral beam system of ITER for the entire ITER pulse is foreseen, with pulse lengths extending up to 1 h. Operation for such a pulse length is entirely new for neutral beam systems and the associated sub systems. This paper reviews some of the experience gained so far in the operation of the MANTIS test bed where long pulse operation of the ITER reference type of negative ion source is being studied. The three identified adverse effects of long pulse operation—reduced negative ion yield, reduced plasma grid temperature effect (see below) and increased caesium consumption—are still being studied, but some tentative explanations for each effect are given. In addition to the aforementioned effects, some operational difficulties associated with long pulses will be discussed, and some areas where caution will be needed in the future long pulse, high power operation of the ITER injectors are indicated.

Numerical simulations and experiments of simultaneous acceleration of positive and negative ions in a radio frequency quadrupole

The simultaneous acceleration of ${\mathrm{O}}^{+}$ and ${\mathrm{O}}^{\ensuremath{-}}$ beams in a radio frequency quadruple accelerator has been numerically simulated and the results are presented in this paper. The micropulses of ${\mathrm{O}}^{+}$ and ${\mathrm{O}}^{\ensuremath{-}}$ beams have been measured in dual beam acceleration experiments, which verified the numerical simulations and feasibilities of dual beam acceleration.

Production of carbon stripper foils for high-power cyclotrons

TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics operates three industrial negative ion cyclotrons for commercial radioisotope production. Two of these cyclotrons (TR30 and TR30-2) can deliver 30 MeV protons at beam currents in excess of 1000 μA and are designed for optional dual beam extraction and continuous operation. High-power negative ion accelerators use stripper foils as thick as 500 μg/cm 2. It has been found that very smooth films with nanocrystalline microstructure perform the best in high-current applications. However, high-quality thick foils with good uniformity are difficult to manufacture. To meet our specific requirements, we have developed a carbon arc deposition system capable of producing durable, homogeneous carbon stripper foils. In this paper, we report on the fabrication of carbon foils with a thickness of 100–200 μg/cm 2. The manufacturing equipment capable of producing ∼600 cm 2 of foil in a single run and the process details are described. Properties of these foils and their performance in the cyclotron are discussed.

The operational phase of negative ion beam systems on JT-60U and LHD

This paper reviews the operational phase of the negative-ion-based neutral beam systems on the JT-60U tokamak in Naka, Japan and the large helical device (LHD) stellarator in Toki, Japan. These systems were the first high power negative ion beam systems to be deployed for any application, and thus represented large advances in the state of the art for negative ion sources and accelerators, especially since the ions used were hydrogen and deuterium, which have only a modest electron affinity. This paper reviews the systems, the principal problems encountered, and the improvements they engendered, as well as the progress of these systems to the present time. The role of neutral beams in fusion is also discussed, and some of the contributions of the negative ion systems to the physics programs of JT-60U and LHD are briefly reviewed. These systems have been central to the success of JT-60U and LHD, and the knowledge gained about their characteristics should provide a strong basis for the development of the next generation of negative-ion-based neutral beams for ITER and other large fusion devices.

Development of high performance negative ion sources and accelerators for MeV class neutral beam injectors

The operation of an accelerator at low pressure is an essential requirement to reduce the stripping loss of negative ions, which, in turn, results in high efficiency of the neutral beam systems. For this purpose, a vacuum insulated beam source (VIBS) has been developed at Japan Atomic Energy Research Institute, which reduces the gas pressure in the accelerator by enhanced gas conductance through the accelerator. The VIBS achieves a high voltage insulation of 1 MV by immersing the whole structure of the accelerator in vacuum with a long (∼1.8 m) insulation distance. Results of the voltage holding test using a long vacuum gap of 1.8 m indicate that a transition from vacuum discharge to gas discharge occurs at around 0.2 Pa m in the long vacuum gap. So far, the VIBS succeeded in accelerating a 20 mA (H−) beam up to 970 keV for 1 s. It has been demonstrated that the high voltage holding capability of the 1 MV bushing surrounding the VIBS accelerator could be drastically improved by installing new large stress rings that reduces the electric field concentration at the triple junction. After implementing this change, the VIBS sustained 1 MV stably for more than 1200 s. Acceleration of ampere class H− beams at high current density is to be started soon to demonstrate ITER relevant beam optics.The operation of a negative ion source at low pressure is also essential to reduce the stripping loss. However, it was not very easy to attain high current density H− ions at low pressure, since the destruction cross-section of the negative ion becomes large if the electron temperature is >1 eV in low pressure discharge. Using a strong magnetic filter to lower the electron temperature, and introducing higher arc discharge power to compensate for the reduction of plasma density through the filter, an H− ion beam of 310 A m−2 was extracted at a very low pressure of 0.1 Pa. This satisfies the ITER requirement of current density at one-third of the ITER design pressure (0.3 Pa).

Simulation of the Accelerating Systems of Cyclotrons for Applications

The results of a comparative numerical investigation of three types of resonator systems of a cyclotron for acceleration of negative hydrogen and deuterium ions up to maximum energy 30 and 15 MeV, respectively, are presented. It is shown that a vertical system with two half-wave resonators excited in-phase has the smallest active losses and the smallest differential of the amplitude of the accelerating voltage along the edge of the dees and is effective for accelerating particles at dual working frequencies.

The operational performance of the Yale ESTU

The ESTU began operation in 1988 and achieved the design voltage of 20 MV in 1990. Since that time, improvements to the gas handling system, negative ion injector, accelerator terminal and control system have greatly increased its capability and reliability. Today, the ESTU can efficiently produce an extensive assortment of stable ions at wide-ranging energies in support of low-energy nuclear physics.

SINGAP: The European concept for negative ion acceleration in the ITER neutral injectors

In contrast to the multiaperture–multigrid concept of the International Thermonuclear Experimental Reactor (ITER) injector reference design in which D− acceleration to 40 A, 1 MeV is achieved through seven grids containing 1280 apertures each, the European SINGAP (single gap–single aperture) concept proposes acceleration of 1280 preaccelerated (20–50 keV) beamlets to 1 MeV in a single step. During acceleration the beamlets are merged into 16 groups of 80 beamlets. These 16 so-called hyperbeamlets then emerge from an exit grid containing only 16 very large apertures. This concept is expected to reduce cost and increase the tolerance to mechanical errors and stray magnetic fields. The SINGAP physics is described here. The SINGAP beam optics is calculated to provide good beam transmission (86%) to the ITER.

The next step in the development of a negative ion beam plasma neutralizer for ITER NBI

Deuterium atom beam injectors developed for ITER plasma heating and current drive are based on negative ion acceleration and further neutralization with a gas target. The maximal efficiency of the gas stripping process is 60%. The replacement of the gas neutralizer by a plasma one must increase the neutral yield to 80%. An overview of experimental studies of microwave discharges in a multicusp magnetic system chosen as the base device for plasma neutralizer (PN) realization and design development of PNs for ITER neutral beam injectors (NBIs) is presented. The experimental results achieved with the PN model PNX-U are discussed. Plasma confinement, gas flows and ionization degree were investigated. High density plasmas with ne ∼1018 m-3 with low electron and ion temperatures ( ≈ 5-6 eV) and a high ionization degree (not less than 40%) at its centre have been generated in operation with argon.

The isochronous cyclotron: principles and recent developments

The principals of a cyclotron are described. A magnetic field guides the ions in circular paths, while an electric field accelerates them. The main problem in any accelerator is not to accelerate ions, but to focus them. An isochronous cyclotron overrules the problems related to relativistic mass increase during acceleration. Harmonic operation and negative (vs positive) ion acceleration (and extraction) are explained, as they make dedicated PET cyclotrons a simple, reliable, and suitable tool. The characteristics of such PET cyclotrons are described, as well as their technical implementation. The IBA 18/9 PET cyclotron is given as an example.

Optimization of Cs deposition in the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection system

A compact cesium deposition system was used for direct deposition of cesium atoms and ions onto the inner surface of the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection (LHD-NBI), system. A small, well defined amount of cesium deposition in the range of 3–200 mg was tested. Negative ion extraction and acceleration were carried out both in the pure hydrogen operation mode and in the cesium mode. Single Cs deposition of 3–30 mg to the plasma chamber has produced temporary 2–5 times increases of H− yield, but the yield was decreased within several discharge pulses to the previous steady-state value. Two consecutive 30 mg depositions done within a 3–5 h/60 shot interval, produced a similar temporary increase of H− beam, but reached a large H− yield steady-state value. Deposition of larger 0.1–0.2 g Cs portions with a 20–120 h/150–270 shot interval improved the H− yield for a long (2–5 days) period of operation. Directed depositions of Cs to the various walls of the plasma chamber showed approximately the same H− increase. Deposition of 0.13 g Cs to a surface polluted by a water leak, produced a temporary increase, and a H− steady-state level similar to that from a single 30 mg cesium deposition. Deposition of 0.1 g with a cesium plasma produced one half the H− yield obtained by deposition of the same amount of cesium atoms. A higher steady-state H− current value and a smaller rate of H− yield decrease was recorded during the eight filament discharge operation, as compared to the 12 filament operation at the same discharge power.

European contributions to the beam source design and R&D of the ITER neutral beam injectors

The article reports on the progress made by the ITER European Home Team in strong interaction with the ITER Joint Central Team and the Japan Atomic Energy Research Institute regarding several key aspects of the beam source for the ITER injectors: (1) Integration of the SINGAP accelerator into the ITER injector design. This is a substantially simpler concept than the multiaperture, multigap (MAMuG) accelerator of the ITER NBI reference design that has the potential for significant cost savings and that avoids some of the weaknesses of the reference design such as the need for intermediate high voltage potentials from the high voltage power supply and pressurized gas insulation. (2) High energy negative ion acceleration using a SINGAP accelerator. (3) Long pulse (i.e. >1000 s) negative ion source operation in deuterium. (4) RF source development, which could reduce the scheduled maintenance of the ITER injectors (as it uses no filaments), and simplify the transmission line and the auxiliary power supplies for the ion source.

Calculations of stray electron trajectory for designing negative ion accelerators of neutral beam injector

A simulation code named “STELOC2” has been developed to calculate the stray electron trajectories of negative ion sources for neutral beam injectors (NBIs). The electrons stripped from the negative ions were treated as one of the main sources of stray electrons. This code was applied to a negative hydrogen ion accelerator for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). Calculation results showed that stripped electrons and electrons secondarily produced by stripped electron impacts on the extraction grids account for the main part of accelerated electrons. By using STELOC2, the effect of electron suppression grid was confirmed.