The UCLA RF photoinjector at low charge and small spatial and temporal scales J.Maxson, D. Cesar, P. Musumeci UCLA Department of Physics and Astronomy HBB Workshop, Havana, Cuba March 29, 2016

Outline The UCLA Pegasus photoinjector Pegasus path to generating ultra-low transverse emittance: Decreasing source size: oblique laser incidence Concepts for decreasing source temperature Diagnostics: TEM grid based full + slice phase space diganostic + High resolution YAG:Ce Pegasus path to ultra small longitudinal emittances

The UCLA Pegasus Photoinejctor Quads Exp. Chamber Gun Sol 1. Sol. 2 Deflector Linac Spec. 3-14 MeV beamline Optimized for ≤𝟏𝟎𝟎 fC MeV UEM: Contrast created by charge density. Sub-𝜇𝑚 beam sizes with 10’s of fC required DLA: 400 nm x 1 mm x 1mm aperture, accelerating bucket <3 fs BNL/SLAC/UCLA style 1.6 cell gun, new brazeless design: D. Alesini et al., PRSTAB 18, 092001 (2015)  MeV single shot ultrafast 𝑒 − microscopy see D. Cesar’s talk Dielectric laser acceleration 1 mm 400 nm V. aperture see J. England’s talk …ultra-low 6D emittances required!

Generation of nm-scale emittances Must reduce source size or source “temperature” Source size: Utilize oblique (72 deg) incidence port on 1.6 cell gun + strong final focus lens Use 2 photon emission enhancement (in UV!) to adjust lens position 175 mm lens -7mm 0 mm 7mm Laser in Beam 25 mm stage UV on Virtual Cathode ~ 8 x 18 μm Cu MTE @ 266 nm ∼ 350 meV, implies 𝜖 𝑥 𝜖 𝑦 ≈10 𝑛𝑚

Source temperature 𝜙 Practical limit: Ultrafast heating of metals? Reduce wavelength to reduce source photoemission temperature. 𝜖 𝑡ℎ = 𝜎 𝑥 𝑘 𝑇 𝑒 𝑚 𝑐 2 = ℎ𝜈− 𝜙 𝑒𝑓𝑓 3 𝑚 𝑐 2 J. Feng et al., Appl. Phys. Lett. 107, 134101 (2015): minumumphotoemissionmomentum spread set by materialtemperature 𝒌𝑻 𝒎 . Reduceh𝝂 𝒂𝒏𝒅 𝒌 𝑻 𝒎 !Pay a price in QE… Fermi Sea Energy 𝐸 𝑣𝑎𝑐𝑢𝑢𝑚 𝐸 𝐹𝑒𝑟𝑚𝑖 𝜙 𝑘 𝑇 𝑚 Practical limit: Ultrafast heating of metals? For ultrafast lasers at high fluence (low QE) Two temperature model (electrons and lattice) predicts fast rise and ps scale decay of electronic temperature. Long “cigar” beam aspect ratio takes advantage of electron-lattice coupling and lower peak intensity Calls for higher QE photocathodes! Alkali antimonides near threshold: Near room temperature electrons ( 35 meV) generated with 10 −4 QE with NaKSb ~10 mJ/𝑐 𝑚 2 , QE = 10-5 , Q = 10 pC 𝑡𝑖𝑚𝑒 (𝑝𝑠) 𝑇 𝑒 ~ 250 meV ! J. Maxson et al., APL 106, 234102 (2015)

Measurement of nm-scale emittances Highest resolution: in-vacuum optical microscope objective coupled to thin (30 𝜇𝑚) YAG:Ce. Infinity corrected CCD in air. Single shot transverse phase spaces: TEM grid point projection imaging, “inverse pepperpot” technique. 3 𝜇𝑚×3 𝜇𝑚 beam size, 5 nm normalized emittance observed High Transmission 4D Phase space information Skew quadrupole Nonlinearites Sensitive to psf. Phase space “width” -> bar shadow blur 25 𝜇𝑚 thick Cu grid electron shadowgraph 𝑒 − beam 85 𝜇𝑚 Method benchmarked in: R. K. Li et al., PRSTAB 15, 090702 (2012)

New ideas for extending the TEM grid method Interpolated beamlet phase space (linear correlation removed) Shadowgraph Emittance vs. charge Emittance vs. fraction 7.25 mm 4.125 mm 25 fC 50 fC 200 fC Gulliford et al. APL 2015 : Beam phase space core contains information about cathode physics/thermal emittance. stronger transport invariant than 100% rms emittance. Invariance could break down with new cathode physics! More info: Single shot slice phase space: grids +slit +deflector. XTCAV TEM Grid Slit (10 𝜇𝑚) ~4.5 ps Preliminary analysis: need modeling of psf. “visualize” 𝜃 𝑥 (𝑥, 𝑡)

Ultrasmall bunch lengths & longitudinal emittance RF-induced long. phase space curvature mi to reach sub-fs bunches or ~10 −5 relative energy spread: Two options at Pegasus: X-band linearization cavity: Small energy spreads for UEM 20 fC charge Frequency 9.6 GHz Shunt Impedance 95 MOhms/m Quality Factor 9,000 On-Axis Epeak 5.3 MV/m RF Power(critical coupling) ~9kW Sub-fs beam Or 20 eV rms energy spread R. K. Li et al., PRSTAB 15, 090702 (2012) Funded for STTR Phase II

How to detect ultra short beams? XTCAV have finite length (~20 cm): Pegasus stretcher mode case has average 𝜎 𝑡 ~ 10 𝑓𝑠 within XTCAV Similar to what can be done with on crest bunching (also can be ~10 fs at 20 fC) Previous on-crest velocity bunching at Pegasus: X. Lu et al. PRSTAB 2015, measurement using CTR diagnostic Now: bunching with direct deflector measurment + grid emittance. First results from Pegasus velocity bunching with direct deflection Jitter creates distribution of deflector on/off shots—challenging rigorous analysis. Median width shots (25 fC) with deflector on/off shown: At instrument resolution (limited by klystron forward power) All caveats of non-Gaussian distributions applied: 𝜎 𝑓𝑖𝑡,𝑜𝑛 2 − 𝜎 𝑓𝑖𝑡,𝑜𝑓𝑓 2 ≈35 𝑓𝑠

Summary Source size reduction via oblique incidence Approaching new limits in source temperature reduction nm-scale emittance measurements High spatial resolution solenoid scan Phase space reconstruction with TEM grids Ultrashort beams accessible with low charge and velocity bunching ~10 mJ/𝑐 𝑚 2 , QE = 10-5, 𝑡𝑖𝑚𝑒 (𝑝𝑠) 𝑇 𝑒 ~ 250 meV !

Acknowledgements UCLA Pegasus Laboratory group PI: Pietro Musumeci D. Cesar, E. Pyrez, E. Curry. Graduate students; A. Ody, G. Calmasini, Y. Sun undergraduates. Collaborators: R. K. Li, D. Alesini, G. Andonian, F. Carbone, D. Filippetto, J. Luiten, J. B. Rosenzweig, J. Spence Funding sources Radiabeam Technologies. DE-FOA-0001164: SBIR/STTR FY 2015 Phase I award # 0000215429 NSF grant #1415583