acceleration schemes for pamela carl beard astec, daresbury laboratory

3rd September 2008 FFAG08 Carl Beard

Acceleration Schemes for PAMELA Carl Beard

ASTeC, Daresbury Laboratory


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


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…


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


Options for consideration1. Cavity type

• Normal Conducting \ Superconducting – Single Cell (Fixed Frequency)– Ferrite Loaded Cavity– Travelling Wave Structure

2. Scheme– Broadband - NCRF– Modulated RF Cavity – NCRF

– Harmonic Jumping Scheme• Fixed Frequency – SRF/NCRF

3. Power Sources– Tetrodes – low frequency <300 MHz– IOTs/Klystrons High Frequency >300 MHz

4. LLRF Control System


Examples of Cavity Types

N.B. Bespoke systems recommended


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.


PoP FFAG RF Structure• High Gradient RF Cavity• “Finemet” Magnetic Alloy

Cores• Low Q

• Superimposed Frequency (Coupled cavity)

Frequency 0.61 – 1.38 MHz

Rep Rate 1 KHz

Voltage 1.3 – 3 kV

Rsh 82 Ohms

1.1m

0.7m

0.64m

?


Compact, Tunable RF CavitiesNew 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.

Muons, Inc.

5/24/2008 9Compact, Tunable RF Cavities


Test Cavity

Compact, Tunable RF Cavities 10

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.

5/24/2008

Muons, Inc.


Test Cavity


Muons, Inc.


Test Cavity-Ferrite-Liquid


Muons, Inc.


Li, Rimmer, 805 MHz Cavity

16cm16cm

36cm

• Power coupler is very large

• SRF strucfture would be much larger


805 MHz Cavity Parameters

•Normal conducting – still high Q

•High gradient


Travelling Wave Structure

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

- Transmission line


Travelling Wave Structures1) 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 damage4) 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 continuous dispersion. Broad bandwidth


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)

0

1

2

3

4

5

6

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00

Energy (MeV)

Po

we

r (k

W) Power(kW)

Fre

qu

enc

y


Harmonic (Number) Jumping


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


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


Constant Harmonic Jump

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

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250 300

Energy (MeV)

Ene

rgy

gain

(ke

V)

1

•Fixed RF Frequency

•Harmonic Jump of 1

Demonstration Purposes


Fixed Harmonic Number JumpFrequency \ FIXED HN \ Energy versus Turn

0

20

40

60

80

100

120

140

1 251 501 751 1001 1251

Turn

MH

z \ H

N

0.000

0.050

0.100

0.150

0.200

0.250

En

erg

y (G

eV)

HN

Energy

Frequency



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.


Controlled HNJFrequency \ HN \ Energy versus Turn

0

5

10

15

20

25

30

35

40

45

1 251 501 751 1001 1251

Turn

MH

z \ H

N

0.000

0.050

0.100

0.150

0.200

0.250

En

erg

y (G

eV)

HN

Energy

Frequency



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


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

acceleration schemes for pamela carl beard astec, daresbury laboratory

Documents