High Gradients in Dielectric Loaded Wakefield Structures Manoel Conde High Energy Physics Division Argonne National Laboratory AAC 08 – Santa Cruz, CA
Outline • AWA & wakefield generation • High gradient excitation • Upgrades and future goals • Wakefield excitation in SLAC / KEK X-band structure
Accelerator R&D at Argonne National LaboratoryHigh Energy Physics DivisionAWA Group • Chunguang Jing • Richard Konecny • Wanming Liu • Marwan Rihaoui • John Power • Zikri Yusof • Wei Gai • Sergey Antipov • Manoel Conde • Felipe Franchini • Feng Gao 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.
Drive Beam Dielectrics Accelerated Beam Deceleration Acceleration 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 is king! Energy Charge Bunch length Emittance e Q 2a 2b Cu Wakefields in Dielectric Structures (a short Gaussian beam)
Wakefield Structure Linac & Beam Optics Drive Gun Spectrometer Quads Experimental Chambers 4.5 m YAG3 YAG4 YAG1 YAG2 YAG5 ICT1 Dump/ Faraday Cup ICT2 GV GV Slits BPM AWA Drive Beamline Single bunch operation • Q = 1-100 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) • 16 - 64 bunches x 50 - 100 nC 10 - 50 ns long (future)
The Argonne Wakefield Accelerator (AWA) 15 MeV 8 MeV Experimental Area YAG1 Linac Faraday cup magnetic lenses Quads Spectrometer Laser In λ =248nm rf-gun Direction of beam propagation YAG2 ~ 1 meter
43 nC 100 0 -100 0 2 4 6 time (ns) Experimental Setup for High Gradient Tests Monitor for breakdown WF signal RF field probe (- 60 dB) e Q Cu Infer Gradients from MAFIA Goal: Test breakdown thresholds of dielectric structures under short RF pulses.
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) Measured and simulated Er probe signals TM013 (14.1GHz) HEM111 (12.4GHz) TM014 (16.2GHz) HEM112 (14.7GHz) Measurement TM012 (13GHz) Measurement Freq (GHz) TM013 (14.3GHz) HEM111 (12.2GHz) TM014 (16GHz) Simulation TM012 (13GHz) HEM112 (14.7GHz) Simulation Freq (GHz) 86 nC → 43 MV/m
Wakefield Measurements: Structure #3 (C5.5-28) TM013 86 nC → 78 MV/m
MAFIA Simulation of Structure #3 (C5.5-28) 28mm 7.5mm 2.5mm E-field pattern Wz > 1MV/m @ 1nC for 10GHz Structure Wz (V/m)
Wakefield Measurements: Structure #4 (Q3.8-25.4) TM012 TM013 TM014 HEM111 75 nC → 100 MV/m
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: • 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. • 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)
Witness Gun (12 MW) 8 MeV 0 29.1 Future AWA Facility (25 MW + 25 MW = 50 MW) D.U.T. Drive Gun (12 MW) Linac 1 (10.5 MW) Linac 2 (10.5 MW) D.U.T. 7.5 MeV 25 MeV 15.75 MeV 0 29.1 cm 225 cm 351.6 cm 455 cm 581.6 cm 650 cm Single Bunch: 50 - 100 nC Bunch Train: 16 – 64, total charge 1 – 2.5 µC *all distances in cm
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 5×D t 5×Rb Rpipe a_pipe b_end b_cpl b_conv b1 b3 b2 a_cpl 4×a 5×ellips_r Dimensions for 20 deg. C V.A.Dolgashev, 2 March 07
Measured voltage signal q = 34.6nC voltage - Volt t -μs 13.825GHz 3900 7.783GHz Q ≈ 6700 11.427GHz 5500 8.361GHz 3600 voltage spectrum 9.408GHz 4600 13.711GHz 1700 f – GHz
The wakefield in the ceramic vacuum break for ICT may contribute to some of the peaks spectrum of the wakefield in the vacuum break for ICT (simulated) vacuum break f – GHz 13.825GHz 7.783GHz 11.427GHz 8.361GHz voltage spectrum 9.408GHz 13.711GHz f – GHz
The 11.4GHz component is filtered out 11.427GHz voltage spectrum f – GHz IFFT Vmax q = 34.6nC Q ≈ 5500 voltage - Volt Vmin t -μs
For 11.427GHz component: voltage peak value charge q - nC