# Presentation: 160755

## Interesting Problems that we Can’t Do Yet...

*Presented at the 2004 Advanced Accelerator Concepts workshop*
*John W. Lewellen*

## Murphy’s Law in Context:

**Any really new idea is bound to break at least one simulation code.  Why?  Because it’d be a feature of some code, somewhere, if someone thought of it already.**
**Lewellen’s Corollary:  You will probably break at least one simulation anyway, just trying to do something old, only better.**

## Outline

**Structure Scales and What Matters**
**High-power design needs**
**Ultra-low emittance design needs**
**Next-generation cathode simulation needs**
**Wrap-up**

## Structure Scales

**Consider a canonical S-band 1.6-cell photoinjector (********=10.25 cm)**
**	****- one cathode cell, length 0.6 * 1⁄2 ******** 3 cm**
**	****- one full cell, length 1⁄2 ******** 5 cm**
**	****- radius ~ 4 cm**

- z

- r

- “Area” in z-r space ~ 33 cm2
- “Volume” in z-r- space ~ 400 cm3

- 

## Two Beam Scales of Interest

- _“__Canonical” S-band Gun Beam_
- n:  1 m
- r’ ~ 1 mrad
- r ~ 1 mm
- z ~ 3 mm (10 ps)
- Aspect ratio ~ 1:1
- “Area” in z-r space:  0.03 cm2
- “Volume” in z-r- space: 0.01 cm3

- _“__RF Microscope” Gun Beam_
- n:  1 nm
- r’ ~ 1 mrad
- r ~ 1 m
- z ~ 3 mm (10 ps)
- Aspect ratio:  ~ 1000:1
- “Area” in z-r space:  3·10-4 cm2
- “Volume” in z-r- space:  9·10-7 cm3

## So what...?

**A uniform-grid PIC code is not a good idea, especially for the microscope beam**
**The “microscope” beam will also be more susceptible to field-related problems, e.g. head-tail kicks:**

- 1 mm

- 1 mrad

- 1 m

- 1 mrad

-  ~ 10-3

-  ~ 2

## New Structures Pose New Problems

**SRF Photoinjector with Cathode-Region Focusing**
- Development at Rossendorf (“Drossel Collaboration”)
- Further studies & design done at AES

- Multipactoring is an immediate concern with a cathode-stalk design

## What matters for all designs?

- Everything matters more as beams become more extreme
- Multi-length scale, multi-time scale codes are already necessary
- Some codes already support multi-frequency, multi-field overlays
- Methods for better handling wakefields, surface effects, etc., are required
- Photoemission process modeling will require true multiphysics approach, as well as raw data for the models
- “Integrated” codes that can also do, e.g. mutlipactoring, field emission, etc., are also needed.

## High-Power Design Needs

**Cavity-beam power exchange**
- transient build-up
- wakefields & beam break-up
**Cathode-region charge flow (for surface heating modeling)**
**Multipactoring / Arc Modeling / Field Emission**
**Halo Formation & Tracking**

## “Nano-Beam” Design Needs

**Large-aspect-ratio beam space-charge models**
**Very precise knowledge of cavity fields**
**Detailed halo formation**
**Field emission / arcing / multipactoring**
**Detailed cathode emission model**

## Cathode Emission Models Needed For...

**Ultra-Low Emittance Beam**
- Needle tip emission modeling, field effect modeling, etc.
- Very fine-scale surface roughness modeling
**ANL Cathode-Region Focusing**
- requires modifications to image-charge handling
**BNL Super-QE Cathode Design**
- needs to be a materials science code as well as an injector code
- multiple stages of electron emission, beam strike, secondary emission, etc.
**Thermal emittance prediction and inclusion, especially!**

## Beam-Strike Effects

**Inputs:**
- Beam power:  9 kW
- Spot size:  100 m
- Translation rate:  2 m/sec
- Penetration depth:  0.15 mm
- Material:  6063 Aluminum
**	******** **286 GW / m2

- 4 mm

- 2 mm

- 1 mm

## Wrap-Up

**Simple scaling suggests modeling injectors is hard**
**Existing injector codes work with today’s beams, but...**
- “microscope” beams are very different in a number of ways
- high-power beams require integrated multipactoring, wake analysis, etc.
- all need better emission modeling
**More data / experiments are needed for cathode model data**
**Some of the implications are very interesting!**