X-Band: test stand and structure program

Outline Gradient status XBox status Collaborators Structures under test Longer-term objectives Concrete Generalize Wakefields and tolerances I will not have time to cover the excellent progress which has been made in operation and data analysis .

State of the art Operating frequency 12 GHz Accelerating gradient 100 MV/m Peak surface electric field 250 MV/m Pulse length 200 ns Repetition rate 50 Hz Input power 50 MW Breakdown rate <10-7 BD/pulse/m One structure showed 120 MV/m, so full margin for beam loading (about -15%). But no damping features. Numerous structures around 100 MV/m. Technical issues (probably) identified, feeding back on design and production. Parameters (rounded values)

X-band test stands at CERN XBox-1: beam test area XBox-1 XBox-2 XBox-2 and 3: Bunker XBox-3

Technical Specifications klystron Peak power pulse length Modulator Rep rate XBox-1 CPI 50 MW 1.5 µs Scandinova 50 Hz CTF3 beam test capability XBox-2 XBox-3 Toshiba 4x6MW 5 µs 400 Hz up to 4 testing slots! Cavity-type pulse compressors on all the XBoxes. Full computer control, data acquisition and on-line analysis for all three XBoxes. XBoxes test prototypes but also are prototypes of linac rf units for a wide variety of applications: initial energy stage CLIC, XFEL (low and high rep rate), Compton/Thompson sources, transverse deflectors, linearizers, photo injectors etc.

Status XBox-1 – Operational XBox-2 – Operational XBox-3 All four klystron/modulators at CERN 3a – Operational 3b,c,d – Site acceptance testing week of 12 October followed by commissioning Waveguide network, LLRF under preparation, complete for end of year

Structures under test right now XBox-1 – CLIC prototype, re-conditioning, beam loading test to follow (TD26CC). XBox-2 – Structure from halves, conditioning (T24-open) XBox-3 – 3-D printed Ti waveguide under medium (5 MW-range) power test

Collaborators Now a brief look at collaborators with major X-band rf infrastructure; KEK, SLAC, Tsinghua and SINAP. Many more collaborators make important contributes to the effort: University of Valencia, University of Lancaster, University of Uppsala, Sincrotrone Trieste, PSI, SACLAY, CIEMAT, Melbourne University/Australian Light Source, University of Helsinki, University of Tartu, Hebrew University of Jerusalem, University of Eindhoven, University of Oslo, Argonne.

NEXTEF – prototype and single cell testing KEK Outstanding record of structure production and testing. Close coordination with CLIC study. CERN designed, KEK/SLAC built prototype. Single-cell test cavity NEXTEF – prototype and single cell testing

SLAC Extensive, independent high-gradient program.

Tsinghua University Choke-mode cavity Working towards new generation Thompson scattering source. Choke-mode cavity

SINAP X-band deflector Currently building soft X-ray XFEL test facility at C-band. Plan to propose hard X-ray XFEL, X or C decision not taken yet. Building up in-house capability for high-gradient and high-frequency cavities. They will build: X-band deflector T24 CLIC structure Four XFEL-optimized accelerating structures X-band deflector To be tested at KEK

CLIC prototype in XBox1/dog-leg First try with beam loading experiment was only partial success. Experiment established but structure developed hot front end, perhaps due to accumulated beam loss. Re-aligned collimator. Re-use TD26CC to minimize conditioning time. Pulse compressor was very hard to run. Modified and now runs far better. Currently re-conditioning structure, behaving well. 110 MV/m, 100 ns Comparison to initial conditioning

T24 open in XBox2 Milled structures have huge potential advantages - cost, treatment, materials. Early tries with quadrants yielded unsatisfactory results, but don’t believe this was end of story. We’re back! Over 70 MV/m, 100 ns and behaving well so far. Similar behaviour to disk-based. Sc comparison shown. Structure in halves built by SLAC

Baseline program CERN production CLIC prototypes: Second ready but needs leak repair. Objectives, refine CERN production, reproducibility (TD26CC#2 and 3). CLIC prototype with SiC absorbers: a missing element (TD26CC-Sic) T24s to confirm KEK result: Essential question of effect of damping features on gradient. Do they lower? If so why? And how to counteract? New 3 TeV baseline series: Absorb lesson learned so far, redesign for higher performance at lower cost (CLIC-G*) Collaborator production Three CLIC prototypes to be built and tested at KEK. Still unbeaten production! One TD26CC built by CIEMAT One T24 built by SINAP

Alternative program Milled structures: Potentially much lower cost. If current progress continues follow up with: Bonded irises Open with hard copper Many other options! Two T24s built by PSI in their production run. Vacuum brazing alternative Potentially lower cost. Choke-mode damping by Tsinghua. Potentially much lower cost. Deflector by SINAP Four XFEL structures by SINAP. Connection to XbXFEL Pulsed dc system –presentation later in morning

Schedule

Overall high-gradient objectives Consolidate existing results – solve issues uncovered as we move towards higher gradient, statistics, operational experience, conditioning Investigate a high-potential, performance and cost, alternatives Critically evaluate and optimize baseline fabrication procedure Validate new baseline structure Run klystron/pulse compressor units up to full specification – feed two structures in parallel

Wakefield suppression Wakefield from 1.5 m long, 6 structure length, prototype measured directly with beam at the FACET facility at SLAC. The agreement between measurement and simulation is a spectacular validation of our design capabilities and we meet our beam dynamics requirements!

Tolerances Component tolerances achieved. We order such parts regularly from industry. We already are feeding back on the rf design to make the parts less expensive based on what we have learned. Assembly tolerances, mainly straightness, need work. Priority has been to produce high-gradient test structures and we choose to minimize time/risk associated with optimizing alignment. More work is needed on this.

I would like to acknowledge the excellent work which has been done by colleagues at CERN and around the world and express my sincere appreciation.

More information Our high-gradient high-gradient and X-band applications community recently held a workshop in Beijing: https://indico.cern.ch/event/358352/.