THE MICE RF SYSTEM

J.F.Orrett* j.f.orrett@dl.ac.uk, A.J.Moss,

ASTeC, Daresbury Laboratory, WA4 4AD, UK

Accelerator Science and Technology Centre www.astec.ac.uk

The Muon Ionisation Cooling Experiment (MICE) at the Rutherford Appleton Laboratory uses normal conducting copper cavities to re-accelerate a muon beam after it has been retarded by liquid hydrogen absorbers. Each cavity operates at 200MHz and requires 1MW of RF power in a 1ms pulse at a repetition rate of 1Hz. In order to provide this power, a Thales TH116 triode, driven by a Burle 4616 tetrode is used, with each amplifier chain providing ~2.5MW. This power is then split between 2 cavities. The complete MICE RF system is described, including details of the low level RF, the power amplifiers and the coaxial power distribution system. Testing of the amplifier chain, power supplies and low level RF is described

Ionisation cooling is the process by which a particle beam is passed through a material medium (absorber) and individual particles lose energy (momentum) through ionisation interactions. The particle beam is then accelerated longitudinally using RF cavities. As the particles are losing transverse and longitudinal momentum but the RF cavities are only replacing longitudinal momentum this results in an overall reduction in the particle beam emittance.

The MICE experiment consists of three liquid hydrogen absorbers and eight RF accelerating cavities. Detectors and tracking devices before and after the cooling channel are used to measure the effectiveness of the ionisation cooling process.

MICE Cooling Channel

MICE RF REQUIREMENTSMICE will operate at 201 MHz, 1ms pulse width at 1 Hz and require RF power levels of ~ 1 MW per cavity to produce a measurable amount of cooling. Peak power levels of this magnitude, in this frequency band require conventional high power vacuum tube technology, triodes and tetrodes. Four chains of three amplifiers will each produce a final stage RF pulse power level of ~ 2.5 MW. This will be split using a 3 dB hybrid or coaxial matched tee to deliver ~ 1 MW of RF power to each cavity. The following RF amplifier stages will be used;• 4 kW Dressler solid state amplifier, this will be operated at ~ 2.5 kW output power. • Burle 4616 tetrode, this will be operated at ~ 250 kW output power.• Thales TH116 triode, this will be operated at ~ 2.5 MW output power.

MICE RF System (2 cavities shown)

4616 Circuit

The first Burle 4616 RF circuit has now been refurbished at Daresbury Laboratory. Ancillary equipment such as HT Power Supplies, Grid Power Supplies and Crowbar Protection have been built or procured.

4616 Power Supplies and Crowbar

The rebuilt 4616 circuit has been tested at Daresbury Laboratory. This test was carried out using the 4616 tube which was fitted in the RF circuit when it arrived from LBNL. Despite the age of the tube, and its lack of use for several years the circuit produced 170 kW of power, proving the serviceability of the RF circuit and ancillary equipment.

TH116 Circuit

The first TH116 circuit is now approaching the end of its refurbishment and rebuild. The High Voltage Power Supply and other ancillaries are also approaching completion. It is hoped to test the TH 116 circuit in the near future.

TH116 Power Supplies and Crowbar

Low Level Radio Frequency (LLRF)

The MICE RF Test Stand at Daresbury Laboratory will be used to evaluate a digital LLRF system. This system is based on the LLRF4 card designed by Larry Doolittle of LBNL.

LLRF4 Evaluation Board

MICE RF Power Distribution

MICE RF power distribution will be achieved using rigid co-axial line. The line will be pressurised with N2

.In addition to increasing voltage holdoff at the narrower sections of coax, this will also provide a useful interlock in the case of disconnected co-ax.

RF Power Distribution, LBNL Circuit

RF Power Distribution, CERN Circuit

Refurbishment

The 4616 circuits required relatively little refurbishment, apart from replacement of ageing hoses and electrical cabling. The TH116 circuits were in a far more dilapidated condition. RF surfaces were pitted, fingerstock was damaged and the cavity tuning mechanisms were seized. The first circuit has been completely stripped down and cleaned, damaged components have been replaced and RF surfaces have been silver plated. Also the hand wheel operated tuning mechanisms have been replaced with motors to enable remote tuning.

Input Cavity Tuning Annulus after silver plating and replacement of fingerstock

Grid Circuit Pre and Post Refurbishment

Abstract

MICE RF Requirements

Concept