Acceleration Schemes for PAMELA
Carl Beard
ASTeC, Daresbury Laboratory
3rd September 2008
FFAG08
Carl Beard
Pamela
•
Conceptual design study of a combined
proton and light-ion Charged Particle
Therapy (CPT) facility
– PAMELA must accelerate both carbon and
protons
– From 50 to 250 MeV extraction energy
Protons
– 70 MeV/u to 450 MeV/u for Carbon
•
•
Energy range (beta) 0.2 – 0.7
2 or 3 rings concentric rings
3rd September 2008
FFAG08
Carl Beard
Practical Considerations
• Facility
– Cyclotron footprint
– Services
• Power Supplies (magnets & RF)
• RF
• Diagnostics
• Vacuum
• Control system
– Complex acceleration scheme!!!
• Cryogenics \ Cooling
• RF system
– Large energy range (velocity factors)
• Conversely – Achieve the design parameters, and then consider the
practical aspect…
3rd September 2008
FFAG08
Carl Beard
Design Constraints
• Longitudinal Space (0.6 – 1m)
• Aperture (10 – 15cm)
• Energy range (beta) 0.1 (Carbon) – (0.7 Proton)
– Energy gain/range per ring, undefined
• Energy gain per turn \ cavity
– 50 KV – 5MV
– Voltage change
• Frequency range??
–
–
–
–
Low frequency
(up to 40 MHz)
Medium Frequency
(200 MHz say…)
High Frequency
(800 MHz up to 1.3 GHz)
Rate of change of Frequency
• Phase and Amplitude stability – this will depend on the acceleration
regime
• System has to be simple to operate
– No in-house RF engineers planned to supervise the system
3rd September 2008
FFAG08
Carl Beard
Options for consideration
1.
Cavity type
•
–
–
–
2.
Single Cell (Fixed Frequency)
Ferrite Loaded Cavity
Travelling Wave Structure
Scheme
–
–
–
Broadband - NCRF
Modulated RF Cavity – NCRF
Harmonic Jumping Scheme
•
3.
Fixed Frequency – SRF/NCRF
Power Sources
–
–
4.
Normal Conducting \ Superconducting
Tetrodes – low frequency <300 MHz
IOTs/Klystrons High Frequency >300 MHz
LLRF Control System
3rd September 2008
FFAG08
Carl Beard
Examples of Cavity Types
N.B. Bespoke systems
recommended
3rd September 2008
FFAG08
Carl Beard
Single Cell Broadband Cavities
•
•
Compact
Ferrite loaded cavity to increase bandwidth
•
•
•
•
•
Low Q
Low – high Frequency
Can maintain High R/Q even considering an
aperture 10-15cm (Low f)
Tetrodes have can have ~200MHz Bandwidth
Higher frequency sources limited bandwidth
–
•
Exception; TWT
If acceleration scheme allows, SRF Cavity
could be used.
3rd September 2008
FFAG08
Carl Beard
PoP FFAG RF Structure
• High Gradient RF Cavity
• “Finemet” Magnetic Alloy
Cores
• Low Q
0.7m
• Superimposed Frequency
(Coupled cavity)
Frequency
0.61 – 1.38 MHz
Rep Rate
1 KHz
Voltage
1.3 – 3 kV
Rsh
82 Ohms
3rd September 2008
0.64m
1.1m
?
FFAG08
Carl Beard
Muons, Inc.
Compact, Tunable RF
Cavities
New developments in the design of fixed-field alternating gradient (FFAG)
synchrotrons have sparked interest in their use as rapid-cycling, high intensity
accelerators of ions, protons, muons, and electrons. Potential applications include
proton drivers for neutron or muon production, rapid muon accelerators, electron
accelerators for synchrotron light sources, and medical accelerators of protons and
light ions for cancer therapy. Compact RF cavities that tune rapidly over various
frequency ranges are needed to provide the acceleration in FFAG lattices. An
innovative design of a compact RF cavity that uses orthogonally biased ferrite or
garnet for fast frequency tuning and liquid dielectric to adjust the frequency range
and cool the cores is being developed using physical prototypes and computer
models.
The first example will be to provide 2nd Harmonic RF for the Fermilab Booster
Synchrotron.
rd September 2008
5/24/2008
3
Compact,
Tunable RF Cavities
FFAG08
Carl Beard9
Muons, Inc.
Test Cavity
Fig. 1: Conceptual design of a compact, tunable RF cavity for FFAG and other applications.
Ferrite cores (black) and liquid dielectric (yellow) surround a ceramic beam pipe (green) with an RF iris as shown. Coils
(red) outside of the cavity generate a solenoidal magnetic field that is transverse to the RF magnetic field. A laminated
iron return yoke (black) localizes the field.
rd September 2008
5/24/2008
3
Compact,
Tunable RF Cavities
FFAG08
10
Carl Beard
Muons, Inc.
rd September 2008
5/24/2008
3
Test Cavity
Compact,
Tunable RF Cavities
FFAG08
11
Carl Beard
Test Cavity-Ferrite-Liquid
Muons, Inc.
rd September 2008
5/24/2008
3
Compact,
Tunable RF Cavities
FFAG08
12
Carl Beard
Li, Rimmer, 805 MHz Cavity
36cm
16cm
16cm
• Power coupler
is very large
• SRF strucfture
would be
much larger
3rd September 2008
FFAG08
Carl Beard
805 MHz Cavity Parameters
•Normal conducting – still high Q
•High gradient
3rd September 2008
FFAG08
Carl Beard
Travelling Wave Structure
- Transmission line
Particle velocity < c, Guide velocity = c
Guide velocity slowed to match particle
•Typically broadband (linear dispersion)
•Efficiency reduced over large spread in beta
•Small apertures for low velocities
3rd September 2008
FFAG08
Carl Beard
Travelling Wave Structures
1) TWS can have more cells as for SWS (No trapped HOMs)
2) TWS require lots more drive power power exits through the output
coupler.
3) When a cavity has a breakdown a TWS will absorb RF power causing
extra damage
4) In NC cavities SWS should get higher fields in theory but field
enhancement around the coupler prevents this. SLAC are still
working on it.
5) TWS can sometimes have lower surface fields.
6) Beam loading is much higher in TWS meaning for an acc gradient of
50 MV/m in the NLC you need an unloaded gradient of 70 MV/m for
example.
7) Damping wakfields in long TWS has been demonstrated. SWS should
be just as good but it hasn't been proven.
8) By nature travelling wave structures require small irises to maintain a
relatively modest R/Q. Spacing critical for low beta structures
9) TWS is more compact because it has less couplers and is also
cheaper. It is also less sensitive to mechanical errors as it has a
rd
continuous
bandwidth
3 September
2008 dispersion. Broad
FFAG08
Carl Beard
Energy \ Frequency Requirements
• Limitations -Energy gain per turn increases
– Ramps from very low power to 5kW in a few
microseconds… Power (kW) vs Energy (MeV)
6
Power (kW)
Frequency
5
4
Power(kW)
3
2
1
0
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
Energy (MeV)
3rd September 2008
FFAG08
Carl Beard
Harmonic (Number) Jumping
3rd September 2008
FFAG08
Carl Beard
Harmonic Number Jumping
• Acceleration Schemes so far
require frequency modulation
• Scheme for fixed frequency highly
desirable
• Pre-programmed Phase and
voltage
– To ensure arrival at each RF station
an integer number of wavelength
later
– Energy Increases
• Velocity increases
– Number of Harmonic jumps
decrease
3rd September 2008
FFAG08
Carl Beard
Harmonic Jumping
• Fixed RF frequency
– High frequency option possible
– Stability may be an issue – LLRF Control
– As velocity increases TTF changes
• Acceleration per cavity will change
• Could be advantageous – starting further off phase
• Superconducting RF is a possible solution
– Larger beam apertures by default
• Stray (High) fields
– heating flanges etc.
– Local BPMs
3rd September 2008
FFAG08
Carl Beard
Constant Harmonic Jump
•Fixed RF Frequency
Demonstration Purposes
•Harmonic Jump of 1
3
Energy Gain Per Turn for 1 Harmonic Jump
Energy Gain (MeV)= 2.988E-05xEnergy(MeV)2 + 2.606E-03xEnergy(MeV) - 2.622E-02
R2 = 1.0
Energy gain (keV)
2.5
2
1.5
1
1
0.5
0
0
50
100
150
200
250
300
Energy (MeV)
3rd September 2008
FFAG08
Carl Beard
Fixed Harmonic Number Jump
Demonstration Purposes
Frequency \ FIXED HN \ Energy versus Turn
140
0.250
Frequency
120
Energy
0.200
0.150
80
Energy (GeV)
MHz \ HN
100
60
HN
0.100
40
0.050
20
0
0.000
1
251
501
751
1001
1251
Turn
3rd September 2008
FFAG08
Carl Beard
HNJ & Frequency sweeping
• Frequency sweep of multiple octaves
required
– Could limit the energy gain possible
• Simplified Control System
– Finite frequency shift
– Smaller Harmonic jumps
– Improved stability
• Large energy range.
3rd September 2008
FFAG08
Carl Beard
Controlled HNJ
Demonstration Purposes
Frequency \ HN \ Energy versus Turn
45
0.250
Frequency
40
Energy
0.200
35
30
Energy (GeV)
MHz \ HN
0.150
25
20
HN
0.100
15
10
0.050
5
0
0.000
1
251
501
751
1001
1251
Turn
3rd
September 2008
FFAG08
Carl Beard
Frequency & HNJ Modulation
• Reduction in operating bandwidth
– Achievable for Ferrite loaded and broadband cavity
• Increased efficiency
– Frequency returns back to initial frequency to allow
continuous operation
• Constant energy gain.
– Fixed Power per cavity
• Stepped “controlled” ramping of the Harmonic
number
3rd September 2008
FFAG08
Carl Beard
Summary
• Standard acceleration scheme
– Modulated RF
– Broadband
• Ramping of RF power limits the use
• Bandwidth could be a number of octaves
• Harmonic Number Jump
– Large advantages
• Reduce the required bandwidth
• Fixed frequency
– Low Level RF Control looks possible, but difficult
• Hybrid of HNJ + Cavity (Modulated or Broadband)
– Looks promising
– Could this system work independently and reliably?
– More comprehensive study required
3rd September 2008
FFAG08
Carl Beard