Post on 15-Jan-2016
description
transcript
PAMELAan overview
Takeichiro Yokoi
JAI, Oxford University
Introduction
PAMELA(Particle Accelerator for MEdicaL Applications ) aims to design particle therapy accelerator facility for proton and carbon using NS-FFAG with spot scanning Prototype of non-relativistic NS-FFAG (Many applications !! Ex. proton driver, ADS) It also aims to design a smaller machine for biological study as a prototype.Difficulty is resonance crossing acceleration in slow acceleration rate As a practical machine, economy is an issue.
Collaboration
PAMELA (PM: K.Peach) Rutherford Appleton Lab Daresbury Lab. Cockcroft Inst. Manchester Univ. Oxford Univ. John Adams Inst. Imperial College London Brunel Univ. Gray Cancer Inst. Birmingham Univ. FNAL (US) LPNS (FR) TRIUMF (CA)
In this session ….
T.Yokoi … Overview
A.Kacperek … Medical requirement
H. Witte … Magnet option
C. Beard … RF option
S. Sheehy … Lattice
Clinical requirements (1) : Spot scanning
Spot scanning can fully exert the advantage of particle therapy and pulsed beam of FFAG matches well to the treatment
Typical voxel size : 4mm 4mm ~10mm 10mm Energy range : 70MeV~250MeV Typical @patient : ~1m
Extraction scheme : Fast extraction
Beam emittance : ~10 mm mrad (normalized)
Clinical requirements (2): IMPTDose uniformity should be < ~2% To achieve the uniformity, precise intensity modulation is a must IMPT (Intensity Modulated Particle Therapy)
Beam of FFAG is quantized. Good stability of injector and precise loss control are indispensable for medical applications
At the moment, instead of modulating the intensity of injected beam, shooting a voxel with multiple bunches is to be employed.
“How high is the intensity of a beam bunch?”
SOBP is formed by superposing Bragg peak
time
Inte
gra
ted cu
rrent
Synchrotron & cyclotron
Gate width controls dose
time
Inte
gra
ted cu
rrent
FFAG
Step size controls dose
“Analog IM”
“Digital IM”
Medical requirement (2): IMPT To investigate the requirement of injector, formation of SOBP in IMPT was studied using analytical model of Bragg peak
The study of beam intensity quantization tells intensity modulation of 1/100 is required to achieve the dose uniformity of 2%. (minimum pulse intensity:~106 proton/1Gy) Monitor is a crucial R&D item of PAMELA
If 1kHz operation is achieved, more than 100 voxel/sec can be scanned even for the widest SOBP case. 1 kHz repetition is a present goal (For proton machine : 200kV/turn)
InjectorInjector can preferably cope with proton and heavy ion injection
(ICL group lead by J.Pozinsky investigating the scheme )
Two injectors are to be employed: cyclotron for proton, RFQ for HI
Typical beam emittance from injectors : 1 mm mrad (normalized)
Tracking study of RFQ line is undergoing. (transmission efficiency> 75% is achieved
Stability of intensity is typically less than 5%
Lattice
†
At present, two different types of lattice are proposed for NS-FFAG of non-relativistic particle
(1) Linear lattice (by E.Keil et al.) Small excursion, large tune drift, short drift space, ordinary combined function magnet
(2) Non-Linear Lattice (by C. Johnston et al.) * sextupole for chromaticity correction Large excursion, small tune drift, long drift space, wedged combined function magnet
Cells Tunes for 30-400 MeV Tune-stablized FFAG
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.2 0.4 0.6 0.8 1
Momentum (GeV/c)
nux/cell-modelnuy/cell-modelnux/cell-approxnuy/cell-approx
In lattice design study, we are now focusing on the understanding of dynamics of proton NS-FFAG : dynamics of slow resonance crossing acceleration, field quality, tolerance etc…
Test Lattice
Wedge shaped combined function magnet (quadrupole)
small number of cell (#cell:14), and long straight section(>1m)
Long excursion(>80cm) variable energy extraction, rf cavity
Relatively weaker field gradient(4.5T/m), Max dipole field:1.5T (on orbit)
As a test lattice, tune stabilized lattice proposed by C. Johnston was employed
Tune of test latticeCells Tunes for 30-400 MeV Tune-stablized FFAG
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.2 0.4 0.6 0.8 1
Momentum (GeV/c)
nux/cell-modelnuy/cell-modelnux/cell-approxnuy/cell-approx
Using ZOGUBI, lattice building was carried out.
Horizontal tune can be well reproduced. However, to reproduce vertical tune, wedge angle was needed to be tweaked. The source of discrepancy must be identified. One possible source is the fringing field model
The beam dynamics is basically subjected by the tune As long as tune is similar, the dynamics can be discussed in a similar way.
Original design
ZGOUBI result
Acceleration (perfect lattice)
Horizontal beam blows up slightly ( amplitude wise:~6% for 400MeV acceleration
It is caused by the transverse kick by rf acceleration due to the tilted orientation of accelerating field to the beam axis. Arrangement of rf cavity could affect the intrinsic horizontal beam blow up, But this effect is not important
210keV/turn
Acceleration (Vertical) The beam acceleration was carried out for vertically distributed beam with various positioning error and accelerating rate (horizontal beam size: 0)
Beam blow-up is clearly observed at integer resonance
V:260keV/turn
‘Microscopic’ study is required to understand the blow-up process
Integer resonance crossing (1)R. Baartman proposed a simple formula to evaluate the amplitude growth during resonance crossing
€
ΔA =π
Qτ
R
Q
Bn
B
Stronger focusing suppresses amplitude growth through smaller
Design parameter Intrinsic parameter of lattice
€
Δ(A2−m )
2 − m=
π
2m−1bn,m
m
Qτ
€
bn,m +1 =1
n
R
B
1
m!
∂mBn
∂xm
€
Qτ = ΔQ / turn
For integer resonance Q, (m=1, n=Q)
€
1
2
β
εΔε ⋅
Qτ
σ pos
= πR
Q
BU :N
B
€
Bn = BU :N ⋅σ pos
€
ΔA =1
2
β
εΔε
(A ≈ βε )
Integer resonance crossing (2)
Tracking study was carried out around integer resonance(Q=4,3)
3 acceleration rate, 2 alignment error were examined
100 different lattice configurations
For single integer resonance crossing, Baartman’s formula can estimate the growth rate 210 210 260 260 320 320
70 70 7090 90 90
210 210 260 260 320 320
70 70 7090 90 90
kV/turn
(m)
kV/turn
(m)
Theoretical value
Half integer resonance crossing
€
logA f
Ai
=π
2
R
nB
∂Bn
∂x (=2Q)
€
logA f
Ai
=π
2
R
nB
∂Bn
∂x
1
Qτ (n=2Q)
€
R ≡ logA f
Ai
Qτ
σ fld
=π
2
R
nB
∂BU :N
∂xDesign parameter
.
By introducing focusing error to individual magnet, blow-up rate was estimated
100 different error settings were examined
Baartman’s formula can somehow evaluate the blow-up rate of half integer resonance
Lattice parameter
Structure resonance
Q=4 Q=3.5
Dynamic aperture
Q=3
Q=2.5
Dynamic aperture 20mm mradQ=3.5 Q=2.5
Ncel 14 4Q=14 (2Q=7) is structure resonance
Even with only positioning error, resonance is excited at Q=3.5 **Field gradient error caused by the positioning error
is<10-3
210kV/turn
Requirements for lattice
Linear NS-FFAG (200kV/turn, average B0;n,, w/o ∆B1,x=100m)
Up to half integer resonance, Baartman’s formula can somehow evaluate the blow-up rate.
For slow acceleration case, (~200keV/turn) integer resonance crossing should be avoided.
Single half integer resonance crossing would be tolerable
Structure resonance also should be circumvented. ** Contribution of higher order components, ex fringing field, remains for future study
“Is there doable lattice option at the moment ??”
pos(m)
eV(M
eV/t
urn)
Integer resonance (=6,1mm mrad.norm)
Lattice optionS.Machida proposed semi-scaling FFAG for proton therapy (up to decapole)
Tune drift ∆<1 (no integer crossing, no structure resonance crossing)
Orbit excursion ~30cm
Long straight section (>2m)
H.Witte (magnet), S.Sheehy (lattice)
Acceleration Rate(1) Half integer resonance
(2) 3rd integer resonance
Nominal blow-up margin : 5 (1mm mrad 5mm mrad)
With modest field gradient error (210-3), acceleration rate of 50kV/turn can suppress blow up rate less than factor of 5.
For the considered range, 3rd integer resonance will not cause serious beam blow-up
Required accelerating rate : >50kV/turn
1/0-1
eV/turn (MeV)
∆B2/B2
eV/turn (MeV)
1/0
∆B1/B1
∆B1/B1
1/0:50kV/turn
∆B1/B1
1/0 :200kV/turn
∆B1/B1
eV/t
urn(
MeV
)
Acceleration Scheme
€
P =(ΣV )2
Rdt∫
€
(ΣV )2 ≡ (ΣVisin[ f i(t)])2
€
=Σi(Visin[ f i(t)])
2 + Σi≠ j
(Visin[ f i(t)]⋅V jsin[ f j (t)])
€
1
Tdt ⏐ → ⏐ 0∫
time
Energy
1ms
Option 1
time
Energy
1ms
Option 2
Option 1: P Nrep2
Option 2: P Nrep
Multi-bunch acceleration is preferable from the viewpoint of efficiency and upgradeability
Repetition rate: 1kHz min. acceleration rate : 50kV/turn (=250Hz) How to bridge two requirements ??
Low Q cavity (ex MA) can mix wide range of frequencies
Multi-bunch acceleration
2-bunch acceleration using POP-FFAG (PAC 01 proceedings p.588)
∆f 4 fsy
Multi-bunch acceleration has already been demonstrated
In the lattice considered, typical synchrotron tune <0.01 more than 20 bunches can be accelerated simultaneously (6D Tracking study is required)
“Hardware-wise, how many frequencies can be superposed ??”
Test of multi-bunch accelerationExtraction (5.5MHz) 50kV
Injection (2.3MHz) 50kV
PRISM RF PRISM rf can provide 200kV/cavity
It covers similar frequency region
Brf-wise, MA can superpose more than 20 bunches
Now, experiment using PRISM cavity is under planning (possibly in this October)
Summary PAMELA intends to design particle therapy facility to deliver proton and carbon using FFAG.
Intensive study is going on (dynamics, rf, magnet, clinical requirement etc.)
Lattice requirements is now getting clear.
For acceleration, multi-bunch acceleration provides efficient and upgradeable option.
By the end of next year , hope an doable overall scenario is proposed .
Accelerationrf: 5kv/celldx: 100µm(RMS)
dx: 10µm(RMS)
dx: 1µm(RMS)
Acceleration (Horizontal)
V:260keV/turn
The beam acceleration was carried out for horizontally distributed beam (Vertical beam size: 0)
For horizontal motion, beam blow up is controllable. (Half integer resonance affect slightly for the case of positioning error.)
The blow up should be checked with realistic distribution (finite beam size for both direction)