A Kurup, K Long, J Pozimski, P Savage

High power pulsed proton drivers (up to 5MW) will be required for a neutrino factory or the next generation neutron spallation sources and other applications. However, the highest pulsed power machine currently in operation is ISIS which has a pulsed proton beam power of 0.16MW. So developing a 5MW pulsed proton driver provides a significant challenge. Proton drivers typically begin with an H- ion linac section. This facilitates the injection into a circular accelerator or an accumulation ring using charge exchange injection, which strips the two electrons off the proton by using a stripping foil or laser. Using charge exchange injection allows the injected beam to populate the same phase space as the beam already circulating in the accelerator. This allows higher beam currents to be stored in the ring than is delivered by the linac.

The Front End Test Stand (FETS) is an experiment based at the Rutherford Lab built in collaboration between ISIS, ASTeC, Imperial College and the University of Warwick. This experiment will design, build and test the first stages necessary to produce a very high quality, chopped H- ion beam as required for high beam power on the target. The beam parameters are 3 MeV energy and 60 mA beam current at 50 Hz repetition rate and up to 2ms pulse duration . The FETS consists of an H- ion source, a low energy beam transport (LEBT) section, a radio frequency quadrupole (RFQ), a beam chopper and medium energy beam transport (MEBT). The test stand will be equipped with a set of conventional diagnostics and a laser based beam profile and emittance measurement device.

Picture of the Penning ion source
Fig 1. - Picture of the Penning ion source

A Penning ion source is used to produce H- ions by creating a hydrogen plasma in the presence of caesium vapour. The aim of the ion source is to produce a 60mA, 2ms long pulse of H- ions with an energy of 65keV, fifty times a second. High reliability and a very good beam quality are further requirements on the ion source.

The LEBT is a series of magnetic solenoids which focus the beam to adjust the size and angle of the beam envelope delivered by the ion source to fit into the requirements of the first accelerator stage (RFQ).

In order to accelerate the ions by the use of Radio Frequency (RF) fields they need first to be converted from a continuous stream of particles into a series of bunches. The RFQ consists mainly of 4 electrodes which produce a time varying electric field to focus the beam in the transverse direction. At the front of the RFQ the electrodes are slightly modulated to produce a longitudinal field component.

This longitudinal field changes with time and accelerates some ions and decelerates others which forms gaps in the stream of particles and groups the ions into bunches. Gently (adiabatically) bunching the beam keeps more than 95% of the ions in the beam and preserves the quality of the beam. If the bunching was done more rapidly the quality of the beam would deteriorate and lead to beam loss and activation of the components.

After the bunching section the modulation of the electrodes increases and produces stronger electric fields in the longitudinal direction for particle acceleration. All particles in the bunch will be in the right phase with respect to the RF and only see accelerating forces as the decelerating ones occur only in the bunch gaps. The RFQ will accelerate the H- ions from 65 keV up to 3 MeV.

The ions exiting the RFQ are a stream of bunches that are very close to each other, i.e. they are separated only by a few ns. If the beam is injected into a circular accelerator or storage ring after the linac as foreseen for a neutrino factory it would be very difficult to inject such a continuous stream of bunches into the ring.

It is therefore necessary to create a gap (about a few hundred microseconds long) in the bunch train. This is done using a chopper device. The chopper works by applying a high voltage perpendicular to the beam to remove a portion of it. The main challenge for the chopper is to be able to switch this voltage on very quickly, i.e. between the bunches, but have it last for long enough to produced the required long gap in the bunches.

Drawing of a design for the chopper line
Fig. 3 - Drawing of a design for the chopper line

This is done by having two stages to the design-a "fast" (which uses a transmission line as deflection electrode) and "slow" (which uses a plate electrode) chopper. The fast chopper is able to turn on within about 2ns but cannot last for a few microseconds. So, the fast chopper is used to remove the first and last part of the beam until the slow chopper comes online, which then removes a larger portion of the beam. This arrangement will give clean chopping with no partially chopped bunches. To prepare the beam for the chopping process and to condition the beam after the chopping for the next accelerator section a MEBT line, consisting of quadrupoles for transverse focussing and RF buncher cavities for rebunching of the beam, is used.

The design constraints and cost of a large facility like the neutrino factory, depends on the quality of the beam that is produced in the Front End. It is therefore essential to understand what the beam looks like after the chopper. The Front End Test Stand will be equipped with a set of conventional diagnostic devices (e.g. current transformers, Spectrometers, etc.) and newly developed, non-destructive laser based system to measure the beam profile and quality.

Further information can be found on these links:
FETS portal
UK Neutrino Factory Proton Driver