Measurements and modeling of space charge assisted photoemission at - - PowerPoint PPT Presentation

Measurements and modeling

• f space charge assisted photoemission at PITZ

M.Krasilnikov for the PITZ team Experimental optimization of the PITZ photo injector for a nC bunch charge level resulted in machine parameters corresponding to a space charge assisted photo emission from the Cs2Te

• cathode. Several additional dedicated emission studies have been performed in order to study the

charge production as a function of photo injector parameters like rf peak power in the gun, laser spot size at the cathode and laser pulse energy. Results of these studies will be discussed.

Photocathode Physics for Photoinjectors (P3) Workshop - 2012

Photo Injector Test facility at DESY, Zeuthen site

<7 MeV <25 MeV

The Photo Injector Test facility at DESY in Zeuthen (PITZ) focuses on the development, test and optimization of high brightness electron sources for superconducting linac driven FELs:  test-bed for FEL injectors: FLASH, the European XFEL  small etr  stable production of short bunches with small sE  further studies  e.g. cathodes: dark current, photoemission, QE, thermal emittance, …

+ detailed comparison with simulations = benchmarking for the PI physics

PITZ RF gun and photo cathode laser

RFgun: L-band (1.3 GHz) nc (copper) standing wave 1½-cell cavity Peak rf power: up to 7MW Ez@cathode: > 60MV/m Photo cathode (Cs2Te) QE~0.5-5% Cathode laser 257nm ~20ps (FWHM)

FWHM = 25 ps

edge10-90 ~ 2.2 ps edge10-90 ~ 2 ps

birefringent shaper, 13 crystals

OSS signal (UV)

Temporal pulse shaper

Flattop

Emission studies: motivation

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.20 0.25 0.30 0.35 0.40 0.45 0.50 xy-emittance (mm mrad) rms laser spot size (mm)

1nC, meas.(0deg) 1nC, meas.(6deg) 1nC, simulated 1nC, simul. (Ek=4eV)

Cs2Te: EG=3.3eV EA=Evac-EG=0.2eV ET=EG+EA=3.5eV Ek=Eph-ET=4.05eV-ET=0.55eV

• R. A. Powel et. al.

Photoemission Studies

• f Cesium Telluride.
• Phys. Rev. B, 8:

3987–3995, 1973.

?Field enhancement?

6.65 6.67 6.69 6.71 6.73 6.75 6.77 6.79 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

• 60
• 40
• 20

20 40 60 80 100 <Pz> (MeV/c) bunch charge (nC) gun phase-MMMG (deg)

charge (0.3mm, LT=62%) charge (0.3mm, LT=100%) Ez at cathode, arb.units simulated charge (XYrms=0.4mm, Qb=1nC) simulated charge (XYrms=0.3mm, Qb=1nC) measured mean momentum <Pz>

Emission studies: modeling

D.Dowell, J.Schmerge “Quantum efficiency and thermal emittance of metal photocathodes”, PRST-AB 12, 074201 (2009)

𝑅𝐹 ≈ 1 − 𝑆(𝜕) 1 + 𝜇𝑝𝑞𝑢(𝜕) 𝜇 𝑓−𝑓(𝜕) ∙ ℏ𝜕 − 𝜚𝑓𝑔𝑔

2

8𝜚𝑓𝑔𝑔 𝐹𝐺 + 𝜚𝑋 , where the effective work function (Schottky term): 𝜚𝑓𝑔𝑔 = 𝜚𝑋 − 𝑓 𝑓𝛾𝐹 4𝜌𝜁0

The emitted charge:

𝑅 = 1 − 𝑆(𝜕) 1 + 𝜇𝑝𝑞𝑢(𝜕) 𝜇 𝑓−𝑓(𝜕) ∙ 𝑂𝛿 8𝜚𝑓𝑔𝑔 𝐹𝐺 + 𝜚𝑋 ℏ𝜕 − 𝜚𝑋 + 𝑓 𝑓𝛾𝐹 4𝜌𝜁0

2

D.Dowell, PAC 2011 Tutorial  Derivation of Schottky scan function: emitted charge vs. launch phase 2-parameter fit

𝑅 ∝ 𝜃 ∙ 𝑀𝑈 ∙ 1 + 𝑐 𝐹

𝑛

𝑀𝑈 = laser transmission (%) 𝐹 – field at the cathode (MV/m) 𝜃, 𝑐, 𝑛 – fitting parameters

Emission studies: modeling  RF field influence (LT=25%) 𝑅 ∝ 𝜃 ∙ 𝑀𝑈 ∙ 1 + 𝑐 𝐹

𝑛

𝑀𝑈 = laser transmission (%) 𝐹 – field at the cathode (MV/m) 𝜃, 𝑐, 𝑛 – fitting parameters 𝑀𝑈 = 𝑀𝑈0 = 25% (1nC at MMMG phase for 6MW)

RF power (MW) Ecath (MV/m) max <Pz> (MeV/c) 6.02 62.0 6.83 3.54 47.6 5.43

Fitting: Phase range: 1070deg 𝐹= 𝐹𝑑𝑏𝑢ℎ ∙ 𝑡𝑗𝑜𝜒0 h=1.2148E-5 b=10.9222 m=1.8705 (1.8977-2.1081)2 +convolution with lase laser temporal pr profile le Measurements: Laser:

• Temporal  flattop 2/20\2ps
• Transverse  0.3 mm rms

Main solenoid: 400A Charge measured by LOW.ICT1  z=0.9m

0.2 0.4 0.6 0.8 1 1.2 1.4

• 20

20 40 60 80 100 120 140 bunch charge (nC) launch phase (deg) meas.(LT=25%,3.5MW) fitted (LT=25%,3.5MW) meas.(LT=25%,6MW) fitted (LT=25%,6MW)

Emission studies: modeling  RF field

𝑅 ∝ 𝜃 ∙ 𝑀𝑈 ∙ 1 + 𝑐 𝐹

𝑛

Simultaneous fitting (LT=13% and 25%): Phase range: 1070deg 𝐹= 𝐹𝑑𝑏𝑢ℎ ∙ 𝑡𝑗𝑜𝜒0 h=8.44E-8 b=205.9 m=1.805

LT=13% LT=25% LT=100%

• Simultaneous fitting  assumptions are not correct?
• Almost no RF impact for low SC density
• RF field impact increases with SC density increase

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

• 20

20 40 60 80 100 120 140 bunch charge (nC) launch phase (deg) meas.(LT=13%,3.5MW) fitted (LT=13%,3.5MW) meas.(LT=13%,6MW) fitted (LT=13%,6MW) 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

• 20

20 40 60 80 100 120 140 bunch charge (nC) launch phase (deg) meas.(LT=25%,3.5MW) fitted (LT=25%,3.5MW) meas.(LT=25%,6MW) fitted (LT=25%,6MW) 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

• 20

20 40 60 80 100 120 140 bunch charge (nC) launch phase (deg) meas.(LT=100%,3.5MW) fitted (LT=100%,3.5MW) meas.(LT=100%,6MW) fitted (LT=100%,6MW)

Measured cathode laser shapes

Emission studies: LT scans and ASTRA simulations

0.2 0.4 0.6 0.8 1

• 12-10 -8 -6 -4 -2 0

2 4 6 8 10 12 laser intensity (arb.units) t (ps)

Temporal profile

OSS measurement FT-fit (2.18/19.88\2.43ps) SG-fit(2.34/19.74\2.34ps)

• 0.8
• 0.6
• 0.4
• 0.2

0.2 0.4 0.6 0.8

• 0.8
• 0.6
• 0.4
• 0.2

0.2 0.4 0.6 0.8

x (mm) y (mm)

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Transverse halo modeling in ASTRA

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 bunch charge (nC) ~ laser energy (arb.units)

Charge vs. laser pulse energy

measured(MMMG phase) simulated (ideal transv. flattop) simulated (+ transv.halo)

𝑅 = 𝑅0 + 𝑇𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧 ∙ 𝐹 + 𝑀𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧 ∙ 𝐹

ASTRA simulations: Schottky effect implementation

𝑅 ∝ 𝜃 ∙ 𝑀𝑈 ∙ 1 + 𝑐 𝐹

2

ASTRA: charge of a particle at the time of its emission:

ASTRA input: 𝑅𝑐𝑣𝑜𝑑ℎ, 𝑇𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧  2-parameter fitting 𝑀𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧 = 𝑇𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧

2

𝑅𝑐𝑣𝑜𝑑ℎ 𝑀𝑈 = 𝜊 ∙ 𝑀𝑈0 𝑅𝑐𝑣𝑜𝑑ℎ = 𝜊 ∙ 𝑅𝑐𝑣𝑜𝑑ℎ0 𝑇𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧 = 𝜊 ∙ 𝑇𝑇𝑑ℎ𝑝𝑢𝑢𝑙𝑧0

Schottky constants should be scaled with laser pulse energy 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

• 60
• 40
• 20

20 40 60 80 100 bunch charge (nC) launch phase-MMMG (deg)

measured (LT=25%) measured (LT=100%) simul.(Qb=1.2nC, no Schottky) simul.(Qb=4.8nC, no Schottky)

No Schottky effect applied Schottky parameter fitting

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

• 60
• 40
• 20

20 40 60 80 100 bunch charge (nC) launch phase-MMMG (deg)

measured (LT=25%) measured (LT=100%) simul.(Qb=0.5nC, +Schottky) simul.(Qb=2nC, +Schottky) Qbunch= 0.5nC L_Schottky= 0.0059983 S_Schottky= 0.109529

𝜊 = 100% 25% = 4

10 20 30 40 50 60

• 5
• 4
• 3
• 2
• 1

1 2 3 4 5 beam current (A) z-<z> (mm)

Bunch current profile after emission (z=0.1m)

Qbunch=1.2nC, no Schottky Qbunch=0.5nC, +Schottky

ASTRA simulations: Schottky effect impact

Applied Schottky effect  more smooth charge extraction

0.2 0.4 0.6 0.8 1 1.2 1.4

• 60
• 40
• 20

20 40 60 80 100 bunch charge (nC) launch phase-MMMG (deg) measured (LT=25%) simul.(Qb=0.5nC, +Schottky) simul.(Qb=1.2nC, no Schottky)

Further emission studies: + laser spot size and pulse energy variation

200 400 600 800 1000 1200 1400 1600 1800 2000

• 20 0

20 40 60 80 100120140 Bunch charge, pC gun (ASTRA) phase, deg Q(0.7nJ;6MW;1.2mm) Q(0.7nJ;3.5MW;1.6mm) 200 400 600 800 1000 1200 1400 1600 1800 2000

• 20 0 20 40 60 80 100120140

Bunch charge, pC gun (ASTRA) phase, deg Q(1.4nJ;6MW;1.2mm) Q(1.4nJ;3.5MW;1.6mm) 200 400 600 800 1000 1200 1400 1600 1800 2000

• 20 0

20 40 60 80 100120140 Bunch charge, pC gun (ASTRA) phase, deg

Q(6.1nJ;6MW;1.2mm) Q(6.1nJ;3.5MW;1.6mm)

𝑄𝑠𝑔 ∙ 𝑀𝑏𝑡𝑓𝑠𝑇𝑞𝑝𝑢𝐸𝑗𝑏𝑛𝑓𝑢𝑓𝑠 = 6 ∙ 1.2 = 3.5 ∙ 1.6 = 3 = 𝑗𝑜𝑤? 𝐹catℎ ∙ 𝜏𝑦

𝑚 ∙ 𝜏𝑧 𝑚 = 𝑗𝑜𝑤?

Further emission studies: Ecath·LaserSpotSize=const

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

• 60
• 40
• 20

20 40 60 80 100 bunch charge (pC) launch phase -MMMG (deg) 302um, 6.5MW; 57% 312um, 6.0MW, 52.6% 327um, 5.4MW, 48.2% 327um, 5.0MW, 43.8% 327um, 4.6MW, 39.5% 381um, 4.0MW, 35.1%

Parameters in legend: (𝜏 ,

𝑦𝑧 𝑚𝑏𝑡𝑓𝑠 𝑄 𝑠𝑔,𝑕𝑣𝑜, LT)

𝜏 =

𝑦𝑧 𝑚𝑏𝑡𝑓𝑠

𝜏𝑦 ∙ 𝜏𝑧 - rms spot size of the cathode laser 𝑄𝑠𝑔,𝑕𝑣𝑜 - peak rf power in the gun cavity LT – laser transmission was always tuned to keep laser pulse energy constant

# 𝑸𝒔𝒈,𝒉𝒗𝒐, MW 𝝉 ,

𝒚𝒛 𝒎𝒃𝒕𝒇𝒔

mm LT, %

𝑸𝒔𝒈,𝒉𝒗𝒐 ∙ 𝝉𝒚𝒛

𝒎𝒃𝒕𝒇𝒔 1 6.49 0.302 57.0 0.769 2 5.99 0.312 52.6 0.764 3 5.45 0.327 48.2 0.763 4 5.00 0.341 43.8 0.762 5 4.55 0.361 39.5 0.770 6 3.99 0.382 35.1 0.762 D= 48%

• 24%

STDEV=0.49%

Simultaneous variation of the rf field and the space charge density at the cathode by keeping the laser pulse energy and 𝑭𝒅𝒃𝒖𝒊𝟏 ∙ 𝝉𝒚𝒛

𝒎𝒃𝒕𝒇𝒔

constant yields very similar extracted bunch charge for a rather wide range of the launch phase.

Conclusions

> Studies of the space charge assisted photoemission at PITZ:

• L-band, Cs2Te, Ecath0>60MV/m
• Basic measurement = launch phase scan for a bunch charge
• Experimental optimum (w.r.t. beam emittance) conditions  space charge assisted

emission

• Simulated conditions ≠ experimental
• Schottky-like effect is stronger pronounced for higher space charge densities
• Simple (simultaneous) fitting of the macroscopic Schottky model does not work
• ASTRA simulations of the phase scans:
• Cathode laser halo implementation  rather small effect
• Simultaneous simulations of different machine conditions are hard and still delivering

generally smaller charges than experimentally obtained

• Applied Schottky-like effect resulted in a more smooth charge extraction
• Further experimental photoemission studies:

𝑭𝒅𝒃𝒖𝒊𝟏 ∗𝝉𝒚𝒛

𝒎𝒃𝒕𝒇𝒔

~inv?

• Several other measurements have been taken (e.g. Gaussian vs. flattop laser pulses)

have been done, treatment is ongoing