Experience with X band klystrons and solid state modulators at CERN Gerry McMonagle & Xbox team International Workshop on Breakdown Science and High Gradient Technology (HG2015)

At the CLIC workshop in 2007 CERN decided that to complement the testing being done at KEK and SLAC a high power X band power source is necessary for testing structures on site at 12GHz Market survey showed that there was no source available from industry Initial contact with SLAC klystron department for a collaboration to develop a 12GHz klystron based on the existing 11.4 GHz tube was positive Wheels in motion for a collaboration between CERN and SLAC with financial contributions from CERN (1 year administrative delay). PSI and TRIESTE joined collaboration. 5 klystrons ordered (1 CERN and 2 each PSI and Trieste)

Installation and Requirements Klystron 50 MW 1500 ns 50 Hz test slot 200 MW, 0 ... 100 ns or 100 MW, 0 ... 350 ns phase modulation hybrid Modulator pulse compression CTF2 Gallery Specification for modulator based on XL5 parameters form SLAC In kind contribution for modulator from France (CEA Saclay) Only limitation in call for tender was size and volume, not the technology Scandinova won the bid with solid state technology

XL5 X Band Klystron spec Parameters Specifications units RF Frequency 11.9942 GHz Peak RF power 50 MW RF gain dB Efficiency % RF pulse length 1.5 µs Pulse repetition rate 50 (100) Hz Duty cycle Klystron voltage 410-470 kV Klystron current A Inverse klystron voltage 100 max Heater voltage 20 max V Heater current 30 max Heater start up surge current ?max Preheating period 60 min

Modulator delivered to CERN (only tested in dummy load waiting for klystron)

XL5 performance at SLAC (Feb 2010) Voltage, Current and rf waveforms at 57MW 1.25us Tube was damaged in manipulation after tests. 2nd tube delivered to CERN late 2010

Klystron installation in modulator with help from our SLAC collaborators Many teething problems with modulator calibration, interlocking etc. But we got first pulse on klystron before end of the year, stopped to do some modifications to modulator

Klystron testing Vacuum leak developed in klystron Repaired in situe at CERN (several months) Several issues with believing current and voltage calibration of the modulator But tube was pulsing !!! If we believe the figures from the voltage and current waveforms the perveance on the tube is lower than what was seen at SLAC There was a problem with the tube but diagnostics were difficult as we could not believe our measurements. Modulator was not tuned for klystron perveance, lots of overshoot and not a flat pulse. Did not help that the current measurement transformers were on the ”low” side of the secondary pulse transformer windings enhancing the overshoot at the beginning of the pulse due to capacitive coupling. Eventually did a mechanical squeeze of gun to correct perveance

While repairing klystron some additional measurement systems were implemented in the modulator to help with our calibration doubts Started pulsing klystron at short pulse width on 30th August 2011 All seems fine running at around 370kV 50Hz By 2nd September we got to near nominal pulse width at 50 Hz and at approximately 370kV

Some comments on the operation of Xbox1 modulator Nice pulse to pulse stability Pulse shape better once klystron problems were resolved but not yet optimised as we never got to nominal voltage due to testing schedule. GUI Not very user friendly Hexidecimal inputs and readings for interlock levels etc. No realtime and date stamp for interlock events that occurred. No accumulative counter for high voltage hours Only counts from when modulator is turned on and reset if powered down Initially no ΔT body temperature measurement, resolved by Scandinova

Xbox2 modulator and Klystron CERN decided to buy commercially produced XL5 klystron from industry (CPI) New modulator from Scandinova (after call for tender) Current measurement transformer now on high side of secondary windings  Still unfriendly GUI  Again late delivery of the tube meant that factory acceptance test was done with resistive load and also initial site acceptance tests. First klystron tests were performed using SLAC XL5 (limited peak voltage and pulse width)

50 Hz rep rate klystron operates in diode ok 1150 V IGBT , 50 Hz rep rate klystron operates in diode ok Pulse width reference 0.5µS Vkly 348kV Pulse width reference 0.6µS Vkly 348kV Pulse width reference 0.8µS Vkly 343kV Still some modulator issues to be resolved. To limit overshoot IGBTS were delay triggered for this mode of operation. Charging voltage had to be increased to compensate. Confident that klystron can run at this level until new CPI one arrives for crab cavity tests.

CPI Klystron Installed

233kV on klystron Instability starts Klystron voltage approximately 220 kV , dark blue trace and current Pulse width 1.5 μs 233kV on klystron Instability starts

V klystron 236 kV V klystron 242 kV V klystron 250 kV V klystron 262 kV

1000 V reference Klystron Current Klystron Voltage Tank Probe 1 MHz bandwidth zoom 700 MHz gun oscillation

Example of the pulse with ~ 5 MW RF peak power expected: CPI2-2 Klystron in Xbox2 First diode tests (no RF) of CPI#2 tube showed that 0.7 GHz gun oscillations starts at about 240 kV and even generate RF power from the input cavity: 12 GHz pulse from the input cavity Fortunately, going higher in voltage, the instability zone moves towards the rise/fall time, so the plat top can be used now. Example of the pulse with ~ 5 MW RF peak power expected:

Operation of XBOX2 modulator at full power Decided that even with oscillations tube can be run to finish crab cavity tests Modulator issues IGBT switch and charging supply interlocks and breakages at nominal klystron voltage initially caused by delay triggering but also a design modification was needed on the capacitor charging power supplies. Now operating correctly Problem with ΔT measurement on body. Very sensitive with instantaneous changes to the input temperature from cooling station. Input became a lot greater or lower than output measurement. Resolved by using temperature controlled chiller on body

Gun Oscillation problem with klystron not modulator No oscillations in XBOX1 with first CPI tube Only difference is the geometry of high voltage tank housing the pulse transformer and klystron Proposition for Gun Oscillation Suppression in CERN XBOX-2 Oil Tank from CPI Method 1: replace existing parts on base of gun with steel parts to provide magnetic loss. Method 2: in combination with method 1, add ferrite tiles to walls and bottom of oil tank. Method 3: in combination with method 1, add plates to mimic the internal shape of the XBOX-1 oil tank. Plates are made of steel to additionally provide magnetic loss. Pieces arrived at CERN, modification and tests to start 29th June

XBOX 3 Klystron specification 6 MW , 6μS, 400 Hz ( 4 required) Decided to do call for tender on a turnkey solution with both klystron and modulator under the responsibility of klystron manufacturer. Toshiba won contract, with the modulator subcontracted to Scandinova. 1st prototype tube successfully tested at Toshiba and turnkey solution delivered to CERN with the modulator already tuned for pulse shape. Excellent flat top and pulse to pulse stability

XBOX3a pulse shape Klystron current 92 A Klystron voltage 152 kV

Phase measurement 0.19 degrees fluctuation - Preliminary RF phase measurements last Thursday Klystron LLRF phase HPRF phase PXI measurement + - Difference measured to eliminate TWT phase fluctuations RMS measurement and mean value over 2 minutes 0.19 degrees fluctuation

XBOX 3 New modulator control system (SCANDICAT) Great improvement on older models  Nice GUI User friendly Time and date stamps of all events memorized Fixed hour counter for all high voltage time Interlocks levels can easily be changed via GUI Commissioning ongoing, may have some issues with klystron or modulator performance, not conclusive yet to report here

Some comments on Solid State modulators Unfortunately the capacitor charging power supplies and switch unit design has evolved and there is no backward compatibility i.e. extra spares needed for each type of modulator Flat top on voltage is far superior to line type modulators Instantaneous adjustment of voltage pulse width is a big advantage Line type modulators give the possibility of adjusting PFN to compensate for overshoot at lower voltages, solid state does not But due to the very good flat top and pulse to pulse stability working in the non saturated regime of the klystron is possible

Other news A new solid state klystron driver prototype has just been delvered to CERN 330 W , 6 μS, 40 Hz Performance will be evaluated in the next few weeks. CPI klystron number 3 under test. Once delivered XL5 klystron will be returned to SLAC for repair. XBOX1 modulator has had two faulty solenoid supplies in the last week (over temperature) still to be evaluated The supplies are out of their 5 year manufacturer warranty.

Conclusions Progress in high power solid state modulators is still evolving. Scandinova and hopefully other manufacturers, will listen to our positive and negative feedback and implement the changes required by our community in order to improve the already encouraging performance and reliability of the modulators.

Thank you for your attention