Latest results of MICE amplifier testing

1. Latest results of MICE amplifier testing Andrew Moss DaresburyLaboratory August 2013

2. People Over the last couple of months a intensive period of RF testing and power supply optimisation by the following people has brought good results, thanks too:- • Kevin Ronald, Colin Whyte, Alex Dick –Strathclyde Uni • Chris White, Tim Stanley, Steve Griffiths – STFC • SaadAlasri – Imperial/STFC

3. Test system at Daresbury

4. Status of Amplifier s and power supplies • In June 2012 using an old ISIS test tube we reached 1MW RF output power. • New tubes were then put in the 4616 tetrode and 116 Triode amplifier cavities so that we could push towards 2MW demonstration. • However many power supply issues developed which limited our ability to sustain high voltage for periods of time, and by October 2012 we could not generate more than 300kW without crowbarring of the HV power supply • When we removed the new MICE tube from the amplifier cavity, it was clear that issues were occurring in the HV tank, the valve was burnt on its HV input contact • To speed up testing parts of the HT input of CERN amplifier were used on the LBNL amplifier cavity – causing incorrect location of the valve in the socket. Thankfully this damage is light and not a problem

5. Original LBNL design, water in , steam out CERN design has two water pipes, MICE must do the same – to speed this up we used some CERN parts This lead to a dimension change here and aloud the tube to sit too far down in the socket – resulting in finger strips sitting on painted area of triode tube, hence anode was not correctly connected to the supply

6. Restart of testing • During November 12 to January 13 new parts were manufactured to remove the incorrect valve seating issue and the amplifier rebuilt with the old ISIS tube again • Testing restarted however the limit of 300kW and power supply crowbarring was still present • Power supply testing revealed that the crowbar device itself was at fault and this was replaced with an old thyratron with a limited capability – max 30kV and 1/10th the current handle capability we needed, it would do but was at risk of being destroyed if a fault occurred • This allowed us to go back to RF testing with the old ISIS tube and we were able to get back to 1MW RF output power – the gain of the old tube was limiting us at 9dB – 200kW drive = 1MW output

7. Temporary crowbar solution • Colin Whyte had been negotiating with e2v over a replacement full spec crowbar, and was offered 3 prototype versions, condition unknown, for us to try as a fast non guaranteed solution rather than 6 months for a new crowbar • With prototype number one fitted (June 2013) we were able to standoff 38kV DC with reliable crowbar operation in test conditions without the RF amplifier in circuit • With the MICE Triode valve refitted to the amplifier circuit we were in a position to attempt to pass the previous high power record of 1.2MW

8. Going for power • Continuous measurement of X-ray and Microwave radiation was performed during high power operation – only background levels for in both cases • Steady improvements in power numbers started to appear over the following weeks, however there were still problems with crowbarring and low gain • Mark Keelan from ISIS alerted us to an adjustment we had not previous tried to use – ¼ wavelength tuning stub on the HT line – this should be adjusted to minimise RF going back towards the power supply and firing the crowbar

9. 2MW • Several attempts at tuning the HV quarter wavelength stub were made and in combination with adjusting the RF pulse structure we were able to minimise RF sent back inside the power supply • This is properly the key to stopping the problem crowbar events • Immediately we were able to run at 35kV and – • Continued to run at this level for a good period of time and preform other measurement's • Multiple runs at this power level

11. RF Parameters • Tetrode 18kV 15 amps 217kW drive • Triode 35kV 130 amps = 4.5MW electrical power • Electrical conversion efficiency – 48% • Gain – 10.3dB • Output power – 2.06MW • Very pleased with results and system looks stable at maximum power

14. The Next step • Tiara milestone needs to be achieved, this requires the amplifier system to be operated in the MICE hall • Amplifiers and power supplies are currently being striped out of test area ready for transport to RAL on 19th Aug • Program of work to rebuild system is understood but is very high work load • Components being delivered from Don Summers Mississippi – coax and test loads so that system can be run at a maximum of 1MW • In fact a huge amount of equipment is in transport to Don and then to RAL

16. Summary • Many electrical problems over the past year have been resolved • Crowbar device is temporary and needs a solution in the future for the MICE experiment – possibly a solid state device – tests will be done • 2MW RF power has been achieved with amplifier system after many years of work, lots of experienced gained and new RF people now on hand to dedicate their time • Amplifier system will be installed in experimental hall in the near future and operate