1. Challenges of Dual Harmonic RF Systems
John Thomason
ISIS Synchrotron Group

2. Second Target Station
50 pps
40 pps

3. Increased Beam Intensity
1) The RFQ Accelerator
• 4-rod 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)

4. 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

5. 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)

6. 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 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

7. 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

8. 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

9. 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

10. 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 Beam to TS-2

11. 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  

12. 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)

13. 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)