Interpretation of beam current experimental results in HoBiCaT Gun0

Vladimir Volkov

General Experimental Results• Measured spot size of the beam focused on the screen and

the corresponding emittance are several times larger than the predicted ones by dynamic calculations. The measured data spread is very high.

• Dependence of the photo emitted charge on the RF field phase and amplitude can not be perfectly explained by Shottky effect

• Whether the emittance and spot size can be explained by the microstructure of the cathode surface?

• Whether the charge vs. phase dependence can be explained on the base of tunneling effect (Fowler-Nordheim equation)?

HoBiCaT Experimental SetupSolenoid scan emittance measurement method

and Charge vs. RF phase measurement method

Laser wave length 250 nm

Laser spot size in Ø 0.8 mm

Laser Power 1 mW

Laser rms length 2 ps

Bunch charge 1 pC

Emittance measurement results

E, MV/m Bsol,/ T E/ MeV εx,y / μm σx,y,/ μm

20 0.12658 1.809 235 86

18 0.11534 1.597 237 96.8

16 0.10196 1.375 242 98.9

14 0.08841 1.139 247 113

12 0.07223 0.885 259 127

Calculated parameters of the beam focused to the screen. Smooth cathode thermal emittance is 212 μm

Image of the cathode emitting surface.

(Courtesy R.Barday)

Focused screen image has uniform density that indicates the laser uniform density at the cathode

(Courtesy J. Völker )

What is the source of the emittance?

Calculations show: cathode field uniformity due to surface micro profile may be the reason

No solenoid offset. No cathode offset. No steering coil offset. No uniform cathode charge density etc.

(Courtesy R.Barday)

SLANS field modeling of 200 μm blobs and 200 nm knobs randomly distributed along the cathode surface

E, MV/m εx/y , μm σx/y, μm

1 blobs (β=4.2) 20 1.20 883 blobs (β=2) 20 1.07/1.13 122/1347 Knobs(β=5.4) 20 0.349 61/60.114 knobs(β=5.4 20 0.427 72.4/69.73 blobs (β=2) +14 knobs (β=5.4

20 1.41/1.42 98.9/11218 1.46/1.43 113/11516 1.54/1.45 136/14314 1.66/1.50 171/17412 1.81/1.59 230/228

Bunch top view at 2 mm away from cathode

BlobKnobs

Beam current measurement results

10 0 10 20 30 40 50 60 70 80 90 1000

5 10 9

1 10 8

RF Phase/degree

Bea

m c

urre

nt/A

O 20 MV/m∆ 19 MV/m◊ 18 MV/m□ 16 MV/m+ 14 MV/m× 12 MV/m

(Courtesy J. Völker )

FITTING POINTS

• The experiments were made during two days, first day 12 MV/m, then 14, 16, 18, 19, 20 MV/m.

• Beam current dependence is very sensitive to the laser driven locality on the cathode.

Shottky fitting of thermo emission cathodes

10 0 10 20 30 40 50 60 70 80 900

5 10 9

1 10 8

Phase/degree

Cur

rent

/A

O 20 MV/m∆ 19 MV/m◊ 18 MV/m□ 16 MV/m+ 14 MV/m× 12 MV/m

Zero

RF

phas

e

at

I 0=0

E, MV/m A 10∙ 10 B 10∙ 9 Accuracy/%12 -6.142 3.996 3.57714 -5.135 4.841 1.43216 -3.480 4.294 1.72518 -4.253 4.595 1.85519 -4.494 4.999 1.85720 -3.654 4.616 2.216

6 4 2 0 28

6

4

2

0

2.1

2.15

2.2

2.25

2.3

Photo Current/nA

RF

Phas

e/de

gree

Acc

urac

y/%

Accuracy

Phase

Fowler-Nordheim fitting

10 0 10 20 30 40 50 60 70 80 900

5 10 9

1 10 8

Phase/degree

Bea

m c

urre

nt/A

E, MV/m A 10∙ 8/A B, MV/m Accuracy/%12 0.839 2.586 2.26114 1.453 4.133 1.26716 1.562 5.251 1.72818 1.605 5.596 2.09219 1.761 5.632 2.02820 1.789 6.311 2.589

12 14 16 18 200

2

4

6

8

E, MV/m

A 1

0**8

/A, B

/ MV

/m

High Power Processing?

1 0 1 2 31.9

2

2.1

2.2

2.3

Fitting accuracy

Photocurrent/nA

Acc

yrac

y/%

I 0 =0.05nA

0 2 4 6 8 101.9

2

2.1

2.2

2.3

2.4

Fitting accuracy

Initial Phase/deg

Acc

yrac

y/%

Φ=5⁰

O 20 MV/m∆ 19 MV/m◊ 18 MV/m□ 16 MV/m+ 14 MV/m× 12 MV/m

A

B

Zero

RF

phas

e

Possible explanation: A → A1+A2 E∙ 2/φ, where

A1»A2 -if the laser is switched on.

A1=0 -if the laser is switched off

Why B value becomes lower if the laser is switched on?Possibly, the work function (φ) of laser exited electrons becomes lower because according to FN formula

B[MV/m]=6830φ1.5

The perfect fitting of the experimental data by complete FN equation is impossible. The perfect fitting of the experimental data by complete FN equation is impossible.

Conclusion

On the base of the experimental results we can conclude:

•The main reason for beam emittance dilution is the photocathode field imperfection induced by field emitters that change the local electric field.•The beam current experimental data is well fitted by Fawler-Nordheim equation. But additional experiments are required to exclude the time factor during the experiments.