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THE MICE RF SYSTEM
J.F.Orrett* [email protected], A.J.Moss,
ASTeC, Daresbury Laboratory, WA4 4AD, UK
Abstract
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
4616 Circuit
MICE RF Power Distribution
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.
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.
Concept
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.
RF Power Distribution, LBNL Circuit
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.
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.
MICE Cooling Channel
MICE RF MICE
Requirements
RF REQUIREMENTS
MICE 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.
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.
Input Cavity Tuning Annulus after silver plating and
replacement of fingerstock
Grid Circuit Pre and Post Refurbishment
MICE RF System (2 cavities shown)
LLRF4 Evaluation Board
Accelerator Science and Technology Centre
www.astec.ac.uk