RF systems for MICE Andrew Moss The MICE RF Group and the TIARA WP7 Team Contributions include Daresbury, RAL, CERN, LBNL, LANL, FNAL, Strathclyde & Sheffield Universities, and Imperial College NVEC 2014
Contents The amplifier system for MICE – Modernisation and recommissioning of amplifier systems – Operation of the amplifier systems towards achieving high power operation
The amplifier chain At Lawrence Berkeley National Laboratory (LBNL) there existed some amplifier systems from the Bevelac last used ~ MHz tetrode and triode systems capable of up to 2MW pulsed operation
Amplifier cavity 4616 super power tetrode Tube Cavity Socket Input cavity Output tuning stub Cavity
4616 Tetrode testing 60db directional coupler 200kW CW water load Starting at 10kV Grid one 250V Grid two 1800v 1kW drive 1mSec 1Hz pulsed operation Ideal 50 Ohm load 4616 data sheet
Tetrode testing The tetrode has adjustments for cavity frequency, output coupling and input frequency tuning, all of which effect each other Initial setup using an old tube to check that all systems were ok, 100kW achieved With new tube fitted and conditioned we were able to optimise the output up to 234kW at 1Hz 1mSec pulse operation with good gain and electrical conversion
HT inputTriode HT tank Water input with steam output Triode socket in HT tank Tuning stubs
Ad hoc and undocumented modifications had been made to improve the operation of the systems Considerable reverse engineering and revision was required Heater terminal Grid Anode Tap RF capacitor
RF input and movable Grid tap RF capacitor & resistor stacks 1 metre long cooled with compressed air
Parts ready for rebuild
The re-build process With the amplifier completely stripped down we were able to inspect all the components Change from grid modulated to cathode modulated operation All the spring fingers needed to be replaced as these are key to the operation of the amplifier system Many of the sections of the amplifier were silver plated to improve the surface condition of the parts Motorisation of the operation of the tuning sections, grid tap and anode tap which tune the performance of the amplifier system
Moveable coax sections inside the amplifier structure HT blocking capacitor Grid section with RF input Anode section
HT tank under modification HT input to tank had become distorted Reformed to correct shape and position using a heat and a 2 Ton winch Inner surface of tank was polished to remove high points
Assembly The amplifier was assembled in stages using many jigs that were made to align the fragile spring fingers into the coaxial structures of the amplifiers RF output 8 inch coax Triode socket in HT tank Motor drive anode tap
Output capacitor Water in and out at 35kV HT input 6 inch coax RF power out
Triode tube socket Moveable output capacitor TH116 triode We operate at constant HT with cathode modulation – cathode switched by IGBT between -400V to earth to switch tube into conduction
Triode testing June 2012 With an old triode valve fitted (to be safe) at 32kV 72A = 2.3 MW electrical power per pulse we achieved 1.0MW RF power into a test dummy load No significant microwave of X-Ray hazard was observed from the amplifier system while it was on Triode tube in HT tank HT and water
Power supply problems After a number of technical issues, power supply trips at 32kV power supply tube protection device (crowbar) was finally identified as the problem 140uF Capacitor bank (110kJ sorted energy) must be discharged in 5uS to prevent damage to tube in event of an arc New crowbar had to be fitted Amplifier system running into test load Crowbar protection device
June 2013 – MICE RF parameters archived – First triode final stage amplifier had demonstrated 1ms pulses at 1Hz, MHz – Up to 34kV bias, operation with good gain achieved Performance achieved: 2.06MW output RF 34kV bias voltage 129A forward average current =46% (electronic) Gain 10.8dB Input port return loss -12.5dB VSWR 1.6 – Drive from Tetrode 170kW output RF 18kV bias voltage 15.5A forward average current =61% (electronic) Gain 19dB – Drive from SSPA 2.27kW
The challenge has been achieved
MICE experimental hall