Progress on dual harmonic acceleration on the isis synchrotron

Abstract

The ISIS facility at the Rutherford Appleton Laboratory in the UK is currently the most intense pulsed, spallation, neutron source. The accelerator consists of a 70 MeV H^- linac and an 800 MeV, 50 Hz, rapid cycling proton synchrotron. The synchrotron beam intensity is typically 2.25 times 10¹³ protons per pulse, corresponding to a mean current of 180 muA. The synchrotron beam is accelerated using six, ferrite loaded, RF cavities with harmonic number 2. Four additional, harmonic number 4, cavities have been installed to increase the beam bunching factor with the potential to raise the operating current to 300 muA. The dual harmonic system has now been used operationally for the first time, running reliably throughout the last ISIS user cycle of 2006. This paper reports on the hardware commissioning, beam tests and improved operational results obtained so far with dual harmonic acceleration.

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PROGRESS ON DUAL HARMONIC ACCELERATION ON THE ISIS
SYNCHROTRON




A. Seville, I. Gardner, J. Thomason, D. Adams, D. Bayley, C. Warsop,
STFC, Rutherford Appleton Laboratory, Chilton, Didcot, UK.


Abstract
The ISIS synchrotron at the Rutherford Appleton
Laboratory in the UK is currently undergoing an RF
upgrade. Four h=4, or second harmonic (2RF), cavities
have been installed in addition to the existing six h=2,
fundamental frequency (1RF), cavities and should be
capable of increasing the operating current from 200 to
300µA. Two of the four cavities have been in operation
for the last 2 user cycles of 2007 improving trapping
losses and increasing operating currents beyond 200µA.
The remaining two cavities were commissioned in the
spring of 2008. This paper reports on hardware
commissioning, beam tests and beam simulation results.
INTRODUCTION
Over the last twenty years, acceleration of the ISIS
synchrotron beam has been provided by six two-gap RF
cavities. With this arrangement ~2.75×1013 protons can be
held in the synchrotron throughout the 10ms accelerating
cycle from 70 to 800 MeV during which the RF sweeps
from 1.3 to 3.1 MHz. The maximum mean beam current
which can be accelerated by the synchrotron is ~220µA,
although for ease of active maintenance, beam loss limits
the operational beam current to ~180µA. The addition of
a second harmonic component [1,2] to the RF waveform,
as shown in figure 1, should allow the acceleration of
higher currents by extending the phase stable region and
therefore increasing the bunching factor.
Figure 1: Addition of 1RF and 2RF components.
The longitudinal phase acceptance is increased due to
the addition of the 2RF component, giving a higher
trapping efficiency. Simulations indicate that up to
~3.75×1013 protons, or ~6 µC of protons, can be held and
accelerated using this technique. In ISIS the 2RF
component is provided by four 2RF cavities, installed in
super-periods (SP) 4, 5, 6 and 8. The cavities are similar
in design to the existing fundamental frequency cavities,
04 Hadron Accelerators
but are approximately half the length. As with the 1RF
cavities, the resonant frequency of the 2RF cavities has to
sweep throughout the acceleration cycle (at twice the
fundamental frequency, 2.6 to 6.2 MHz) to match the
changing rotational frequency. This sweeping is effected,
by loading the 2RF cavities with ferrite and then
sweeping the ferrite bias current throughout the
acceleration cycle to change the permeability of the ferrite
and hence the inductive element of the equivalent L-C
circuit. The hardware necessary for driving the new 2RF
cavities is based on that used very successfully over the
last twenty years for the fundamental cavities, but the
electrical and electronic hardware has been updated where
appropriate.
COMMISSIONING THE 2RF SYSTEMS
Since the last reported results [3], there has been much
work done on the 2RF systems. Shortly after the
successful operation of the 2RF systems giving dual
harmonic accelerated beam of over 200μA in December
2006, further machine physics tests were hampered by a
failure in the transformer of one of the 2RF anode power
supply for the 2RF system in SP6, which rendered the
SP6 2RF cavity unusable. Fortunately, the long shutdown
from December 2006 to October 2007, scheduled for
preparatory work on ISIS Target Station 2, gave an
opportunity to replace the anode power supply with a
spare unit. Other work on the 2RF systems was also
carried out during the long shutdown. The low power RF
(LPRF) systems were re-styled and housed in vertical
CAMAC style crates, as shown in figure 2, in order to
reduce over-heating seen on the previous units. The
bandwidths of the level control loops and tuning loops
were adjusted to reduce the effect of loop oscillations on
the RF signals. Further improvements were made to the
four anode power supplies by replacing the banks of 88
small electrolytic capacitors by 2 oil-filled polypropylene
film capacitors, as shown in figure 3, to reduce the
likelihood of the power supply tripping on current surges,
and also to improve reliability of the power supplies.
The replacement power supply in 2RF system 6 was
not fully commissioned by the end of the long shutdown,
so during the ISIS operational cycle 2007/1 beam could
only be accelerated with two second harmonic cavities
(systems 4 and 5) in operation. However, even with only
two systems providing the second harmonic component of
the accelerating field, successful acceleration of beam
pulses containing 2.93×1013 protons was achieved during
machine physics tests on November 1st, 2007. This beam
intensity was achieved at low repetition rate (1.6Hz), but
pulses of the same intensity would give an equivalent
50Hz beam current of 234μA.
-150-100-50050100150
-100
-80
-60
-40
-20
0
20
40
60
80
RF Phase (degrees)
Accelerating Volts per turn (kV)
-1RF Volts
-2RF Volts
-Total RF Volts
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Figure 2: The replacement second harmonic low power
RF units installed in the ISIS diagnostics room.

For these beam tests, the peak accelerating 2RF voltage
per accelerating gap had been increased from 7.2kV to
9kV. The total accelerating voltage per turn throughout
the 10ms acceleration period is shown in figure 4.
0
-0.005
20
40
60
80
100
120
140
160
180
00.005 0.01 0.015
time (ms)
Total Accelerating Gapvolts (kV)
1rf peak (kV)
2rf peak (kV)

Figure 4: Total 1RF and 2RF accelerating voltages used
to accelerate 234μA equivalent beam.

Further machine physics tests were planned and
systems 6 and 8 we also made operational by increasing
the accelerating voltage per gap from 7.2kV to 9kV.
However, just before tests with beam could be made, a
repeat of the anode power supply transformer failure (this
time in system 8) prevented operation. In the three
subsequent operational cycles to date, when dual
harmonic acceleration has been achieved, it has relied on
only two second harmonic cavities.
04 Hadron Accelerators


Figure 3: The replacement second harmonic anode
power supply capacitors.

Nevertheless, dual harmonic acceleration enabled 50Hz
accelerated beam with an average current of over 200μA
during the 24 hour period of February 29th, 2008. During
the scheduled machine physics period from late March to
early April and also in mid May 2008, further 2RF system
tests were scheduled. During these periods, further
commissioning of the beam compensation system was
carried out. The feed-forward beam compensation system
is a copy of that used on the 1RF systems [4] and is
shown schematically in figure 5. It comprises a beam
pick-up electrode signal which is first filtered to generate
a pure 2RF component. The signal is then amplified to an
appropriate level and delayed sufficiently to cancel the
beam loading of the proton bunch on the subsequent turn.
Figure 5: the feed-forward beam compensation system.
An attempt was made to replace the fixed band-pass
filter with two switch-in filters in order to allow beam
Bunch
Bunch
Tetrode
Amplifiers
Intermediate
Amplifier
Subtracter
-
Variable
gain
Variable
delay
2RF Filter




2RF
demand
Function
Generator
Function
Generator
MOPC121Proceedings of EPAC08, Genoa, Italy
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compensation to be applied during the early and late parts
of the beam acceleration period. Tests with beam showed
that whilst the system did indeed give cancellation of the
beam induced voltages during the periods when each of
the filters was switched in, the later part appeared
unstable, possibly due to the 1RF component of the beam
signal being passed by the high frequency filter. Further
tests will be made and the possibility of using a tuneable
digital filter is also being explored.
OPERATIONAL DHRF RESULTS
For the recent machine physics tests in May 2008, the
failed transformer in 2RF system 8 had been replaced by
a higher specification unit. However, an unrelated fault on
the anode power supply for system 4 left only 3 operable
2RF cavities. Beam was accelerated at low repetition rate
(1.6Hz) at increasing intensity. Initial tests with 2RF
systems 5 and 8 operating at 8.4kV peak voltage gave an
accelerated beam of 2.67×1013 protons, equivalent to a
50Hz beam current of 212μA. Increasing the peak
voltages to 9kV per gap allowed an equivalent beam
current of 220μA to be accelerated. The further addition
of 2RF system 6 operating at 7.2kV peak gap voltage
allowed an equivalent beam current of 228μA to be
accelerated. Figure 6 shows the beam intensity monitor
signal for these accelerated beam currents together with
that for a single harmonic accelerated beam and the
234μA equivalent beam from November 2007. Figure 7
shows the corresponding beam loss monitor signals.
0
5E+12
1E+13
1.5E+13
2E+13
2.5E+13
3E+13
3.5E+13
-2024
Time (ms)
68 1012
Beam Intensity (protons)
SHRF 175uA
DHRF 212uA
DHRF 220uA
DHRF 228uA
DHRF 234uA

Figure 6: Accelerated beam intensity signals.
-202468 10 12
Time (ms)
Beam Loss Signal
SHRF 175uA
DHRF 212uA
DHRF 220uA
DHRF 228uA

Figure 7: Accelerated beam intensity signals.
The beam loss monitor signals are shown here as a
qualitative guide to the losses during ISIS acceleration.
The initial peak corresponds to losses during injection and
that just after 0ms to trapping loss. The loss levels in
each case shown are within acceptable limits for ISIS
operation at 50Hz. The trace for 228μA shows some mid-
cycle loss at 3-5ms, which was seen intermittently for the
higher intensity beam. Mid to late cycle beam loss is to be
avoided as the protons in ISIS have much higher energy
later in the acceleration cycle, causing higher activation of
the machine, and this loss prevented acceleration of beam
with higher intensity than 228μA and the high losses were
of a frequency that would prevent 50Hz operation. It has
subsequently been found that these mid cycle losses may
be driven by a ~10kHz oscillation in the beam phase loop
and investigations are being made to remove this fault.
CONCLUSIONS AND FURTHER WORK
The four 2RF cavities and their services have now been
installed in the ISIS synchrotron, and the 2RF systems
commissioned. Reliable operation of the 2RF systems,
particularly the anode power supplies, has proven a
challenge. However, several hardware problems have
been overcome and dual harmonic acceleration has been
successfully achieved during the last 4 ISIS user cycles.
All four 2RF systems are yet to be utilized together.
However, beam has been successfully accelerated with
different pairs of the 2RF cavities of sufficient beam
intensities to provide the increased beam current required
by the addition of the second target station at ISIS.
The application of beam compensation on the 2RF
systems has so far proven successful but is only applied
early in the acceleration cycle, due to the fixed bandwidth
of the filter used. It is intended in the near future to use a
tuneable filter to enable beam compensation throughout
the acceleration period.
The “second generation” low power RF equipment
developed for the 2RF system has been optimised and can
now be duplicated and installed as replacements for
ageing low power RF equipment incorporated at present
in the fundamental RF systems. As part of this process, a
replacement digital RF Master oscillator [5] has been
designed and built and will be commissioned during the
next few months.
REFERENCES
[1] M. Harold et al, “A Possible Upgrade for ISIS”,
PAC’97, Vancouver, 1997, p. 1021.
[2] C. Prior, “Studies of Dual Harmonic Acceleration in
ISIS”, ICANS XII, RAL Report 94025, 1994, p. A11.
[3] A. Seville et al, “Progress on dual harmonic
acceleration on the ISIS synchrotron”, PAC’07,
Albuquerque, 2007, p.1649.
[4] P. Barratt et al, “RF system and beam loading
compensation on the ISIS synchrotron”, EPAC 1990.
[5] C. Appelbee et al, “Digital master oscillator results
for the ISIS synchrotron”, PAC’07, Albuquerque,
2007, p.1649.
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