Challenges of Dual Harmonic RF Systems John Thomason ISIS Synchrotron Group

Second Target Station 50 pps 40 pps

Increased Beam Intensity 1) The RFQ Accelerator • 4-rod 202.5 MHz RFQ accelerates 35 keV H− beam from ISIS ion source to 665 keV • Focuses and bunches with ~ 95% transmission efficiency (compared with ~ 60% for the old pre-injector)

Increased Beam Intensity 2) The Dual Harmonic RF System • Fundamental RF system of six cavities (1.3 – 3.1 MHz) gives up to 140 kV/turn • Four additonal RF cavities (2.6 – 6.2 MHz) give up to 80 kV/turn

Increased Phase Stable Regions • Increased phase stable regions, enhanced bunching factors and smaller beam loss • Potentially increase accelerated intensity from 2.5x1013 ppp (200 µA) to 3.75x1013 ppp (300 µA)

2RF System Overview 2nd Harmonic high power RF LPRF Controls: Amplitude, Phase & Cavity Tuning Driver Amplifier Anode Power Supply 2nd Harmonic high power RF Frequency Law Summing Amplifier Frequency Law Generator Master Oscillator 1.3 MHz - 3.1 MHz Dipole Search Coil Beam Phase Loop Radial Loop Trim Function Bunch Length Loop Existing Fundamental RF System  Phase Shifter Super-period Phase Splitter To Other 2nd Harmonic Cavity LPRF Systems Phase Modulator 2 RF Level Controller Func Gen. 2nd Harmonic Cavity Voltage Loop Func. Gen Phase Detector (Cavity Lock) Cavity Lock LPF RF Detector RF Limiters Phase Detector (Cavity Tune) Gated Summing Amplifier Delay Unit RF Summing Amplifier Phase Networks 2nd Harmonic Cavity Phase Loop 2nd Harmonic Cavity Tuning Loop Digital Signal Processor 2nd Harmonic Beam Compensation Loop Sum Electrode Func. Gen. Func. Gen. LPF Variable Delay Variable Gain Frequency Doubler 2.6 MHz – 6.2 MHz 2nd Harmonic low power RF HPD Cavity Cavity Bias Regulator Low Voltage Power Supply Cavity Lock Servo Cavity Gap A Voltage Monitor Summing Amplifier Phase Modulation and Delay Chassis 1RF 2RF A single Burle 4648VI tetrode is used to drive the 2RF cavity

2 × 2RF Cavity Running to TS-1 Operating Regime Trapped beam intensity (protons) Total loss Equivalent current to TS-1 (µA) 50 pps 40 pps 1RF (50 pps) 2.30×1013 2.76×1012 184 148 +2 × 2RF 2.65×1013 1.60×1012 212 170 (50/32 pps) 2.93×1013 2.70×1012 234 187

2RF Hardware Failures Anode Power Supplies But also significant problems with intermediate amplifiers, bias regulators, cavity interlocks, LPRF modules, etc. Problems not evident on test rig, only with beam User cycles limited to 2 × 2RF cavities to give some spares capacity

2RF Challenges Installation has to contend with very stringent space constraints, particularly in the ISIS ring, but also in cable trenches and RF hall Additional 2RF systems are notoriously difficult to get working at all on proton machines (see experiences of IPNS team at Argonne) System is very complex Reliability of equipment can only be proved under limited conditions on test rigs Very little dedicated commissioning time with beam available Much of this has had to be spent repairing hardware rather than actually setting up the systems Because ISIS can continue to run without 2RF (unlike any other of the accelerator systems) so far whenever 2RF systems have failed during a run operational/user pressure has dictated that it is left until the end of the run to mend them, hence compounding the lack of running with beam and having to use machine physics sessions to mend hardware Target constraints on TS-1 have limited achievable intensity during some user runs

2 × 2RF Cavity Running to TS-1 and TS-2 2 × 2RF cavities have been run for most of the time during the last 10 ISIS user cycles (starting with Cycle 2006/3), admittedly with some reliability issues and problems with mid-cycle (~ 5 ms) beam loss TS-1 began routine running at 40 pps in Cycle 2008/3 with beam to TS-2 being phased in during Cycles 2008/3 and 2008/4 End of Cycle 2008/4 Synchrotron average current 210 µA Beam to TS-1 Beam to TS-2

2 × 2RF Cavity Machine Physics (@ MS1/32) 3.27×1013 ppp injected (96%) 2.90×1013 ppp accelerated (93%) 2.90×1013 ppp extracted (100%) 20 December 2008 Average horizontal position R2BLM3 BLM sum BLM sum –(SP1+SP2) Losses were well controlled and we ran for the majority of the 8 hour shift at this intensity Well within BLM trip tolerances, but inhibiting on intensity Linac pulse length was 290µs, so in theory we could operate up to this level for next user run  

Future Work Try to start Cycle 2008/5 (10 February 2009) with 180 µA to TS-1 at 40 pps and 45 µA to TS-2 at 10 pps using 2 × 2RF cavities Gradually improve intensity and reliability running 2 × 2RF systems during user cycles Optimise running with 3 × 2RF cavities and 4 × 2RF cavities during machine physics sessions Eventual optimised running with 4 × 2RF cavities during user cycles (when we have a spare 2RF APS again) Install new Master Oscillator: new FPGA unit provides the RF frequency sweep for 1RF & 2RF cavities. Applies all phase modulation (delta, anti-phasing, geometrical offset)

2RF Feed-forward Beam Compensation Now – Fixed Band Pass Tried – 2 x “switch-in” Later – Tuneable Bunch 1 Bunch 2 Tetrode Amplifiers Intermediate Amplifier Subtracter - Variable gain delay 2RF Filter 2RF demand Function Generator Induced 2RF gap volts: No beam compensation Single (low frequency) Switched (low and high frequency)