Significant progress has been made over the past decade by studies of normalconducting linear colliders NLCJLC and CLIC to raise achievable accelerating gradient from the range of 2030 MVm up to 100120 MVm ID: 806439
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Slide1
W. Wuensch8-9-2012
High-Gradient Accelerating Structures for Proton Therapy Linacs
Slide2Significant progress has been made over the past decade by studies of normal-conducting linear colliders, NLC/JLC and CLIC, to raise achievable accelerating gradient from the range of 20-30 MV/m up to 100-120 MV/m.
The
gain has come through a greatly increased understanding of high-power rf phenomena, development of quantitative high-gradient rf design methods, refinements in cavity fabrication techniques and through development of high peak rf power sources.
CLIC accelerating structure
High-gradient test summary
Slide3With successful demonstration of a CLIC baseline structure we now ask - what other applications could benefit from this development? Because:
Spreading the technology will broaden and strengthen the technological base which would one day be needed to support construction of a linear collider.
It’s a new challenge plus it would feel nice to see our ideas on a timescale shorter than that of a linear collider.
Among the application which would benefit from our high-gradient technology:
Linacs for proton and carbon ion cancer therapy.
High repetition rate FELs (Free Electron Lasers) for the ‘photon-science’ community which encompasses biology, chemistry, material science and many other fields.
Compton-scattering
gamma ray sources providing MeV-range photons for laser-based nuclear physics (nuclear-photonics) and fundamental processes (QED studies for example). There are also potential applications such as nuclear resonance fluorescence for isotope detection in shipping containers and
mining.
Slide4If our theoretical models are correct we should be able to increase gradient for medical proton therapy linacs up to around 50 MV/m from the current 27 MV/m in LIBO linac tanks (in CABOTO, the medical linac designed by the TERA foundation)
.
We wish to do this for specific a target application so - design, build and high-power test two accelerating structures targeting use in TULIP, an idea of
Ugo Amaldi which is being studied by the TERA foundation.TULIP is a gantry-mounted proton therapy linac (more details in a moment) which means linac length is extremely important, and where increased gradient could decrease cost.
More generally what we are doing is transferring high-gradient technology developed for relativistic electron acceleration to low
β heavy particle acceleration. But via a specific application.In parallel we are testing our high-gradient ideas in a parameter space far from that where they were developed. What we learn may in turn feed back to improved performance for electrons.
So we see a host of mutual benefits.
Objectives
Slide5Synchrotron-based proton therapy at CNAO in Pavia
Slide6Target project - TULIP
Proposal
from
the TERA Fondation lead by Ugo Amaldiproton therapy
single room facilitycompact machine (accelerator and
gantry together)cyclinac
concept
with
fast
actively
energy
modulated beam
Slide7More detail
Slide8TULIP: the initial idea
Slide9Basic linac parameters
Input energy: 35 (24) MeV Output energy: 230 MeV
Input beta: 0.2658 (0.2219) Output beta: 0.5958
Geom beam emittance: 5 pi mm mradNorm beam emittance: 1.3 pi mm mradAcceptance affected by:
Max distance between PMQsNumber of cells per tankInter-tank distanceRF field defocusing effect
Quadrupole strength beam aperture
Slide10radius= 0.85 m
angle = 30 deg
radius= 0.5 m
angle = 30 deg
radius= 1.35 mangle = 45 deg
W= 70.3 MeV
W= 230 MeV
W= 24 MeV
The linac that we are trying to improve
Very low energy acceleration needs a different technology. Maybe later…
Slide11The current design of the basic cell geometry for low velocity acceleration
(still under optimization)
And a micron-precision CLIC cell
We plan to improve it with a novel high-gradient backward wave structure based closely on the successful CLIC geometry and technology:
Slide12In addition to the expected higher gradient, our backward travelling wave is simpler mechanically.
Geometry of LIBO structure
Slide13For rf enthusiasts – here you can see how we design for high gradient
Surface electric field
Surface magnetic field
Modified
Poynting
vector
Our current understanding of high-gradient limits
Slide14Low energy structure
High energy structure
Energy [MeV]
76
213
Aperture radius [mm]
2.5
2.5
Active length [mm]
180
330
The accelerating structures we propose to build
We propose to build a structure at the main linac injection energy and another at the final energy to determine the range of gradients which can be accessed (lower energy is trickier).
The structures will be designed using the high-gradient theory we have developed in the CLIC study.
The structures will be fabricated in the same way as prototype CLIC structures – diamond turning and milling, bonding at 1050⁰C in a hydrogen atmosphere followed by a 650 ⁰C vacuum
bakeout
.
We will build the structures at 3 GHz so that they can be tested using CTF3 klystrons. The optimum frequency is however 5.7 GHz, C-band but we don’t have such a power source.
S
tructure parameters
Slide15Resources
[kCHF]
Diamond machined cells structure 1
24
Diamond machined cells structure 2
24
Coupler units
8
Bonding and heat treatment
10
Jigging and specialized supports
5
Vacuum equipment
4
Rf components
5
Total
80
The main CERN participation is– Walter Wuensch,
Alexej
Grudiev
and Igor
Syratchev
(we’re in BE-RF
)
– and we will lead
the project and guide
technical
work .
Our time comes out of CLIC studies with the full support of
Steinar
and
Erk
. The synergy and benefits for CLIC and the rf group are clear.
The TERA project will take on the bulk of the technical work – making drawings, assembly, rf measurements. In this way technology transfer is maximised. We have
Ugo
Amaldi’s
agreement on this.
T
he benefits for TERA are clear.
Funded manpower would reduce project risk. I don’t know if this is within the scope this funding.
Slide16Testing – what do we do with the completed cavities?
The cavities will be designed so that they can be
high-power tested using one of the 3 GHz klystrons currently used for CTF3, so all the necessary the hardware and expertise is in place.
However we should expect that we may need a few hundred hours of testing time per cavity so priorities with CTF3 may be an issue.
On the other hand we have a year and a half to prepare (assuming approval), there are operating modes which do not use all klystrons etc.We do not request any resources for testing in this request. I am confident that this can be supported out of existing CLIC (including work implicating other departments and groups) work packages and BE-RF activities.
Slide17Example with varying coupling made for CLIC in 2008
3GHz structure example (no beam loading):
Q = 12 000
L = 0.2 m
T fill = 60 ns ( Vg ~ 0.01 C)
Eff. in WG loop = 0.98
Eff. in the structure = 0.91
*
Coupling optimised to cancel RF power in the load
Power normalized
Time, ns
Power gain ~ 9
From klystron
To the structure
Into the load
Something new: In
the past few weeks we have realized that
operation in
“recirculation” mode
could give an enormous reduction in required peak power. To add this to the test structures would
require around another
20
kCHF
in rf hardware.
Slide18Thank you for your support!
And thanks to Alberto
Degiovanni for helping to prepare the slides.
Slide19Extra slides
Slide20The LInac BOoster
protoype (LIBO)
Module of 4 tanks, tested with proton beam at the
Laboratori
Nazionali del Sud - INFN , Catania
Collaboration TERA with
INFN (Mi- Na)
and CERN
1999-2002
C. De Martinis et al
V.
Vaccaro
et al.
E.
Rosso
et al
project leader M. Weiss
62 MeV
74 MeV
4 MW at 3 GHz
Proton trajectory
15 MV/m
Slide21The Cell Coupled Linac
19/04/2011 TULIP Meeting
A. Degiovanni
21
RF mode
: pi/2 (accelerating and coupling
cells)
Beam mode: pi
Accelerating Modules (
Tanks+Bridge
Couplers)
FODO lattice (PMQs)
HFSS simulation
Electric field distribution (HFSS)
excited cavity
un-
excited
cavity
~30
cm
acc. cell on axis
coupl. cell on side
TANK
acc. tanks
space for quadrupoles
Slide22Side Coupled Linac – half cells
HFSS v13.0
Nose cones
Slide23Field distributions
(ANSOFT HFSS13.0)
(ANSOFT HFSS13.0)