TERA Foundation on behalf of U. Amaldi, J. Bilbao de Mendizábal, R. Bonomi, A. Degiovanni, M. Garlasché and P. Magagnin Silvia Verdú Andrés 3 GHz Cavity Test Results of the first TERA cavity test at CTF2 and requests for a next run 1 CLIC meeting January 13, 2012

Motivation – TERA activities The hadrontherapy community demands compact, reliable accelerators with the appropriate beam performances for tumour treatment with ions.

TERA’s proposal: cyclotron + high-freq. linac = cyclinac Silvia Verdú-Andrés Cell Coupled Linac RF frequency: 5.7 GHz 18 accelerating modules - Length of each module ~ 1.3 m High gradient : 40 MV/m (TERA+CLIC collaboration) Cell Coupled Linac Standing-wave structure RF frequency: 5.7 GHz 2.5  s-long pulse at 300 Hz 3 (CArbon BOoster for Therapy in Oncology) CABOTO

C-band linac Section 1 cyclotron Beam dose delivery RF rotating joints Line with 2% momentum acceptance C-band linac Section 2 RF Power sources W0W0 W1W1 W2W2 SM 1 SM 2 TULIP (TUrning LInac for Protontherapy) Cell Coupled Linac Standing-wave structure RF frequency: 5.7 GHz 2.5  s-long pulse at 100 Hz TERA’s proposal: cyclotron + high-freq. linac = cyclinac

Motivation – TERA activities appropriate beam performances The hadrontherapy community demands a compact, reliable accelerators with the appropriate beam performances for tumour treatment with ions. Active energy modulation High repetition rate: 3D spot-scanning technique with multipainting + 3D feedback system moving organs treatment + CABOTO – 300 Hz TULIP TULIP – 100 Hz

Hadron therapy: the basics Charged hadron beam that loses energy in matter 27 cm Tumour target 200 MeV - 1 nA protons 4800 MeV – 0.1 nA carbon ions (radioresistant tumours) Courtesy of PSI Silvia Verdú-Andrés tail httt://global.mitsubishielectric.com/bu/particlebeam/index_b.html 6 Depth-dose distribution

Treating moving organs requires... Silvia Verdú-Andrés7  Fast Cycling machine (high repetition rate ~ Hz) Tumour MULTIPAINTING  Fast Active Energy Modulation (a couple of ms) Fast 3D correction of beam spot position in depth Single ‘spot’ pencil beam Lateral scanning with magnets: 2 ms/step 3D conformal treatment Depth scanning: ACTIVE ENERGY MODULATION

TERA’s proposal: cyclotron + high-freq. linac = cyclinac Silvia Verdú-Andrés Cell Coupled Linac RF frequency: 5.7 GHz 18 accelerating modules - Length of each module ~ 1.3 m High gradient : 40 MV/m (TERA+CLIC collaboration) Cell Coupled Linac Standing-wave structure RF frequency: 5.7 GHz 2.5  s-long pulse at 300 Hz 8 (CArbon BOoster for Therapy in Oncology) CABOTO

TERA’s proposal: cyclotron + high-freq. linac = cyclinac 120 MeV/u 400 MeV/u Silvia Verdú-Andrés Cell Coupled Linac RF frequency: 5.7 GHz 18 accelerating modules - Length of each module ~ 1.3 m High gradient : 40 MV/m (TERA+CLIC collaboration) Cell Coupled Linac Standing-wave structure RF frequency: 5.7 GHz 9 (CArbon BOoster for Therapy in Oncology) CABOTO Higher accelerating gradients cost  Reduce size and cost!

TERA’s proposal: cyclotron + high-freq. linac = cyclinac 120 MeV/u 400 MeV/u Silvia Verdú-Andrés Cell Coupled Linac RF frequency: 5.7 GHz 18 accelerating modules - Length of each module ~ 1.3 m High gradient : 40 MV/m (TERA+CLIC collaboration) Cell Coupled Linac Standing-wave structure RF frequency: 5.7 GHz 10 (CArbon BOoster for Therapy in Oncology) CABOTO Higher accelerating gradients  Reduce size and cost! Common goals for TERA and CLIC Cell shapeE S /E 0 E 0 [MV/m] E s max [MV/m] BDR required [bpp/m] TERA linacs 4—54—535— reliability ( reliability ) CLIC structures

Operation limit for S-band cavities  BreakDown Rate BDR limit described by surface field E S (Kilp.) mod. Poynting vector S C Scaling laws (E S, S C, pulse length, temperature, frequency) evaluated for X, K and C bands Applying found limit to future designs ensure reliable operation optimize RF structures (efficiency, length, cost)  TERA high-gradient single-cells test program Our interests

3 GHz -One prototype -Preliminary high-power test in High power in TERA High Gradient Test Program 5.7 GHz -3 prototypes for testing: - BDR behaviour, - eventual hot spots for BD activity - BD as function of machining procedure (i.e conventional vs. diamond turning) -Under tuning and brazing High power test of high-gradient performance single-cell cavities at two different frequencies:

prototype layout 3 GHz test: prototype layout Single accelerating cell (two unsymmetrical half cells) H-coupled to WR284 waveguide. Two lateral plates for structure cooling. CF flanges for data acquisition. Cell Parameters MaterialC10100 Copper Dimensional tolerance band20 μm Surface roughness (Ra) 0.4 μm

prototype production 3 GHz test: prototype production Brazing (Bodycote, France) :Machining (VECA, Italy):

Preliminary Test – February 2010 at CTF3 CLIC collaboration in low power measurements and preliminary high-power testing at CTF3 in February 2010 Breakdown identification from Faraday Cup signal BD event

Gradient limitations for high frequency accelerators, S. Döbert, SLAC, Menlo Park, CA 94025, USA (2004) Limit in copper to surface field by breakdown surface damage TERA Preliminary Test – February 2010 at CTF3 … and the cavity was stored under nitrogen for more than one year until September 2011.

Test September 2011

Experimental Set-up at CLIC Test Facility (CERN) o Rf circuit: klystron no.30, 3 GHz, 5 Hz, 50 MW peak power o Vacuum station (ion pump): mbar o Cooling system: thermalized water (30°C), constant water flow (5L/min) o Diagnostics: Faraday cup Incident and reflected RF power (amplitudes and phases) Vacuum gauge Temperature of cavity and coolant (thermistors and Pt100, respectively) o Remote control of the incident RF pulse performaces o Automatic data acquisition system

RF window bidirectional coupler towards 3 GHz TERA single-cell cavity vacuum system from klystron 30 3 GHz TERA single-cell cavity

from klystron 30 3 GHz TERA single-cell cavity from klystron 30 cavity vacuum port aside Faraday cup vacuum port upstream cavity  vacuum lecture Faraday cup vacuum port aside Faraday cup Faraday cup cavity

from klystron 30 cooling system PT100 thermistors PT100 in out

Experiment timeline About 250 RF hours (total: ~ 10 6 seconds), equal to 4.5 million RF pulses which led to more than 3000 breakdowns used to condition 1 dm 2 of copper surface. (*) No availability to supervise test so run at low field (interesting for TERA applications) (+) Scaling laws evaluation Circulator installation E S = 350 MV/m

Circulator installed to avoid interference of the power reflected by the cavity with the incident RF pulse. -L waveguide is 76 m -v group is 0.72c Therefore, signal reflection expected at 0.7  s. Circulator installed to avoid interference of the power reflected by the cavity with the incident RF pulse. -L waveguide is 76 m -v group is 0.72c Therefore, signal reflection expected at 0.7  s. Incident RF pulse (no circulator) Incident RF pulse (circulator)

Experiment timeline -- evolution of electric field with RF-on time -- max Gradient limitations for high frequency accelerators, S. Döbert, SLAC, Menlo Park, CA 94025, USA (2004) TERA

pulse length incident RF pulse estimated reflected RF pulse « dynamic » stored energy measured reflected RF pulse RF pulse characteristics -- Normal operation -- (12-20% of power in   ~0.9) field emission current signal (< 8  A peak within 2.5  s-long pulse) negative current: electrons!

pulse length incident RF pulse estimated reflected RF pulse « dynamic » stored energy measured reflected RF pulse RF pulse characteristics -- Normal operation -- (12-20% of power in   ~0.9) -- Breakdown -- field emission current signal (< 8  A peak within 2.5  s-long pulse) negative current: electrons! a) reflected power increase: b) field-emission current burst: breakdown! 0 0 electrons positive ions

Breakdowns which do not lead to a field recovery. Breakdowns which lead to a field recovery.

-- Accelerating gradient evaluation -- RF pulse characteristics -- Accelerating gradient evaluation --

Conditioning -- accumulated breakdowns with RF-on time -- About 250 RF hours (total: ~ 10 6 seconds), equal to 4.5 million RF pulses which led to more than 3000 breakdowns used to condition 1 dm 2 of copper surface.

The field enhancement factor  after “conditioning” is about 40 (assuming f = 4.5 V). Conditioning -- field enhancement factor  --

higher field higher current / field  worse performance -- field enhancement factor  with time -- Conditioning -- field enhancement factor  with time --

BDR measurements of the last 2 days of the test -- Compared to measurements in bd, 8.3h 6bd, 4.6h 22bd, 1.5h 1bd, 5.6h

BDR measurements of the last 2 days of the test -- Breakdowns identified from FE current burst or reflected RF energy --

-- field emission current burst OR reflected RF energy?-- The number of breakdowns identified by a field-emission current burst is extremely sensitive (1 order of magnitude) to the threshold used to define a breakdown event from this signal. Next test: use photomultipliers to detect breakdowns from light emission. The number of breakdowns identified by a field-emission current burst is extremely sensitive (1 order of magnitude) to the threshold used to define a breakdown event from this signal. Next test: use photomultipliers to detect breakdowns from light emission.

2.2  s 1.5  s  1.0  s  

BDR measurements of the last 2 days of the test Scaling laws: BDR  E 13  21 t P 1.6 Scaling laws: BDR  E 13  21 t P 1.6

Operation limit to high gradient performance -- Modified Poynting vector Sc power law-- A. Grudiev, S. Calatroni, and W. Wuensch, “New local field quantity describing the high gradient limit of accelerating structures”. Phys. Rev. ST Accel. Beams 12, (2009): The square root of S C has been scaled to t pulse = 200ns and BDR = bbp/m X-band TWS X-band SWS 30 GHz TWS

Operation limit to high gradient performance -- Modified Poynting vector Sc power law-- A. Grudiev, S. Calatroni, and W. Wuensch, “New local field quantity describing the high gradient limit of accelerating structures”. Phys. Rev. ST Accel. Beams 12, (2009): The square root of S C has been scaled to t pulse = 200ns and BDR = bbp/m DESIGN

Operation limit to high gradient performance -- Stress model exponential law-- F. Djurabekova et al., “Multiscale modeling of electrical breakdown at high electric fields”. Talk in the International workshop on Mechanisms of Vacuum Arcs MeVArc, Helsinki, Finland (2011): Defect volume  V [m 3 ] Dislocation loop radius r loop [nm] Other experimental data [0.8,13]* [13,40]

TERA 3 GHz SCC Test 2.9* Operation limit to high gradient performance -- Stress model exponential law-- F. Djurabekova et al., “Multiscale modeling of electrical breakdown at high electric fields”. Talk in the International workshop on Mechanisms of Vacuum Arcs MeVArc, Helsinki, Finland (2011): Defect volume  V [m 3 ] Dislocation loop radius r loop [nm] Other experimental data [0.8,13]* [13,40] Test results are consistent with other experimental data

TERA 3 GHz SCC Test 2.9* Operation limit to high gradient performance -- Stress model exponential law-- F. Djurabekova et al., “Multiscale modeling of electrical breakdown at high electric fields”. Talk in the International workshop on Mechanisms of Vacuum Arcs MeVArc, Helsinki, Finland (2011): Defect volume  V [m 3 ] Dislocation loop radius r loop [nm] Other experimental data [0.8,13]* [13,40] Test results are consistent with other experimental data  interest

Breakdown Timing within RF pulse 450 kW, E S = 265MV/m, t p = 2.2  s 6 events 600 kW, tp = 1.0  s 12 events 600 kW, E S =325MV/m, t p = 2.2  s 22 events 450 kW, E S = 265MV/m, t p = 2.2  s 1 event

Hadrontherapy application: E S = MV/m with t flat-top = 2.2  s BDR < bpp/m (requirement) Consequences for TERA Cavity tests: E S = 265 MV/m with t flat-top = 2.2  s BDR measured (reflected RF energy) =5·10 -4 bpp/m

Hadrontherapy application: E S = MV/m with t flat-top = 2.2  s BDR < bpp/m (requirement) Consequences for TERA Cavity tests: E S = 265 MV/m with t flat-top = 2.2  s BDR measured (reflected RF energy) =5·10 -4 bpp/m BDR measurement at E S = MV/m with t flat-top = 2.2  s is important for TERA applications, no need to apply scaling laws.

Hadrontherapy application: E S = MV/m with t flat-top = 2.2  s BDR < bpp/m (requirement) Consequences for TERA X=20  BDR ~ bpp/m X=10  BDR ~ bpp/m For operation at 100 Hz: XTime/event 1017 hours 156 days months! Cavity tests: E S = 265 MV/m with t flat-top = 2.2  s BDR measured (reflected RF energy) =5·10 -4 bpp/m BDR measurement at E S = MV/m with t flat-top = 2.2  s is important for TERA applications, no need to apply scaling laws.

Strategy for February dreamed scenario -- Testing time: Testing time: 17 daysSchedule: -Start + continue conditioning (1+2 days) -BDR measurements at high field interesting for CLIC comparison (2-3 days, scaling law evaluation) -BDR measurement at low field interesting for TERA applications (12 days) * Dark current measurements at different stages

Summary and Conclusions -Debugged Control and Data Acquisition Systems -More diagnostic intrumentation included -Some conditioning -First interesting measurements of field enhancement factor and breakdown rate, and evaluation of scaling lawsHowever, -Conditioning process has not been completed yet: 3 days -Comparison with scaling laws used by CLIC: 2-3 days -BDR measurement at TERA field: days at 100 Hz

Test February Lessons learned from the latest test - To do list before the next test - Strategy for next test

Comments for the next test -- lessons learned from this experience-- Use photomultipliers to detect breakdowns from light emission. Be careful with saturated signals. Measure dark current for field enhancement factor calculation before conditioning starts. Connect ALL signals to the same data acquisition system to ease synchronization.

Conclusions We are happy, we are grateful and we would like more testing time (it would make us much happier and much more grateful!).

Acknowledgements We would like to express our gratitude to the CTF3 group for permission to run experiment in their test facility and technical and scientific support (Gerry McMonagle, Jan Kovermann, Roberto Corsini, Stephane Curt, Stephane Rey, Frank Tecker, Esa Paju, Ghislain Rossat, Wilfrid Farabolini, Thibaut Lefevre, Aurelie Rabiller, etc.) to prepare and perform the experiment. We also acknowledge the CLIC RF structure development group (Walter Wuensch, Igor Syratchev, Alexej Grudiev, Jiaru Shi, etc.) for the enriching discussions about the preparation, development and analysis of the test. We are also grateful to Rolf Wegner, who leaded the design, prototyping and first high- power test of the cavity, for his valuable discussions on the continuation of the high power tests. Special thanks go to Alexey Dubrovskiy and Luca Timeo, for their special involvement in the experiment. We also acknowledge Javier Bilbao de Mendizábal and Paolo Magagnin for their precious time spent in long shifts and Eugenio Bonomi for the support on the temperature measurement system preparation. Thanks to Vodafone Italy foundation for the fundings received to produce the test cavity. Thanks also to the CERN PS group and CERN General Services for the technical support.

[Degiovanni et al.] A. Degiovanni et al., « TERA High Gradient Test Program of RF Cavities for Medical Linear Accelerators ». NIM A 657 (2011) 55-58: &_check=y&_origin=&_coverDate=21-Nov-2011&view=c&wchp=dGLzVlk- zSkWz&md5=401347aa3fed9fd2706dc8d36b049a94/1-s2.0-S main.pdf &_check=y&_origin=&_coverDate=21-Nov-2011&view=c&wchp=dGLzVlk- zSkWz&md5=401347aa3fed9fd2706dc8d36b049a94/1-s2.0-S main.pdf [Wang&Loew] J. W. Wang and G. A. Loew, “Field Emission and RF Breakdown in High-Gradient Room-Temperature Linac Structures”. SLAC-PUB-7684 (1997): [Grudiev et al.] A. Grudiev, S. Calatroni, and W. Wuensch, “New local field quantity describing the high gradient limit of accelerating structures”. Phys. Rev. ST Accel. Beams 12, (2009): ab.aps.org/pdf/PRSTAB/v12/i10/e102001http://prst- ab.aps.org/pdf/PRSTAB/v12/i10/e [Djurabekova at al.] F. Djurabekova et al., “Multiscale modeling of electrical breakdown at high electric fields”. Talk in the International workshop on Mechanisms of Vacuum Arcs MeVArc, Helsinki, Finland (2011): Bibliography