Photoemission Studies of the PITZ Photoinjector: Bunch ...€¦ · ⃰ CST Particle Studio Repeat...

09-07-2014 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen | 1

Photoemission Studies of the PITZ Photoinjector: Bunch Charge Extraction

DESY-TEMF Collaboration Meeting SR 200, Building #24

DESY Hamburg, July 9th 2014

Ye Chen, Erion Gjonaj, Wolfgang Müller, Thomas Weiland TU Darmstadt, TEMF


Contents

Emittance studies (continuations)

− Emittance results at EMSY1: full EM simulations vs. Measurements

Charge extraction studies (new)

− Charge extractions in CST PS (SCL )

− Multiple comparisons: CST PS simulations vs. Measurements vs. Astra simulations

− Influence of laser spot size, laser transmission (LT) and RF field at the cathode

− Discussions: Qmax (laser), LT (SCL), Schottky-like effect

− Further photoemission studies: simultaneous variation of multi-parameters

Conclusions CST Particle Studio

Space Charge Limited


Emittance Studies (continuations)

CST Particle Studio

Repeat all previous simulations up to EMSY1 using CST PS

Comparisons of emittance at EMSY1 between CST PS simulations, DG-FEM simulations, Astra simulations, and the measurement data

Numerical procedures:

− All bunches only tracked up to 3 cm behind cathode in CST PS, then continue tracking with Astra using the bunches obtained in CST PS

CST PS Simulations:


Conclusions: − There is a modeling error in

Astra − Still no explanation for the

systematic shift w.r.t. laser spot size (observed in all simulations)

Probable causes for the shift: − actual laser spot sizes

smaller than reported in the literature

− bunch transverse size generated at the cathode ≠ laser spot size

Emittance Studies (continuations)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.90

0.5

1

1.5

2

2.5

3

Measured, 1 nC Measured, 2 nC Simulated, CST PS, 1 nC Simulated, CST PS, 2 nC Simulated, DG-FEM, 1 nC Simulated, DG-FEM, 2 nC Simulated, Astra, 1 nC Simulated, Astra, 2 nC Simulated, Lienard-Wiechert, 1 nC

1nC

2nC full bunch

extraction at XY_rms = 0.3mm

Same optimum spot size and curve pattern

Laser Spot Size (mm)

ε x (m

m m

rad)


Photoemission Studies: Charge Extraction

Charge extractions in CST PS Comparisons: Simulations vs. Measurements Influence of laser spot size, LT and RF field at the cathode Simultaneous variation of multi-parameters


Charge Extraction : Motivation & Assumptions

Motivation: Astra simulations predict space charge limit at less than 1 nC for XY_rms = 0.3 mm, whereas 1 nC and even higher bunch charges were detected experimentally.

Assumptions

II. Laser produced just the maximum number of particles that can be emitted at the cathode without space charge limitations

I. Laser produced as more particles as one can inject at the cathode

Main Simulation Parameters: Q0: initial total bunch charge, to be injected at cathode Qb: total emitted bunch charge XY_rms = 0.3 mm, flat top, 2.2/21.46\2.2 ps

Q0=arbitrary, Qb = ?

Qb = Q0 = ?

M. Krasilnikov, PITZ: Simulations versus Experiment, 2013 M-S: Measurement-Simulation

Total Bunch Charge vs. Gun Phase-MMMG

∆(M-S)

(total charge calculation)

Simulations based on assumption Ι Q0 = arbitrary, Qb = ?

Example: Q0 = 2 nC, 1.2 nC, 1 nC XY_rms = 0.3 mm


Charge Extraction — CST PS Simulations (assumption Ι) (1)

XY_rms = 0.3 mm, Q0 = 2 nC

0 5 10 15 20 25 30 350

0.5

1

1.5

2

t (ps)

Qb

(nC

)

Qb1, 10µmQb2, 5µmQb3, 2µmQb4, 0.5µmQb5=1.2nC, 0.25µm

Convergence Tendency

Non-convergent

Parameters in legend: total emitted charge / mesh resolution

Qb ≈ 1.2 nC


XY_rms = 0.3 mm, Q0 = 1.2 nC

0 5 10 15 20 25 30 350

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

t (ps)

Qb

(nC)

Qb1=1.1998nC, 10µmQb2, 5µmQb3, 1µmQb4, 0.5µmQb5, 0.25µmQb6=1.01nC, 0.15µm

Convergence Tendency

Qb ≈ 1.01 nC

Non-convergent

Parameters in legend: total emitted charge / mesh resolution



XY_rms = 0.3 mm, Q0 = 1 nC

• No numerical convergence unless the initial charge assumed to be close to the space charge limit

Parameters in legend: total emitted charge / mesh resolution /simulation tool

0 0.25 0.5 0.75 100.20.40.60.8

11.21.41.61.6

Qb

(nC)

z (mm)

Qb1 = 1 nC, dz = 20 μm, CST PS Qb2 = 1 nC, dz = 10 μm, CST PS Qb3 = 1 nC, dz = 5 μm, CST PS Qb < 1 nC, Astra

Convergent

Qb = 1nC



• Simulation Scheme

Assumption ΙΙ: Qb = Q0 = ?

Q0: initial total bunch charge, to be injected at the cathode Qb: total emitted bunch charge Charge scanning range: (0-2) nC Charge resolution: ~ 50 pC Laser spot size: XY_rms = 0.3 mm Temporal Profile: flat top, 2.2/21.46\2.2 ps

for comparison purpose

the injected charge at cathode is fully extracted

Main idea: calculate the maximum injected bunch charge which can be fully extracted (SPCH-Limited) using CST PS

Charge Extraction : Main Idea for Assumption ΙΙ

Q0 = Q0 – N × ∆Q

Generate Initial Particle Distribution in Astra-Generator

Loading Particle Distribution into Particle Import Interface in CST PS

Bunch Emission in CST PS

Particle Lost? Y

N = N

+ 1

Qb = Q0

N

astra2cst code

Q0 = 2 nC, N = 0


Charge Extraction — CST PS Simulation vs. Astra Simulation (1)

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.5

1

1.5

2

XY_rms = 0.3 mm LT = 100%

Qb (

nC)

Gun Phase-MMMG (deg)

Simulated, CST PS, z = 2 mm, XY_rms = 0.3 mm, LT = 100%

Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 100% Simulated, Astra, ICT1, XY_rms = 0.3 mm, LT = 100%

Ez at the cathode arbitrary units

Most Particles lost on beam tube but

not at the cathode!

Charge at z = 2 mm

Resolution: ∆Q ≈ 50 pC

XY_rms: rms laser spot size LT: laser transmission


Charge Extraction — CST PS Simulation vs. Measurement (1)

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.5

1

1.5

2

XY_rms = 0.3 mm LT = 100%

Qb (

nC)

Measured , ICT1, XY_rms = 0.3 mm, LT = 100% Simulated, CST PS, z = 2 mm, XY_rms = 0.3 mm, LT = 100%

Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 100% Ez at the cathode arbitrary units

LT = 100%


Charge at z = 2 mm

Most Particles lost on beam tube but

not at the cathode!

Gun Phase-MMMG (deg) J.Li et al, EMISSION STUDIES OF PHOTOCATHODE RF GUN AT PITZ, 2012


Charge Extraction — Discussion (1): Schottky-like effect

With Schottky-like effect

J.Li et al, EMISSION STUDIES OF PHOTOCATHODE RF GUN AT PITZ, 2012

Without Schottky-like effect

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.5

1

1.5

2

Q

b (nC

)


XY_rms = 0.3 mm


Measured, ICT1, LT = 100% Simulated, CST PS, ICT1, LT = 100%

Total emitted bunch charge simulated with CST PS without Schottky-like effect can be very close to the measured total bunch charge.

To explain the M-S difference in produced total bunch charge, Schottky-like effect probably not very important??


Charge extractions in CST PS Comparisons: Simulations vs. Measurements Influence of laser spot size, LT and RF field Simultaneous variation of multi-parameters

Photoemission Studies: Charge Extraction


Charge Extraction — CST PS Simulation vs. Astra Simulation (2)

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.5

1

1.5

2

+ Laser Spot Size XY_rms = 0.35 mm

LT = 100%


Qb (

nC)


Simulated, CST PS, ICT1, XY_rms = 0.35 mm, LT = 100% Simulated, Astra, ICT1, XY_rms = 0.35 mm, LT = 100%

missing corresponding measurement data


Charge Extraction — Assumption for Lower-LT (<100%) case

LT

Qmax Assumption

SPCH Limited

M. Krasilnikov, ICFA Workshop on Future Light Sources, 2012

Q0 (nC) LT (%) 0 15 30 40 55 70 80 100

very close to the saturation level in measurements

Measurement data for XY_rms = 0.3 mm, MMMG Phase, various LTs

Gun Phase

Qb LT-1 LT-2 LT-3 LT-4

0

Laser produced Qmax = ? LT (spch limit) = ?

∆(M-S)

Qb ≤ Qmax

SPCH Limited

CST PS



Measured, ICT1, XY_rms = 0.3 mm, LT = 100% Measured , ICT1, XY_rms = 0.3 mm, LT = 62%

Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 100% Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 62%

+ Laser Transmission LT = 62%

XY_rms = 0.3 mm

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.5

1

1.5

2


Qb (

nC)

Gun Phase-MMMG (deg) J.Li et al, EMISSION STUDIES OF PHOTOCATHODE RF GUN AT PITZ, 2012

Source Limited

SPCH Limited



+ RF field, Ecath = 60, 45 (MV/m) XY_rms = 0.3 mm, LT = 100%

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

22

Measured, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 60 MV/m Simulated, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 60 MV/m Simulated, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 45 MV/m

Qb (

nC)



Ongoing calculations for XY_rms = 0.35 mm


Charge Extraction — CST PS Simulation vs. Measurement (4) + RF field and LT Ecath = 45 MV/m

LT = 14% XY_rms = 0.3 mm

J.Li et al, EMISSION STUDIES OF PHOTOCATHODE RF GUN AT PITZ, 2012

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

22

Measured, ICT1, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 60 MV/m Measured , XY_rms = 0.3 mm, LT = 14%, Ecath ≈ 45 MV/m

Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 60 MV/m Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 100%, Ecath ≈ 45 MV/m

Simulated, CST PS, ICT1, XY_rms = 0.3 mm, LT = 14%, Ecath ≈ 45 MV/m

Qb (

nC)



SPCH Limited

Source Limited


Charge Extraction — Test Results with Assumption Ι: RF Field Impact

Preliminary results − RF field impacts for two cases: XY_rms = 0.3 mm and

0.35 mm by applying different voltages

− Tests based on assumption Ι: no numerical convergence

− Calculations with assumption ΙΙ still in progress

• Relative changes in total bunch charge showed stronger RF field impact for XY_rms = 0.3 mm (testing).

-40 -20 0 20 40 60 80 1000.5

0.7

0.9

1.1

1.3

1.51.5

60 MV/m

45 MV/m Ecath ≈ 60 MV/m Ecath ≈ 45 MV/m

XY_rms = 0.3 mm fixed Q0, z = 2 mm

-40 -20 0 20 40 60 80 1000.5

0.7

0.9

1.1

1.3

1.51.5

Ecath ≈ 60 MV/m Ecath ≈ 45 MV/m

XY_rms = 0.35 mm fixed Q0, z = 2 mm

60 MV/m

45 MV/m

Qb (nC) vs. Gun Phase-MMMG (deg)

61 60 58 56 54 52 50 48 46 441.3

1.32

1.34

1.36

1.38

1.4

1.42

1.44

1.46

XY_rms = 0.35 mm XY_rms = 0.3 mm

Qb (nC) vs. Ecath (MV/m)

XY_rms = 0.3 & 0.35 mm fixed Q0 and gun phase

XY_rms = 0.35 mm

XY_rms = 0.3 mm


Further Photoemission Studies (ongoing) Simultaneous Variation of Multi-Parameters RF power + laser spot size + laser pulse energy

# σxy /mm LT Prf, gun/MW Prf, gun × σxy

1 0.302 57% 6.49 0.769

2 0.312 52.6% 5.99 0.764

3 0.327 48.2% 5.45 0.763

4 0.341 43.8% 5.00 0.762

5 0.361 39.5% 4.55 0.770

6 0.382 35.1% 3.99 0.762

Measurements by Simultaneous Variation of Multi-Parameters

Measurements


Qb (

nC)

Keeping Ecath·LaserSpotSize = const

M. Krasilnikov, Simulations at PITZ, DESY 2012 LT was tuned to keep laser pulse energy constant

-100 -80 -60 -40 -20 0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

22

Measured , XY_rms = 0.302 mm, LT = 57%, Prf,gun ≈ 6.49 MW Simulated, XY_rms = 0.302 mm, LT = 100%, Ecath ≈ 64 MV/m Simulated, XY_rms = 0.302 mm, LT = 57%, Ecath ≈ 64 MV/m

Qb (

nC)


In progress Resolution: ∆Q ≈ 50 pC

Case 1

Q = 𝝅𝑹𝟐𝜺𝟎𝑬0 𝒔𝒔𝒔𝝋𝟎


Further Photoemission Studies (ongoing)

Testing: Qb = 𝝅𝑹𝟐𝜺𝟎𝑬0 𝒔𝒔𝒔𝝋𝟎 = 𝝅𝑹𝟐𝜺𝟎𝑬cath

Fitting Result:

• R = 0.0009365 m (0.0009306, 0.0009424)

− R-square: 0.9959

0 10 20 30 40 50 60 700

0.5

1

1.5

measurements fitted

Qb (

nC)

Ecath (MV/m)

XY_rms = 0.3 mm LT = 100%

Gun phase: (-40deg ~ +50deg)

Effective Radius: R = 0.9365 mm σxy = 0.46825 mm > XY_rms = 0.3 mm


Conclusions

Emittance studies (continuations) − There is a modeling error in Astra.

− Still no explanation for the systematic shift w.r.t. laser spot size (observed in all type of simulations).

Photoemission studies: bunch charge extraction (new) − Total emitted bunch charge simulated with CST PS fits the measurement data well at

XY_rms = 0.3 mm for different gun phases.

− M-S comparisons for lower laser transmissions and different RF fields showed good agreements.

− Schottky-like effect may not be very important for explaining the M-S discrepancy in produced bunch charge.

− Preliminary tests showed stronger RF impact for higher space charge density.

Next steps: − Interpolation of laser transmission and maximum charge produced by the laser to space

charge limits?

− Further photoemission studies: simultaneous variation of multi-parameters.

M-S: Measurement and Simulation


Thank you for your attention!

Photoemission Studies of the PITZ Photoinjector: Bunch ...€¦ · ⃰ CST Particle Studio Repeat...

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