High current, high reliability negative ion sources for next generation accelerators

An electric filter made of polarized stainless steel grid has been used to effectively separate the plasma chamber in 2 independent zones: plasma generation zone and negative ion production zone. This allowed an important increase of the extracted current.

The computer code nIGUN was developed for an better understanding of the extraction mechanism in the case of negative hydrogen ions. In the revised sheath theory, accelerated electrons and H- ions, fast protons and thermal ions, like protons caesium and molecular ions can be taken into account by their space charge contribution. Based on a self-consistent theory of the plasma sheath, H- extraction electrodes can be optimized. The world wide distribution of the developed computer code is initiated (see URL) and has raised many inquiries already.

The possibility to build a hydrogen negative ion source using an array of ECR cells, operating at 2.45GHz, implemented in a magnetic multipole has been demonstrated. The magnetic field of the ECR cells provided a sufficient magnetic filtering effect. Thus cold plasma, suitable for negative ion formation by dissociative attachment, was produced in the vicinity of the ECR cells. This concept can be used both for negative ion sources for accelerators, and for sources required by magnetic fusion.

Increased Pulse Power at DESY. With a new collar design it was possible to increase the current from 40mA to 54mA. With an extraction opening 6.5 mm in diameter, the achieved current density is 163 mA/cm2. This is a new international record for an uncesiated H- source. It is the result of test series with different cone, cylinder and cone cylinder set up used on the DESY RF-Volume Ion Source (J. Peters et al.). In parallel, an increase in H production was expected with graphite walls. A test with the DESY collar was prepared and the cone part has been made bias able in order to change the bombarding energy. Finally, no improvement was found compared to the steel version of this collar (J. Peters et al.). Increased Pulse Length of the DESY Source to 3 ms. Moreover, it was important to show that the physics of the H- source also work for long pulses. The DESY transmitter is limited in his power. The usual repetition of 6 Hz for a 100µsec pulse was changed to 0.6Hz for a longer pulse. This way it was possible to demonstrate a pulse of 1ms with a current of 35mA at the beginning and 30mA at the end (30.11.06). It was necessary to use a new gas valve bias system. These tests were made with a more powerful transmitter, which was sent to DESY by a collaboration of SNS and FNAL. The tests were continued and it was possible to reach a 3 ms pulse (20.12.05). This is the longest and highest H- pulse which was produced until now with an uncesiated system. The current is more than 40mA at the beginning and about 30mA at the end. The H- drop is due to the decrease of HV and RF Power to the end of the pulse. It was also the highest current and the longest Hminus pulse of any volume source. In addition it is the first demonstration of such a long and high pulse in a volume source with an external antenna. (J. Peters et al.).

The duty cycle of the ISIS H-minus Penning Surface Plasma Ion Source has been successfully extended from 200us to 1500us at 50Hz. The output current of the ISIS ion source has been increased from 35mA to 70mA. The source is now the leading operational source of its kind. The source will be implemented in the new Front End Test Stand (FETS) being built at RAL. The FETS will be used as a high power driver for Neutrino factory, ISIS upgrades and other future accelerator projects. Work continues at RAL to further develop the source, improving its emittance and lifetime. The Chinese Spallation Neutron Source will be copying the design of the ISIS source for their accelerator.

DCU has primarily carried out work on plasma modeling and plasma diagnostics in an effort to understand H- formation. - A 2D Particle-in-Cell code with Monte-Carlo-Collisions and the Global Model Solver has been developed, with some limitations, for plasma modelling purposes. This model has been benchmarked against the energy distribution functions measured experimentally using an energy resolved quadrupole mass spectrometer on a symmetric, capacitively coupled system operating at 13.56MHz. However, it should be noted that the model fails to predict certain experimental structure in some instances. (D. O Connell, R. Zorat, M.M. Turner and A.R. Ellingboe) - The 2D Particle-in-Cell code with Monte-Carlo-Collisions and the Global Model Solver has also been used to simulate magnetic confinement and unconfinement in the diffusion region of a two-turn ICP rf source operated at 13.56MHz. The expected behavioural trends such as increased negative hydrogen ion density and the peaking of this density at lower pressures with magnetic confinement are predicted by the code. These trends in hydrogen ion density have also been measured experimentally using the Cavity Ring Down diagnostic which has been successfully implemented on this low density negative ion source. The successful implementation of CRD on this source required a protective gas flow near the CRD mirrors, operation of the source in pulsed mode and data acquisition with a fast digitiser. (R. Faulkner, F. Soberón, R. Zorat , M.M. Turner, M.B. Hopkins and A.R. Ellingboe) - A novel optical cavity technique, referred to as the External Cavity Technique has been introduced in an effort to measure line-integrated negative hydrogen ion density in a high density ion source. This technique is a variation of the traditional Cavity Ring Down technique, which was proving difficult to implement on high density ion sources where mirror degradation and cavity misalignment issues became restrictive to the proper application of the technique under ion source operation. (R. Faulkner, M. Bowden and M.B. Hopkins) - Two visits were made to Ecole Polytechnique in 2005 to understand laser photodetachment techniques and to perform such experiments close to the extraction region of the Camembert III source. (R. Faulkner) - Investigations into the existence of a double layer at the boundary between the source and diffusion region were carried out in collaboration with the Australian National University in an effort to increase source efficiency for the production of H-. (A.R. Ellingboe) - Development of an analytical model of a Dual-Frequency-Capacitive (DFC) Sheath was completed with the view to modelling of a small-volume, scaleable H- source. (J. Robiche , P.C. Boyle, M.M. Turner nd A.R. Ellingboe) - The HPNIS website at http://www.hpnis.dcu.ie has been developed at DCU and has undergone significant development over the course of the project.. (R. Faulkner, J. Robiche and A.R. Ellingboe).

FZJ intended to increase the available intensity of polarized ions for the study of structure and dynamics of hadrons at the COoler SYnchrotron facility COSY in Jülich (Germany). The gain in the polarized H- beam intensity provided for experimental runs at the synchrotron rose by a factor of three, compared to the status at the beginning of the funding period in 2002. The magnitude, the width and the shape of the beam pulses have been substantially improved. During the first 2 periods, FZJ started the replacement of the over 35 year old injector cyclotron by a super conducting linear accelerator. For that new injector the ion sources and low energy beam transport have been designed and partly realized. In the very end of 2003 the upgrade of the synchrotron facility was stopped. The activities have been redirected to improve the performance of the existing structures and an intensity upgrade program has been realized. With the new objective, the existing infrastructure has been investigated in detail. The improvement program that has been started years ago is still a work in progress. It can be expected that it will significantly contribute to the quality of the ongoing physics program at the COSY facility also in the future.

Negative ions offer the chance to use a non-destructive method for emittance measurements with good time resolution and with a high number of phase space points. Due to the low binding energy of the additional electron (0.6eV for H--ions) LASER light with a wavelength between 600 and 1100nm will produce a sufficient number of neutral H0-atoms and detached electrons. The neutral beam has the same phase space distribution as the H--beam because neither the laser photons nor the recoiling photodetached electrons transfer a significant momentum to the H0-atoms. A dipole bending magnet separates the neutral atoms from the H--beam. The great majority of negative ions can be used for further applications; the neutral beam will be detected with a scintillator and a CCD camera to determine the emittance.

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