Challenges of DualHarmonic RF Systems

ISIS Synchrotron Group

John Thomason

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)

• 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 Beam Intensity

2) The Dual Harmonic RF System

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)

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

2

RF

Detector

RF

Limiters

Phase Detector (Cavity Tune)

2

Gated Summing Amplifier

Delay

Unit

RF Summing Amplifier

Phase

Networks

2

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

HPDCavity

Cavity Bias Regulator

Cavity Bias RegulatorLow 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

2RF System Overview

OperatingRegime

Trappedbeam

intensity(protons

)

Totalbeamloss

(protons)

Equivalent current

to TS-1 (µA)

50 pps 40 pps

1RF(50 pps)

2.30×1013 2.76×1012 184 148

+2 × 2RF(50 pps)

2.65×1013 1.60×1012 212 170

+2 × 2RF(50/32 pps)

2.93×1013 2.70×1012 234 187

2 × 2RF Cavity Running to TS-1

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

End of Cycle 2008/4Synchrotron average current 210 µA

Beam to TS-1 Beam to TS-2

• 2 × 2RF cavities have been run for most of the time duringthe 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

3.27×1013 ppp injected (96%)2.90×1013 ppp accelerated (93%)2.90×1013 ppp extracted (100%)

Average horizontal position

R2BLM3

BLM sum

BLM sum –(SP1+SP2)

20 December 2008

2 × 2RF Cavity Machine Physics (@

MS1/32)

• 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

• 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)

• 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)

2RF Feed-forwardBeam Compensation

• Now – Fixed Band Pass• Tried – 2 x “switch-in”• Later – Tuneable

Bunch 1

Bunch 2

TetrodeAmplifiers

IntermediateAmplifier Subtracter

-

Variablegain

Variabledelay2RF Filter

2RFdemand

FunctionGenerator

FunctionGenerator

Induced 2RF gap volts:- No beam compensation- Single (low frequency)- Switched (low and high frequency)