interpretation of beam current experimental results in hobicat gun0 vladimir volkov
TRANSCRIPT
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.