FELSI Meeting 3. February 2009

Tuesday 3 Feb 2009, 14:00 → 15:20 Europe/Zurich

WBGB 019 (PSI)

1 Alexander Anghel: Energy and angular distribution functions for thermal emittance simulation

Selected questions and discussion TG: Do you assume normal incidence of the photons? AAng: Yes, I assume normal incidence. FLP: What's the difference? TG: All the momentum conservation rules change. SR: Why is the angle distribution uniform? For a 2d emission process, I would expect higher angles to be more probable. AAng: It follows from the uniform density of states. The angle is determined by the energy, and the energy is uniformly distributed. TG: You consider simple metal photocathodes, but what about semiconductors? AAng: You get a similar picture, but you have to consider the negative electron affinity, which results in an increas of the height of the potential barrier. RG: Do you take into account electron-phonon interactions? AAng: No, it has no influence on the distributions, only on the quantum efficiency. By the way, in your case (single-tip, photo-assisted electron emission) it's not a tunneling effect, it's an optical pumping effect. The electrons can tunnel through the potential barrier in addition. This results in a strong Schottky effect, leading to drastic changes in the distribution functions. SR: You assume a hard edge for the Fermi energy, do you not include temperature? AAng: The assumption of uniform density of state is not a good one for some metals (s? d?); also the assumption of zero temparature should be questioned. FLP: Can't you just shift the Fermi level? AAng: No, you cannot solve it by shifting the Fermi level, because the distribution function changes. Shifting the Fermi level would give an extremely crude approximation.

Slides Presentation1.ppt

2 Yujong Kim: Impact of thermal emittance & nonlinear space charge force on the slice and proj. emittances

FLP: Comment on QE measurement shown on page 3: the second measurement was performed about one month later, not four days as written! TS: The fits are quite bad at low values. YK: It's not a fit, the curves are adjusted to the data by eye. RG: In your formula for thermal emittance, you assume a round beam, but at OBLA, we never really had a round beam, at best it was Gaussian-like. YK: You are right, our beam was not round. It's an assumption. FLP, TS (on p. 6): You choose a laser spot size of 58.2 μm -- why so small, and why so precise? YK: The laser spot size is the result of an optimization taking into account the peak current in the end. FLP: In your optimization the only criterion is slice emittance? YK: No, I simultaneously consider projected, slice emittance, matching between slices and the beta function must all be when optimizing the machine. FLP: You just seem to adapt the laser spot size for a given charge to obtain the best result... RG: The 3D charge density should be roughly the same. The laser spot size is adjusted to achieve the same charge density. AAng: What is the initial distribution used by ASTRA? YK: Longitudinal flat-top, radially uniform, energy uniform. AAng: But this will give you zero thermal emittance!? YK: No, ASTRA adds thermal emittance. AA: Actually, this will be covered in my presentation! TG: By picking a certain K-value, you choose your thermal emittance at will, no? YK: We could use the measured value for K, I use a conservative over-estimate for my simulations. FLP: You change the laser pulse length in your simulation, but can we change it in the injector? CH, RG: Yes, easily. FLP: If we go to very small laser spot, we may enter the ablation regime, and all your calculations will be wrong... YK: We need only small laser power; the laser experts think it's feasible. AO: Has anyone thought about how to measure emittances below 0.1 μm?

Slides Yujong.pdf Yujong.ppt

3 Andreas Adelmann: Implementing thermal emittance in OPAL

Selected questions and discussion AAng: Nobody knows what energy scale is used in ASTRA. When you put numerical values, you get crazy numbers! AA: Yes, it is a mess. TG: The ASTRA implementation seems very similar to what Alexander has shown... AAng: No, the maximum emission angle is not energy-dependent in the ASTRA case!

Slides thermEmit-opal.pdf

4 Discussion

It is decided that AA will first implement the ASTRA model in OPAL, which will allow a comparison between the two codes. AA says he still has to sort out a few question that exist with the ASTRA approach. He is in contact with Floettmann for that. At a later stage, he will implement AAng's more sophisticated emission model. Further complications such as surface roughness will be left to still later studies. BB suggests generate a bunch with AAng's model, then track it with ASTRA, to see the impact of AAng's model. SR: Is the Schottky noise included in ASTRA? AA: This depends on the injection phase and this is not taken into account in ASTRA (only the average gradient). OPAL of course knows about the exact phase of every particle so there it is no problem! FLP asks AAng to look into the ratio of photo-emitted electrons to tunneling electrons. Is the tunneling effect negligible or not? AA is looking forward to an experimental verification (low-charge operation) of the thermal emittance he is going to simulate with OPAL.... Some discussion concerning the equivalence of Schottky effect to field acceleration of electrons inside the cathode.