HighGradient Accelerating Structures for Proton Therapy Linacs W

High-Gradient Accelerating Structures for Proton Therapy Linacs W. Wuensch 8 -9 -2012

Significant progress has been made over the past decade by studies of normalconducting 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

With 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 Me. V-range photons for laserbased nuclear physics (nuclear-photonics) and fundamental processes (QED studies for example). There also potential applications such as nuclear resonance fluorescence for isotope detection in shipping containers and mining.

Objectives If 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.

Synchrotron-based proton therapy at CNAO in Pavia

Target project - TULIP • • Proposal from the TERA Fondation lead by Ugo Amaldi proton therapy single room facility compact machine (accelerator and gantry together) cyclinac concept with fast actively energy modulated beam

More detail

TULIP: the initial idea

Basic linac parameters • Input energy: 35 (24) Me. V • Input beta: 0. 2658 (0. 2219) Output energy: 230 Me. V Output beta: 0. 5958 • Geom beam emittance: 5 pi mm mrad • Norm beam emittance: 1. 3 pi mm mrad • Acceptance affected by: – Max distance between PMQs • Number of cells per tank • Inter-tank distance – RF field defocusing effect – Quadrupole strength – beam aperture

The linac that we are trying to improve radius= 1. 35 m angle = 45 deg W= 70. 3 Me. V radius= 0. 85 m angle = 30 deg W= 230 Me. V W= 24 Me. V radius= 0. 5 m angle = 30 deg Very low energy acceleration needs a different technology. Maybe later…

We plan to improve it with a novel high-gradient backward wave structure based closely on the successful CLIC geometry and technology: The current design of the basic cell geometry for low velocity acceleration (still under optimization) And a micron-precision CLIC cell

In addition to the expected higher gradient, our backward travelling wave is simpler mechanically. Geometry of LIBO structure

For rf enthusiasts – here you can see how we design for high gradient Surface electric field Surface magnetic field Our current understanding of high-gradient limits Modified Poynting vector

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 CTF 3 klystrons. The optimum frequency is however 5. 7 GHz, C-band but we don’t have such a power source. Structure parameters Energy [Me. V] Aperture radius [mm] Active length [mm] Low energy structure 76 High energy structure 213 2. 5 180 330

Resources Diamond machined cells structure 1 Diamond machined cells structure 2 Coupler units Bonding and heat treatment Jigging and specialized supports Vacuum equipment Rf components Total [k. CHF] 24 24 8 10 5 4 5 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. The benefits for TERA are clear. • Funded manpower would reduce project risk. I don’t know if this is within the scope this funding.

Testing – 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 CTF 3, 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 CTF 3 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.

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 k. CHF in rf hardware. 3 GHz 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 optimised to cancel RF power in the load Example with varying coupling made for CLIC in 2008 Power normalized *Coupling To the structure From klystron Into the load Time, ns Power gain ~ 9

Thank you for your support! And thanks to Alberto Degiovanni for helping to prepare the slides.

Extra slides

The LInac BOoster protoype (LIBO) Module of 4 tanks, tested with proton beam at the Laboratori Nazionali del Sud - INFN , Catania 15 MV/m 74 Me. V 4 MW at 3 GHz 62 Me. V Proton trajectory 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

The Cell Coupled Linac Electric field distribution (HFSS) acc. cell on axis coupl. cell on side excited cavity m 0 c 3 ~ TANK HFSS simulation un-excited cavity l l RF mode: pi/2 (accelerating and coupling cells) Beam mode: pi Accelerating Modules (Tanks+Bridge Couplers) FODO lattice (PMQs) 19/04/2011 TULIP Meeting A. Degiovanni acc. tanks space for quadrupoles 21

Side Coupled Linac – half cells Nose cones HFSS v 13. 0

Field distributions (ANSOFT HFSS 13. 0)