Multiphoton photoemission for high brightness electron beam sources
Pietro Musumeci UCLA Department of Physics and Astronomy
P3 workshop, Brookhaven 12-14 October 2010 Outline
• Multiphoton photoemission – “blow-out” regime beam dynamics • Oblique incidence illumination – S and P charge yields • Beyond the cathode emittance limit – Proper use of a collimating aperture • Conclusions UCLA Pegasus Laboratory • UCLA-based advanced photoinjector laboratory – State-of-the-art ultrafast drive laser Ti:Sa – RF streak camera with 50 fs resolution – Electro-Optic Sampling setup • Ultrafast relativistic electron diffraction – New application of high brightness ultrashort electron beams from RF photoinjector. • Uniformly filled ellipsoidal beam distributions – Launch an ultrashort beam at the cathode – Uniformly filled ellipsoid created by self-expansion. • High beam brightness & ultrashort beams !
RF gun Probing electron beam UCLA/BNL/SLAC ~200 fs long 1 pC 3.5 MeV
Photocathode driver pulse Collimating hole Pump pulse
delay line Multiphoton photoelectric effect • Einstein photoelectric effect… o For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light. o For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency • …..and few years of RF photoinjector common practice spent on “getting ready the UV on the cathode” • Two or more small photons can do the job of a big one. • Generalized Fowler-Dubridge theory: photoelectric current can be written as sum of different terms. Fowler function J J selects the dominant n n n where -order of the process n e n n 2 nh e 0 J n n A 1 R I Te F h kTe Send ultrashort IR pulses on the cathode Very early results from Rao et al. and Muggli et al. (mid 90s)…. Recent interest in pancake regime. Ultrashort beam at cathode => uniformly filled ellipsoidal beam. Very high extraction field in RF photoinjector: away from space charge induced emission cutoff. (Early experiments using low gradient setups.) Damage threshold few 100 GW/cm2 at sub-100 fs pulse lengths.
BUT conversion efficiency is ~10%
Measure yield for different spot sizes. Recipe: Use very short (and intense Deflector off !!!) laser pulse on the cathode. What is really new here is: • Shorter beams. Deflector on • More linear phase spaces (transverse and longitudinal). • Control beam tails. • Ease requirements on laser system ! Charge 20 pC. Laser spot size 400 m rms (limited by asymmetry due to image charge)
rms length ~300 fs
Very sharp ellipsoidal beam boundary due to the ultrashort beam on the cathode. Other sources of inspiration
• SPARC scheme for electron beam conditioning at cathode by using electric field of a laser. (Ferrario et al.)
• Transient electric field in Ultrafast Electron Diffraction Laser generated e-cloud • Multiphoton Photoemission generates e-beam spurious time-dependent beam deflections which contaminate UED signal. Diffracted • Hot topic. See PRL comments H. Park and J. e-beam M. Zuo, Phys. Rev. Lett. 105, 059603 (2010) crystal surface and Zewail group reply, Carbone et al. PRL 105, 059604 (2010) Prompt multi-photon photoemission
Is it thermally assisted photoemission? The signature would be a slower response.
Autocorrelation of two IR pulses on the cathode shows promptness of emission.
Charge autocorrelation Nonequilibrium Polarization gating autocorrelation Fujimoto, Liu, Ippen, Electron and Lattice 350 Bloembergen. Phys. Rev. Lett. 53, 1837 - 1840 (1984) Temperatures in 300 Tungsten 250
200
150 Autocorrelation signal (arb. units) (arb. signal Autocorrelation 100 0 100 200 300 400 500 600 700 800 Time (fs) Multiphoton photoemission scaling laws • Electric field on cathode very important • Schottky effect and space-charge saturation. • Current density (charge per unit time per unit area) is proportional to the cube of the laser intensity. • Assuming at emission the electron beam is as long the laser pulse. Effect of MgF2 coating on copper surface
• The multiphoton photoemission experiments were done using a MgF2 coated Cu cathode • Off-axis diamond turning gives mirror- like surface; no tool pressure in critical center of emitting surface.
• Hard MgF2 film on a substrate improves scratch resistance, act as anti-reflective coating • Thickness to give AR coating for 266 nm. • Reflectivity @ 800 nm 15 % (key to multiphoton photoemission results). For 3-photon process we just gained a 3 • Polished uncoated copper surface has factor of (1-R) ~ 40 ! reflectivity > 80 %. • Next step: AR coating @ 800nm ? n e n n 2 nh e 0 J n n A 1 R I Te F h kTe Surface analysis (SEM) • Coating behavior • Good vacuum conditions. • Good RF behavior. • Thermal emittance and dark current levels very comparable for the UV pulses.
• What are the best post-mortem diagnostics?
Polished / uncoated MgF2 coating not irradiated Tested cathode Beam dynamics with IR generated photolectrons
• Excess kinetic energy same as in UV case Ek nh e 0 • Thermal emittance very similar to UV measurements
• Creation of uniformly filled ellipsoidal beam using 800 nm laser pulses. • Very sharp boundary due to prompt emission at the cathode. • Further reducing the charge we measured a minimum bunch length < 200 fs (limited by the RF deflector resolution)
500 fs Study of transverse uniformity and amplitude jitter Virtual cathode Electron beam laser image
Charge density emitted depends on I3
Beam transverse profile very sensitive to laser uniformity.
Charge jitter proportional to E3.
If we are not in saturation within the non linear crystals, UV energy after tripling crystals shows the same cubic dependency. Oblique incidence on the cathode: Physics and Technology background
• Vectorial photoelectric effect – P-polarization is more efficient in emitting electrons. – Surface photoemission.
• Optical Field Enhancement effect – The component of the field pointing out of the surface lowers the work function.
Compensate pulse front tilt for ultrashort laser pulse cathode illumination (blow-out regime at oblique incidence). Remove off-axis mirror for near-normal injection. Oblique incidence: Pegasus setup
For blow-out regime, the recipe is to illuminate the cathode with an ultrashort laser pulse. Need to correct the pulse front tilt if coming at 72 degrees.
Vertically focusing lens Horizontally focusing lens Compensation of ellipticity Pulse fron tilt control and imaging of grating plane Grating
Cathode Mirror Mirror
Laser pulse
Need two cylindrical lenses to compensate dispersion. In Situ Reflectivity measurements
n e n n 2 nh e 0 J n n A 1 R I Te F h kTe
• Absorption of the cathode at 72.5° angle of incidence
0.66 0.64 62 uJ 64% for p-polarization 0.62 28 uJ 0.60
R) 48% for s-polarization - 0.58 s-polarization 0.56 0.54 p-polarization 0.52 3 reflectivity (%) 1 Rs 0.50 2.5 Absorption (1 Absorption 3 0.48 1 Rp 0.46
0 20 40 60 80 100 wave plate angle (deg) Charge yield measurements • Factor of 2.3 difference between s and p. • Same slope and field dependence for both polarization. • Can be fully explained with reflectivity measurements • Taking into account windows transmission, the charge yield is consistent with previous measurements at normal incidence. • Why don’t we see enhancement???
2 10
El = 300 MV/m -5 Al = 5*10 1 2 ) 10 2 Al*m0c = 40 eV
0
10 Charge density (pC/mm Charge S polarizations-polarization slope = 2.08 P polarizationp-polarization -1 10 slope = 1.95
2 20 Intensity (GW/cm2) Repeat measurements for uncoated cathode • Similar results, but in this case the observed enhancement is > 40. • Reflectivity differences can only account for factor 7.5. • But slope and field dependence again is the same for s and p. – Optical field enhancement suggest a field dependence which is not in experimental data. • Field oscillation too fast for photoemission to respond? How fast is photoemission?
coated 72.5 deg s coated 72.5 deg s coated 72.5 deg p coated 72.5 deg p coated normal Space 10000 100 coated normal uncoated 72.5 deg s uncoated 72.5 deg s uncoated 72.5 deg p charge field uncoated 72.5 deg p uncoated normal 3 MV/m 1000 uncoated normal
10 units) (arb. 3 100 Slope = 3
10
1
1 Charge Density (pc/mm^2) Density Charge
0.1 0.1 1 10 R) - (1 / Density Charge 1 10 Intensity (GW/cm^2) Intensity (GW/cm^2) Beam brightness definition • Recent paper by Bazarov et al. PRL, 102:104801 (2009) on “Maximum achievable beam transverse brightness.”
• Transverse brightness is not the all story. • 6D brightness should be a measure of electron beam density in 6D phase space. N N B6D V6D x y z • But, even after being able to measure the x,y,z emittances, 6D brightness is still very difficult to quantify as there are lots of correlations between the different planes. • In the end, it depends on the beam application which brightness is relevant. Maximize 6D beam brightness
From Bazarov et al. . Can it be improved?
. Liouville says not possible. • But beam brightness is the average of the density over the beam. There will be regions of high density and regions of lower density in 6D phase space. . Scraping or collimating the beam we might be able to select the “best quality” particles.
. In Electron Diffraction / Microscopy aperturing the beam helps • To remove dark current ( improving signal to noise-ratio) • Define the probed area of the sample.
. In practice this is done all the time when cutting beam distribution to measure emittance. Local definition of 6D brightness
• To find out in simulations where to cut the beam distribution is not an easy task as in order to simulate the local 6D phase space density, you need to populate it with millions 6 of particles. 2 1 Dist6Di, j xn,i xn, j mn xn,i xn, j m,n 1 • Calculate mean interparticle distance in 6D phase space. • Choose the right metric.
• Need to scale beam dimensions by inverse of beam inverse of beam matrix • The local density defined as Dist 6 D 6 is then invariant for any beam transformation (just like 6D beam brigthness) Collimating aperture
• By aperturing the beam it is possible to
select the area with highest density. 2 ndim x y dens ity GPT simulations. Use a 1 mm diameter hole located at 80 cm from cathode. Emittance measurements ongoing. Very hard to measure ultralow emittances !
0.0e-11
5e-7 Charge Emittance -0.5e-11 4e-7
Q -1.0e-11 3e-7
nemixrms -1.5e-11 2e-7
1e-7 -2.0e-11
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0e-7 GPT avgz 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 GPT avgz
6D brightness 4D brightness
18 3 2 10 1.5 10 B6D j B 3 18 j 1 10 1 10 500 0 0 0.2 0.4 0.6 0.8 1 0 0 0.2 0.4 0.6 0.8 1 zj zj Preliminary measurements of collimated beam • Pepper-pot transverse phase space measurements • Deflector + dipole long phase space measurements • Charge 10 –> 1 pC • Emittance 0.7 –> 0.083 mm-mrad. • Brightness -> 8 times increase. • Wakefield problems are reduced in this low charge regime. – But still need to align well (deflector/spectrometer help a lot !) thres output4 thres Longitudinal phase space max(output4) j npoints inew2 3 Transverse phase space Direct image Reconstructed
2 0 1 output4 output4 output4 2 max output4 Conclusions
• Multiphoton photoemission can be surprisingly useful to generate large amounts of charge from metal cathodes – No more UV on the cathode at Pegasus – Application for high average power guns? With a 5 MHz 2 uJ oscillator (10 W total power) -> 50 pC/pulse.
• Coating effect is critical. Can we improve performances with the right coating?
• Oblique incidence. Is Optical Field Enhancement the right word to describe the effect? How fast is too fast for photoemission?
• Depending on the application it could be possible to select the best part of the beam after photoemission. Works for UED, for FEL need to preserve the small improvement in beam transport. Acknowledgements
• Pegasus postdoc R. Li, graduate students J. Moody and C. Scoby and the rest of the UCLA PBPL. • C. Vicario, L. Cultrera and SPARC collaborators
• Funding agencies – DOE-HEP – DOE-BES – JTO-ONR