HOM Studies at the European Spallation Source

HOM Studies at the European
Spallation Source
Aaron Farricker
18/07/2016
Aaron Farricker (UoM/CI)
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The ESS Collaberation
•The European Spallation Source
is a large cross European project
•The site for the machine is
located in Lund,Sweden.
•Currently 19 countries are
contributing to the project
•The total cost is circa 1.8 Billion
euros
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Why Build ESS?
ESS TDR
The key in most neutron experiments is flux,
the more neutrons the higher the resolution
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Camera
X-rays
Neutrons
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How Does ESS Work?
•ESS is going to be a 5 MW proton
driven spallation source
•A 2 GeV 62.5 mA beam will be
collided with a solid tungsten target
at 14 Hz
•Each proton produces upwards of 30
neutrons per proton on target
•This results in an average neutron
flux of 1.6 x1015 cm-2s-1
•ESS is a long pulse source and
various chopping schemes can be
employed to vary the pulse structure
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The ESS Accelerator
•The ESS accelerator is just over 600 m long and contains 6 normal conducting
structures and 146 superconducting cavities
•96% of the final energy is gained in the superconducting section of the linac
•During the last redesign the final energy was reduced by 500 MeV which meant to
maintain the 5 MW power goal the current had to be increased
•This adds increased technical risk particularly for the power couplers
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ESS Beam Parameters
Parameter
Value
Beam Power
5 MW
Beam Energy
2 GeV
Beam Current
62.5 mA
Pulse Length
2.86 ms
Rep. Rate
Peak Power
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14 Hz
125 MW
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The ESS Warm Linac
•The ESS front end is similar to
most proton accelerator
•The proton sourse is a
Microwave Discharge ion
Source
•Followed by an RFQ which
munches the beam
•Then a DTL accelerates the
beam to an energy that can be
injected into the main linac
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The ESS Cold Linac
Medium Beta Cavity
Spoke Cavity
High Beta Cavity
Parameter
Value
Parameter
Value
Parameter
Value
Frequency
352.21 MHz
Frequency
704.42 MHz
Frequency
704.42 MHz
Geometric Beta
0.5
Geometric Beta
0.67
Geometric Beta
0.86
# of Cavities
26
# of Cavities
36
# of Cavities
84
Max. Acc. Grad.
9 MV/m
Max. Acc. Grad.
16.7 MV/m
Max. Acc. Grad.
19.9 MV/m
Max. Power
335 kW
Max. Power
1.1 MW
Max. Power
1.1 MW
RF
Tetrode
RF
Klystron
RF
IOT
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What is the Wakefield?
The Wakefield
When a charged particle traverses a vacuum with discontinuities or resistance it leaves
EM field behind. These field are the Wakefield and it may cause degradation of trailing
of the bunch itself and all trailing bunches
•The wakefield can be decomposed into a
sum over the cavity eigenmodes (Condon
Method)
•These modes are readily found in EM
eigenmode solvers such as HFSS, CST and
Poisson Superfish
•From these simulations the loss factor and
mode frequencies can be found
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Longitudinal HOMs
Medium Beta
Spoke Cavity
High Beta
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Transverse HOMs
Spoke Cavity
Medium Beta Cavity
High Beta Cavity
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Summary of HOM
Properties
•The maximum voltage induced by a bunch in a cavity is around 260 V in
the monopole modes of the high beta cavities
•Most of these excited fields add incoherently resulting in a relatively
small voltage in the cavities (see next slide)
•There is one modes in the medium beta cavity (1749 MHz) which is close
to the 5th harmonic of the bunch frequency
•There are two modes in the high beta cavity (1419 MHz) which are close
to the 4th harmonic of the bunch frequency
•These could be resonantly driven if their frequency shifts
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Manufacturing Errors
•SRF cavities are often manufactured
using the deep drawing technique
•Turning or milling as done with normal
conducting cavities is not practical
•Further complex stress patters on a
non-uniform media can lead to
deformations in the final shape
•Further errors occur during welding
and processing
•Field flatness and frequency tuning are
done by physically stretching the cavity
For HOMs a 0.38% frequency spread is
expected from empirical studies at SNS
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Cavity Tuning Stand
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Assessing the Impact on the
Beam
•The bunch length at ESS is much smaller than the wavelength of the highest
frequency HOMs considered
•This means the bunch behaves like a point charge and a drift-kick-drift scheme can
be employed
•To characterise the beam a beam emittance is defined as
•The ratio of this both with and without the additional effects of HOMs is used to
characterise the change in the beam quality
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Utilisation of the Condor
Pool
• The analysis in the following slides has been
carried out using the EPS Condor pool
• It allows access to around 3000 CPUs when PC
cluster machines are not in use by
Undergraduates
• For jobs between 30-180 mins it is a great
resource
• http://condor.eps.manchester.ac.uk/
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Sum Wakefields
•The sum wakefield provides a useful
tool to look at the build up of voltage
over an ideal bunch train
•It can be seen that in each case no
extreamly large voltage builds up
•However earlon in the elliptical
cavities a significant voltage is present
•This voltage is due to the mode which
lies nearest to the accelerating mode
known as a passband or same order
mode
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Passband Modes
Cavity
Frequenc
y
Freq. Sep. Max R/Q
Spoke
361.69
MHz
9.48 MHz
97 ohms
Medium
Beta
703.89
MHz
0.53 MHz
179 ohms
High Beta
703.22
MHz
1.2 Mhz
74 ohms
RF Errors
•Below a Q of 107 the effects of the passband modes are much smaller than the
impact of RF errors
•The expected Q for these modes is similar to the accelerating mode which is by
design just below 106
•This means these modes are expected to not be a problem at ESS
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Longitudinal Plane
•The HOMs with the larges R/Q have been used to see if any impact is had on the
beam quality
•Even including the six highest R/Q modes (Over 70% of the total for the first 60
modes) in each cavity only a very small 0.03% increase in the emittance is seen
•It is a strong indication that longitudinal modes do not require damping as found
with previous studies of using earlier linac and cavity designs
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Transverse Plane
•The HOMs with the larges R/Q have been used to see if any impact is had on the
beam quality
•Even including the six highest R/Q modes (Over 70% of the total for the first 60
modes) in each cavity only a very small 0.025% increase in the emittance is seen
•This shows that dipole modes at ESS are likely to have a negligable impact on the
beam regardless of the Q
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Alignment Errors
•Finite tolerances in the alignment of
components within an accelerator are
always present
•Offsetting a cavity or cryomodule from
the beam axis can result in the exciation
of Dipole modes which can degrade the
beam
•To analyse the impact of these errors
the six highest R/Q dipole modes are
included in each cavity and a uniform
random distribution of offsets is applied
to the cavities
•For any physically allowable offset no
discernible degradation is observed
•However testing non-physical offsets
showed that the simulation procedure
does work
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Angular Alignment Errors
•The cavity is allowed to have its axis at
an angle to the beam axis
•This means that the accelerating mode
has a transverse component which can
kick the beam
•From the small angle approximation
1 mrad in a 1m cavity at 12 MV/m is a
12 kV voltage
•This is orders of magnitude more than
expected for the voltage stored in dipole
modes
•This means that the angular alignment
of the cavity is far more important than
the translational alignment with respect
to the transverse growth of the beam
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Driven HOMs
•As discussed previously the frequency
of HOMs can shift significantly due to
manufacturing errors
•This can result in modes shifting to lie
on or near a harmonic of the bunch
frequency
•If this situation arises the voltage can
build up significantly due to the mode
being resonantly driven
•In the worst case this can result in the
severe degradation or loss of the beam
•As ESS does not actively damp HOMs
this is a significant problem
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How Close Can a HOM
Get?
•ESS has set a 5 MHz limit on the
separation of all HOMs from the nearest
machine resonance
•The first two High Beta prototypes
failed to meet this specification
•This has made it a significant risk factor
in the cavity production
•An Extensive study has been performed
to look at how appropriate this limit
actually is
•On the right is the growth due to a
single mode that lay on the nearest
machine line
•Its frequency was moved away insteps
of 200 Hz
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This suggests 5 kHz may be enough!!
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Additional Constraints
•In addition scans to set limitations
on the Q of the HOM on a cavity by
cavity basis were performed
•And also scans of R/Q to allow for
any redesigns to be investigated
quickly
•Indications are that the damping
from the power coupler may be
enough to entirely mitigate the
effects in the high beta cavities
•Studies of this are ongoing
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Additional Cavity Effects
Coupler Kicks
Multipole Components
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Outline
•
•
•
•
The European spallation source
The ESS cavities and linac design
Modal analysis of the ESS cavities
Beam dynamics studies including HOMs
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Summary
• An overview of the ESS accelerator has been
given
• A full cavity mode analysis has been
performed for each of the ESS cavities
• These were used in beam dynamics studies to
show HOM couplers are not required
• It was also shown that the ESS limit of 5 MHz
on mode frequencies is tighter than may be
required
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