RF systems for MICEAndrew Moss

The MICE RF Group and the TIARA WP7 TeamContributions 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 ~1992

• 201MHz 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 testing60db directional coupler

200kW CW water load

Starting at 10kVGrid one 250VGrid two 1800v1kW drive1mSec 1Hz pulsed operationIdeal 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 stacks1 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 coaxRF 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, 2.06MW @ 201.25MHz– 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