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 Setup Solenoid scan emittance measurement method and Charge vs. RF phase measurement method Laser wave length250 nm Laser spot size in Ø0.8 mm Laser Power1 mW Laser rms length2 ps Bunch charge1 pC

Emittance measurement results E, MV/mB sol,/ TE/ MeVε x,y / μmσ x,y,/ μm 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? 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/mε x/y, μmσ x/y, μm 1 blobs (β=4.2) blobs (β=2)201.07/ /134 7 Knobs(β=5.4) / knobs(β= / blobs (β=2) + 14 knobs (β= / / / / / / / / / /228 Bunch top view at 2 mm away from cathode BlobKnobs

Beam current measurement results 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 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/mA∙10 10 B∙10 9 Accuracy/% Accuracy Phase

Fowler-Nordheim fitting E, MV/mA∙10 8 /AB, MV/mAccuracy/% High Power Processing? I 0 =0.05nA 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 phase

Possible explanation : A → A 1 +A 2 ∙E 2 /φ, where A 1 »A 2 -if the laser is switched on. A 1 =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] =6830φ 1.5 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.