High Gradients in Dielectric Loaded Wakefield Structures Manoel Conde High Energy Physics Division Argonne National Laboratory AAC 08 – Santa Cruz, CA

 AWA & wakefield generation  High gradient excitation  Upgrades and future goals  Wakefield excitation in SLAC / KEK X-band structure Outline

Accelerator R&D at Argonne National Laboratory High Energy Physics Division AWA Group  Wei Gai  Sergey Antipov  Manoel Conde  Felipe Franchini  Feng Gao  Chunguang Jing  Richard Konecny  Wanming Liu  Marwan Rihaoui  John Power  Zikri Yusof External Users / Collaborators:  Euclid TechLabs  U.Chicago  NIU  Fermilab  U.Md.  IIT  APS  Yale / Omega-P

Research at the AWA Facility : Advanced Accelerating Structures Current effort has lead to comprehensive knowledge on construction and testing of dielectric based accelerating structures. High Power Electron Beam (~ GW) and RF Power Generation Operating a unique facility to study high current electron beam generation and propagation for efficient beam driven schemes, and high power RF generation. Fundamental Beam Physics and Advanced Diagnostics High brightness beam generation and propagation, phase space measurements, emittance exchange schemes.

Electron Beam Driven Dielectric Wakefield Accelerator  A high current relativistic electron beam passing through a dielectric structure can generate high gradient field, high power microwaves instantaneously. A new way to power accelerating structures by transporting the power in the electron beam.  Justifications for looking at dielectric structures:  Comparable accelerating properties as metal structures.  More material options; possibly higher gradients.  Simpler geometry, simpler construction and HOM damping.  Applications:  Collinear wakefield acceleration  Two-beam acceleration Drive Beam Dielectrics Deceleration Acceleration Accelerate d Beam

Wakefields in Dielectric Structures (a short Gaussian beam) Drive beam is king!  Energy   Charge   Bunch length   Emittance  bb aa  Q Cu

AWA Drive Beamline Drive Gun Linac & Beam Optics Quads Wakefield Structure Experimental Chambers 4.5 m GV YAG1YAG2 Spectrometer YAG5 Dump/ Faraday Cup Slits YAG4YAG3 ICT1 ICT2 BPM Single bunch operation –Q = nC (reached 150 nC) –15 MeV, 2 mm bunch length (rms), emittance < 200 mm mrad (at 100 nC) –High Current: ~10 kA Bunch train operation –4 bunches x 25 nC or 16 bunches x 5 nC (present) – bunches x nC  ns long (future)

The Argonne Wakefield Accelerator (AWA) ~ 1 meter rf-gun magnetic lenses 8 MeV 15 MeV Laser In λ =248nm Linac Quads Spectrometer Faraday cup YAG2 YAG1 Direction of beam propagation Experimental Area

Experimental Setup for High Gradient Tests WF signal RF field probe (- 60 dB) 43 nC time (ns) Monitor for breakdown Infer Gradients from MAFIA  Q Cu Goal: Test breakdown thresholds of dielectric structures under short RF pulses.

Dielectric Loaded Structures Tested SW Structure#1 C10-102#2 C10-23#3 C #4 Q MaterialCordierite Quartz Dielectric constant Freq. of TM01n14.1 GHz 9.4 GHz8.6 GHz Inner radius5 mm 2.75 mm1.9 mm Outer radius7.49 mm Length102 mm23 mm28 mm25.4 mm Wakefield Gradient0.45 MV/m/nC0.5 MV/m/nC0.91 MV/m/nC1.33 MV/m/nC

Wakefield Measurements: Structure #1 (C10-102) 46 nC → 21 MV/m

MAFIA Simulation of Structure #1 (C10-102) Snapshots of wakefield amplitude

Wakefield Measurements: Structure #2 (C10-23) Measurement Simulation Measurement Simulation TM 013 (14.1GHz) TM 014 (16.2GHz) TM 012 (13GHz) HEM 111 (12.4GHz) HEM 111 (12.2GHz) TM 012 (13GHz) TM 013 (14.3GHz) TM 014 (16GHz) HEM 112 (14.7GHz) Freq (GHz) 86 nC → 43 MV/m Measured and simulated E r probe signals

Wakefield Measurements: Structure #3 (C5.5-28) TM nC → 78 MV/m

E-field pattern Wz (V/m) Wz > 1nC for 10GHz Structure 28mm 2.5mm 7.5mm MAFIA Simulation of Structure #3 (C5.5-28)

Wakefield Measurements: Structure #4 (Q ) 75 nC → 100 MV/m HEM 111 TM 012 TM 013 TM 014

 Structure #1 21 MV/m  Structure #2 43 MV/m  Structure #3 78 MV/m  Structure #4 100 MV/m Gradients Reached: Where we go from here: Once the upgrade is complete, the goal is to achieve:  Higher gradient excitation: ~ 0.5 GV/m in structures ( 3 mm apertures)  Higher RF power extraction: ~ 1 GW (10 ns) To improve the drive beam, we are currently upgrading the AWA facility:  A new L-band 1.3 GHz RF station to increase the drive beam energy from 15 to 25 MeV.  Fabrication of Cesium Telluride high quantum efficiency photocathodes to produce long, high charge bunch trains.  A second RF gun to restore two-beam-accelerator capability.

Future AWA Facility (25 MW + 25 MW = 50 MW) *all distances in cm D.U.T. Drive Gun (12 MW) Linac 1 (10.5 MW) 0225 cm351.6 cm581.6 cm29.1 cm455 cm650 cm Linac 2 (10.5 MW) D.U.T. 7.5 MeV MeV 25 MeV Witness Gun (12 MW) MeV Single Bunch: nC Bunch Train: 16 – 64, total charge 1 – 2.5 µC

Some goals of the AWA program: Verify that structures can withstand gradients higher than 100 MV/m. Generation of ~ GW level RF power (25 MeV, 130 Amps, 10 ns, 3 GW beam power). Demonstrate high gradient, broad bandwidth, low cost structures (power extractors and accelerators). Typical parameters: –f = 10 – 30 GHz, Vg = 5 – 20% –Required power: 0.5 – 5 GW peak –Example (for dielectric based structure): 21 GHz, a = 3 mm, b = 4 mm, ε=12, Vg=0.112 For Ez=200 MV/m, it requires P= 1 GW 16 ns RF pulse, structure length : 30 cm, fill time = 8.4 ns Beam loading (fundamental mode):0.75 MV/m/nC

Wakefield Observation on SLAC / KEK X-Band Standing Wave Structure Single bunch charge: up to 80nC. Measurements: Time structure and spectrum of the generated wakefield; Linearity of the voltage-charge curve.

Dimensions for copper only 3-cell standing wave structure experiment t b_conv b_end b1 b2 a_cpl a_pipe 5×Rb Rpipe 5× ellips_r 5×D 2b_end b b b b_cpl b_conv inch 2a a_pipe inch 4×a a_cpl D Rpipe3.00 Rb1.00 t ellips_r Dimensions for 20 deg. C V.A.Dolgashev, 2 March 07 b3 b_cpl

11.427GHz GHz GHz GHz Q ≈ 6700 t - μ s voltage - Volt f – GHz voltage spectrum GHz GHz 3900 q = 34.6nC Measured voltage signal

11.427GHz 9.408GHz 8.361GHz 7.783GHz f – GHz voltage spectrum GHz GHz f – GHz spectrum of the wakefield in the vacuum break for ICT (simulated) The wakefield in the ceramic vacuum break for ICT may contribute to some of the peaks vacuum break

11.427GHz f – GHz voltage spectrum The 11.4GHz component is filtered out t - μ s voltage - Volt q = 34.6nC IFFT Q ≈ 5500 V max V min

voltage peak value charge q - nC For GHz component: