WG3: Laser and High Gradient Structure-Based Acceleration

1. WG3: Laser and High Gradient Structure-Based Acceleration Working group summary by Evgenya Simakov (LANL) Gerard Andonian (Radiabeam)

2. WG3 summary • 19 talks • 4 student posters • 9 talks on dielectric laser accelerators • 2 talks on mm-wave accelerators • 2 talks on breakdown • 3 talks on accelerator technology • PBG accelerating structures, nanostructures, cyclotron autoresonance, and artificial materials.

3. Recent demonstration of record high gradients in dielectric laser accelerating structures Kent Wootton (SLAC) Demonstration of accelerating gradients in excess of 1 GV/m ! • Relativistic electron beam • 800nm laser, 65 fs pulse duration • SLAC (NLCTA): 690±100 MV/m • UCLA (PEGASUS) 1.4 GV/m

4. In addition to acceleration, extending scale of DLAs beyond ~1 mm necessitates focussing Deflecting structure at 45˚ Square pillars, three heights Centre designed for zero net force Supports multiple beams at wavelength spacing. Focusing in dielectric laser accelerators with short-pulse lasers Kent Wootton (SLAC)

5. Silicon Dual Pillars laser accelerating structure Andrew Ceballos (SLAC) • 2D dual pillar DLA structures with improved accelerating field profiles. • >370 MV/m for <100 keV (𝛃~0.5). • highest gradient DLA for subrelativistic beams. Leedle, Ceballos, et al., Opt. Lett. 40.18 (2015)

6. New gratings and staging for nonrelativistic beams in dielectric laser accelerators Joshua McNeur (Friedrich Alexander University of Erlangen) • Si grating instead of SiO2 • 70 MV/m gradient for β= 0.3 • Staging with two laser pulses on a grating

7. Increasing interaction length in a DLA David Cesar (UCLA) Co-propagating structures • Difficult to couple laser • Accumulated nonlinear effect Pulse front tilt • Requires a flat wavefront in the moving-frame Demonstration of accelerating gradients in excess of 1 GV/m !

8. Colonnade structure for dielectric laser acceleration Zhaofu Chen (Unversity of Tokyo) • 50 keV electrons • Optimized structure for the maximum accelerating gradient for the given laser power. • Gradient equal to 0.035 times the peak electric field. • Test is under assembly with a 1030 nm laser. e-1 laser

9. New cathodes and additive manufacturing for dielectric laser accelerators Evgenya Simakov (LANL) • Additive manufacturing of woodpiles (and others) with Nanoscribe Professional. • Diamond field emitter array cathodes • Current > 1 A/mm2, 20 μA/pyramid. • Emittance < 1 mm*mrad. 10 µm

10. Analytic tool to design infrared gratings for laser acceleration Levi Schachter (Technion)

11. Artificial materials for DLAs Adi Hanuka (Technion) Electric • Are dielectric materials optical choice? • Compare to planar Bragg array • PBA consisting of artificial materials (permeable) layers: • For small gap, it manifests higher efficiency than dielectric PBA. • Higher interaction impedance for higher contrast. Bragg Magnetic

12. High gradient mm-wave metal accelerating structures Massimo DalForno (SLAC) CuAg structure • SLAC tested accelerating structures at sub-THz frequencies (100 GHz and 200 GHz). • Exited with an electron beam at FACET. • Statistical properties of the rf breakdown is studied for different structure geometries, accelerating gradient, pulse length, and materials. Breakdown rate measurements: travelling wave Cu 200 GHz structure Solid model of the 200 GHz structure • Massimo Dal Forno et al., Phys. Rev. Accel. Beams 19, 011301, 6 Jan 2016 • Massimo Dal Forno et al., Phys. Rev. Accel. Beams 19, 051302 , 3 May 2016 1 mm Gap 0.5 Gap 1.3 Gap 0.7 Gap 1.5

13. Novel mm-wave metal accelerating structures Sergey Kuzikov (Euclid Techlabs) • Proposed a number of novel structures for acceleration with mm-waves. • Helical acceleration structure preserves small beam’s emittance and energy spread. • High gradients can be achieved with picosecond pulses in a grating structure.

14. Demonstration of complete multipactor suppression in dielectric structures Chunguang Jing (Euclid Techlabs) • Successfully demonstrated the complete multipactor suppression with an external solenoid field • Upcoming studies on grooved structures

15. IREAP Nonlinear structure of the multipactor Rami Kishek (University of Maryland) • A novel comprehensive theory: the details of the trajectory are irrelevant, only the phase at impact matters. • Validated against PIC simulations. • Not device-specific.

16. Phase-locked 100 kW 1.3 GHz magnetron Lawrence Ives (Calabazas Creek) Controlling magnetron phase and amplitude with phase modulation of locking signal. Will significantly reduce the cost of RF power for high Q accelerator cavities. Successfully demonstrated in low power system at 2.45 GHz. Full power system being assembled Full power testing scheduled for September 2016. United States Department of Energy Grant No. DE-SC0011229.

17. Depressed beam collector for multi-beam klystron Sergey Shchelkunov (Yale University) • A klystron with a cost-effective depressed collector to boost the efficiency of the multi-beam klystron to above 75%, or even to 80% • The key components include a grounded element, a magnetic lens, and an electrode held at negative potential. • Successfully tested 6 beamlet multi-beam klystron, 1.3 GHz, ~3MW, 60%, at 60kV.

18. 100 kW CW multi-beam klystron for an electron-ion collider Sergey Shchelkunov (Yale University) • A CW highly-efficient RF source for a future electron-ion collider at TJNAF • Compact (L<850mm). • Multi-beam klystron (MBK) with 12 beamlets. • High efficiency (> 70%) • 100 kW of power at 952.6 MHz • Low beam voltage, ~15kV • Low beamlet perveance (0.544 μAV-3/2), and cathode loading (1.4 Acm-2)

19. X-band photonic band gap structures with elliptical rods and improved wakefields suppression Evgenya Simakov (LANL) Structures with elliptical rods have smaller surface fields and better wakefield suppression HOM suppression demonstrated with beam at AWA Wrapped in foil With SiC absorbers

20. Cyclotron Autoresonance Accelerator (CARA) Jay Hirshfield (Yale University) • Industrial applications for MW avg. power, CW beams (e.g. pollutants in waste water) • Old idea is new again • High accelerator efficiency (97%) • Corrugated helix couples two rotating modes • Chop CARA beams to distribute load on windows x-y x-z x-y

21. TV/m gradient in nano-channel acceleration Young-Min Shin (Northern Illinois University) 1020– 1023 cm-3 1 ~ 30TeV/m • “TeV on a chip” • Channeling in nano-crystals • Beam or laser driven • Strong focusing • Single-stage • Cost-effective • Stable • Planned experiments (FAST and NIU lab) • Beam transmission, modulation, energy shift

22. Conclusion • Dielectric laser accelerator development is exciting, multiple new results. • Still work on multipactor and novel accelerating structures (would be nice to have more on this topic!). • Accelerator technology and components still belong to the group (please keep with us, we love you!). • Novel structures and ideas are always welcome.