Introduction

MICE Magnetic Field Mitigation Review

Potential impact on ISIS of MICE Stray Magnetic Fields

24th September 2013Martin Hughes

Introduction

An ISIS perspective on potential problems arising from stray magnetic fields in and around the MICE hall.

Two main areas of concern:

• Dangers to people

• Dangers to ISIS equipment

MICE Hall SurroundingsGround floor

MICE Hall SurroundingsFirst floor

Personnel IssuesSurrounding the MICE hall are controlled access areas including:

• The ISIS Linac• The ISIS Synchrotron

And open access areas, such as:• The ISIS foyer• The ISIS MCR area and diagnostics room• The MICE control room area• The area outside the MICE shutter door

Many people could potentially be exposed to MICE stray fields in these areas. These include:

• ISIS staff• Non-ISIS STFC staff• Contractors• Facility users• Visitors

Personnel IssuesSTFC health and safety code 39 “working with static magnetic fields” specifies two levels of flux density for which action must be taken if the field extends into a working area:

• 5 Gauss – warning signs must be displayed at all entrances to the area warning of the potential danger to people having implanted medical devices such as pacemakers.

• 2000 Gauss – Access control must be put in place and documented work procedures must be prepared in order to work in the area.

Due to the potential for large numbers of people to pass through the open access areas, the 5 Gauss level can not be exceeded in these areas.

Equipment Issues

ISIS usually runs for between 120 and 160 days per year. It is of great importance to ISIS to always maximise the available beamtime during these user runs. The ISIS target for beam availability is 90%.

The financial cost of beam downtime during a user run can be estimated in the region of £10k/hour.

ISIS equipment in the vicinity of the MICE hall is located in the ISIS MCR, Diagnostics room, Linac, old EHT area, MICE Hall roof, and inside the MICE hall itself.

Equipment IssuesHEDS Beam Chopper

The HEDS (High Energy Drift Space) beam chopper power supply is currently located in the North-East corner of the MICE Hall.

The HEDS chopper is most often used during the beam physics period before a user run to help with beam setup, however access could be required during user runs on short notice and operation at the same time as the MICE solenoids is possible.

The power supply is based on thyratron technology – thyratron manufacturers give a maximum operating environmental field level of 500 Gauss. Also PSU control electronics to consider.

Equipment IssuesDSA Interlocks

The Decay Solenoid Area PPS system is tied into the Synchrotron PPS system, and as such any trip on the DSA interlocks will take off the ISIS beam. The control racks for the DSA interlocks are located in the MICE control room/rack room and contain several PPS crates, each of which contains several 24VDC relays and several hundred PCB relays. A fault on any of these relays could cause ISIS problems.

There are also several ISIS control boxes related to the DSA located inside the MICE Hall, all of which contain relays which could be affected.

Equipment IssuesLinac

Initial concerns were over interference with the H- beam being accelerated in the Linac, however these fears have been eased after the ISIS/MICE meeting held in March, where likely field levels were discussed.

Other equipment of potential concern in the Linac hall is the vacuum pumps which are on the MICE hall side of the Linac, power supplies and amplifiers which are on the far side of the Linac, and interlock racks similar to those controlling the DSA which are at the low energy end of the Linac (West end).

Equipment IssuesOld EHT Area

• Located to the North West of the MICE hall (before the start of the Linac).

• Currently being set up as a Linac test area.

• Same types of equipment as in the Linac.

• Radiation shielding has been installed in order to separate the EHT area from the Linac. This will make the area accessible during ISIS user runs, so personnel issues will be of concern.

Equipment IssuesMICE Hall Roof

The Linac air conditioning plant is currently located on the MICE hall roof.

The equipment is made up of standard air conditioning pumps and related equipment, some of which may be sensitive to strong magnetic fields.

Personnel access will be required to the roof for servicing of the equipment, so field levels must be known to ensure safe access.

Equipment IssuesISIS Main Control Room

The ISIS MCR is manned 24 hours a day, and is open access to all staff so will be covered by the 5 Gauss limit.

At the 5 Gauss level interference with equipment should not be an issue, however were the field higher than this there are still CRT monitors in use in the MCR as well as multiple hard disks, which may be sensitive to strong magnetic fields.

Equipment IssuesDiagnostics Room

Located behind the MCR, directly on the south wall of the MICE Hall.As with the MCR, open access to anybody on site so fields must be below 5 Gauss.

Equipment consists largely of racks of digital electronics used for pulse generation and timing signal generation. FPGA circuits are present, as are several PXI racks which may be sensitive to magnetic fields, however none of this equipment should not be in danger at the 5 Gauss level.

Closing SummaryGiven the projected field levels, the main concern from an ISIS perspective is that field levels in open access areas outside of the MICE Hall do not reach the 5 Gauss level which could present a potential danger to staff and visitors.

It appears that the danger to ISIS equipment in the vicinity is fairly small, with the most high risk items probably being those in the MICE hall itself such as the HEDS beam chopper, and DSA interlock related items.

Upon commissioning of the solenoid coils, ISIS would require that all of the surrounding area field levels are surveyed and recorded, which would have to be carried out in ISIS downtime to allow access to the Linac and other controlled access areas.