1. 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

2. 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

3. 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, (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!

4. 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 266 nm ∼ 350 meV, implies
𝜖 𝑥 𝜖 𝑦 ≈10 𝑛𝑚

5. 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,  (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,  (2015)

6. 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, (2012)

7. 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)

8. 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, (2012)
Funded for STTR Phase II

9. 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

10. 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 !

11. 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 : SBIR/STTR FY 2015 Phase I award #
NSF grant #