M. O. Whitehead

Optimizing the front end test stand high performance H− ion source at RALa)

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.

The front end test stand high performance H- ion source at Rutherford Appleton Laboratory.

The aim of the front end test stand (FETS) project is to demonstrate that chopped low energy beams of high quality can be produced. FETS consists of a 60 mA Penning Surface Plasma Ion Source, a three solenoid low energy beam transport, a 3 MeV radio frequency quadrupole, a chopper, and a comprehensive suite of diagnostics. This paper details the design and initial performance of the ion source and the laser profile measurement system. Beam current, profile, and emittance measurements are shown for different operating conditions.

Understanding extraction and beam transport in the ISIS H− Penning surface plasma ion sourcea)

The ISIS H(-) Penning surface plasma source has been developed to produce beam currents up to 70 mA and pulse lengths up to 1.5 ms at 50 Hz. This paper details the investigation into beam extraction and beam transport in an attempt to understand the beam emittance and to try to improve the emittance. A scintillator profile measurement technique has been developed to assess the performance of different plasma electrode apertures, extraction electrode geometries, and postextraction acceleration configurations. This work shows that the present extraction, beam transport, and postacceleration system are suboptimal and further work is required to improve it.

Electromagnetic modeling of the extraction region of the ISIS H− ion source

The ISIS Penning surface plasma source, which routinely produces 35 mA of H− ions during a 200 μs pulse at 50 Hz for uninterrupted periods of up to 50 days, is regarded as one of the leading operational sources in the world. The ISIS source should provide an excellent starting point for a development program to produce H− ion sources with performances exceeding those achieved today, which will be a key requirement for the next generation of high power proton accelerators. One goal is to produce 60 mA of H− ions from the source without large departures from the optimum conditions for source lifetime or increased emittance. As the ISIS source operates in the space-charge limited mode it is predicted that an increase in extraction potential from 17 to 25 kV should be sufficient to achieve this, and a suitable pulsed power supply for the ion source extraction electrode has been manufactured. An understanding of how extract geometry changes affect beam transport is essential for operation at higher extraction ...

Redesign of the Analysing Magnet in the ISIS H− Penning Ion Source

A full 3D electromagnetic finite element analysis and particle tracking study is undertaken of the ISIS Penning surface plasma H− ion source. The extraction electrode, 90° analysing magnet, post‐extraction acceleration gap and 700 mm of drift space have been modelled in CST Particle Studio 2008 to study the beam acceleration and transport at all points in the system. The analyzing magnet is found to have a sub‐optimal field index, causing beam divergence and contributing the beam loss. Different magnet pole piece geometries are modelled and the effects of space charge investigated. The best design for the analysing magnet involves a shallower intersection angle and larger separation of the pole faces. This provides radial focusing to the beam, leading to less collimation. Three new sets of magnet poles are manufactured and tested on the Ion Source Development Rig to compare with predictions.

Practical experience in extending the ion source and injection system H−-ion source duty cycle

The ion source and injection system H− Penning surface-plasma source is currently being developed on the ion source development rig at Rutherford Appleton Laboratory in order to meet the requirements for the next generation of high-power proton drivers. Finite element modeling has been used previously to study the effect of increasing the duty cycle. The main requirement to allow increased duty cycles is improved cooling. By simply reducing the thickness of a sheet of mica to improve thermal conductance to the cooling system, duty cycles of 1.5ms at 50Hz can be achieved. Slight increase in hydrogen flow rate is required as the duty cycle is increased. As the duty cycle is increased the output current reduces, however, there is no change in beam emittance. The source cooling system is described and the heat flows within the source are discussed.

Testing, Installation, Commissioning and First Operation of the ISIS RFQ Pre-Injector Upgrade

Situated at the Rutherford Appleton Laboratory (Oxon., UK), ISIS is currently the worlds most intense pulse spallation neutron source, delivering 160 kW of 800 MeV protons to a tungsten target at 50 Hz. A major facility upgrade programme involves the construction of a second, 10 Hz target and an increase in the total beam power of up to 50% (i.e. up to 240 kW). To achieve the planned increase in average beam current to 300 μA whilst maintaining the current manageable levels of beam loss, four 2nd harmonic RF cavities have been installed in the synchrotron and the ageing Cockcroft-Walton preinjector in the linac has been replaced with a 665 keV, 202.5 MHz, 4-rod Radio Frequency Quadrupole (RFQ). This paper describes the extensive testing, installation, commissioning and successful initial operation of the RFQ pre-injector upgrade.

Development of the front end test stand and vessel for extraction and source plasma analyses negative hydrogen ion sources at the Rutherford Appleton Laboratory

The ISIS pulsed spallation neutron and muon facility at the Rutherford Appleton Laboratory (RAL) in the UK uses a Penning surface plasma negative hydrogen ion source. Upgrade options for the ISIS accelerator system demand a higher current, lower emittance beam with longer pulse lengths from the injector. The Front End Test Stand is being constructed at RAL to meet the upgrade requirements using a modified ISIS ion source. A new 10% duty cycle 25 kV pulsed extraction power supply has been commissioned and the first meter of 3 MeV radio frequency quadrupole has been delivered. Simultaneously, a Vessel for Extraction and Source Plasma Analyses is under construction in a new laboratory at RAL. The detailed measurements of the plasma and extracted beam characteristics will allow a radical overhaul of the transport optics, potentially yielding a simpler source configuration with greater output and lifetime.

H− ion source test and development capabilities at ISIS

The ISIS ion source is a surface plasma ion source of the Penning type, and routinely produces 35 mA of H− ions during a 200 μs pulse at 50 Hz for uninterrupted periods of up to 50 days. However, because of the constant demands on ISIS from the neutron user community, very little ion source development has ever been possible. It is now necessary to produce sources with enhanced performances for next generation projects such as the European spallation source. Hence a dedicated ion source development rig (ISDR) has been constructed at Rutherford Appleton Laboratory to fully characterize the ISIS ion source and then facilitate appropriate development work. The ISDR has been designed to replicate the beam transport configurations on both the present ISIS preinjector and the proposed ISIS radio frequency quadrupole, while providing additional beam diagnostics equipment. The commissioning of the ISDR will be described, and initial results presented, along with future development plans.

Latest Results from the Front End Test Stand High Performance H− Ion Source at RAL

The aim of the Front End Test Stand (FETS) project is to demonstrate that chopped low energy beams of high quality can be produced. FETS consists of a high power Penning Surface Plasma Ion Source, a 3 solenoid LEBT, a 3 MeV RFQ, a chopper and a comprehensive suite of diagnostics. This paper briefly outlines the status of the project, hardware installation and modifications. Results from experiments running the H− ion source at 2 ms pulse length are detailed: the discharge current is varied between 20 A and 50 A. The discharge repetition rate is varied between 12.5 and 50 Hz. Hydrogen and Caesium vapour flow rates are varied. The effect of electrode surface temperature and beam current droop are discussed. Peak beam currents of over 60 mA for 2 ms pulse length can be achieved. Normalised r.m.s emittances of 0.3 πmm.mrads at the exit of the LEBT are presented for different source conditions.