# Presentation: 626725

## Directions for Electron Beam Injector Development

*John W. Lewellen*
*Accelerator Systems Division*
*Advanced Photon Source*

## Acknowledgements

**Court Bohn (NIU)**
**Charles Brau (Vanderbilt)**
**Jym Clendennin (SLAC)**
**Dave Dowell (SLAC/LCLS)**
**Klaus Fl****ö****ttmann (DESY)**
**Fred Kiewit (Eindhoven)**
**Yuelin Li (ANL)**
**Kwang-Je Kim (ANL) **

**John Noonan (ANL)**
**John Petillo (SAIC)**
**John Power (ANL)**
**John Schmerge (SLAC)**
**Charles Sinclair (Cornell)**
**Frank Stephan (DESY-PITZ)**
**Todd Smith (Stanford)**
**Alan Todd (AES)**

**and many others for interesting discussions on gun design & improvement**

**Katherine Harkay (ANL) – ongoing discussions & support **

## Talk Overview

- What is a high-brightness electron gun, in context?
- What I will ignore
- Areas of interest for new source development
- Linac-based light sources
*X-ray free-electron lasers (X-FELs)*
*Storage-ring replacements (SRRs)*
- IR and UV free-electron lasers
- Electron microscopes
- Common elements
- Ongoing injector development efforts
- Desktop Accelerator Development
- Conclusions & Wrap-Up

## RF Cavity – Accelerator Building Block

## Particle Acceleration

## What is an RF Photoinjector?

- Caution:  laser radiation
- Heartburn may result from use

## RF Gun Beam Dynamics

- Launch phase ~ 35°
- Gradient ~ 120 MV/m
- No space charge included
- No solenoid focusing included

## Why an RF gun?  (Or:  Why not DC guns?)

**Desired beam properties**
- high bunch charge
- good transverse quality
- short bunches
**High bunch charge ******** strong space charge**
**Strong space charge ******** poor beam quality**
**Higher gradients ******** more rapid acceleration ******** less time for space charge to act**
**RF guns are very good at providing high gradients ... at a price.**

## What is emittance?

***What is emittance?***

- Transverse beam quality
- ~ product of spot size and divergence
- Phase space area
- Smaller is better

- x

- x’

- Actual distribution

- RMS fit

## What is brightness?  What’s High-Brightness?

**One canonical definition:**
**Another definition:**
**The actual characteristics of a beam, relative to those which are of interest for the task we wish to perform with the beam**
**In useful terms, brightness is situational.**

## Important but ignored (by this talk)

**Drive laser development efforts**
**High-brightness beam diagnostics (e.g. emittance measurement)**
**Operational reliability – transition from laboratory curiosity to facility keystone**
- service & maintenance features
- mean time between failures
- soft vs. hard failure modes
- etc...

## Directions for Injector Development

- “Big Iron” accelerators
- Linear colliders
- Next-generation x-ray light sources
- “Desktop” accelerators
- Electron microscopy
- Electron beam lithography
- Small laboratory experiments
- “Mini-Me” accelerators
- Radioisotope generation
- High-power free-electron lasers
- Slow positron production
- Pulse radiography

- Courtesy SLAC/LCLS

## Storage-Ring Replacements & X-FELs

- “1st-generation” Linac Based Light Sources

**Storage-Ring**
**Replacement**
**(SRR):**
**Higher Peak Brightness**
**Improved Temporal**
**Resolution**

**X-ray Free Electron**
**Laser (X-FEL):**
**Coherence**
**Temporal Resolution**

## Linac-Based Light Sources – Source Specs

**Single-bunch requirements**
- 0.1 m normalized transverse emittance
- 0.1 nC
- 500 – 1500 A peak current (after linac compressor)
- 0.025% relative energy spread
**Duty factor requirements**
- 10 – 100 mA (SRR gun)
- 120 Hz – 10 kHz (X-FEL gun)

## High-power IR and UV FELs

***High-power IR and UV FELs***

- 7 MeV

- 50 MeV

- 150 MeV

- _Beam Parameters_
- 	Ipeak	~ 200 A
- n		~ 5 m
- E/E		~ 0.1%

- Electron beam power:  150 MW

- Optical beam power:  1 - 2 MW

- RF power used:  7 MW (to dump)

- Average beam current:  1A

- Wallplug efficiency:  ~ 10 – 20%

- :  2 MW (FEL)

## High-power IR and UV FELs

*Injector performance requirements*
- Transverse emittance:  	3 – 5 m
- Longitudinal emittance: 	< 100 keV ps
- Average beam current:  	~ 1 A
- Single-bunch charge:		1 – 1.5 nC

*Some other considerations...*
- Energy gain per gun cavity:  < 2 MeV
- Beam break-up modes
- Drive laser power requirements
- Beam halo

- 99.9% of the beam

*Halo power*
- 	7 kW @ injector
- 	150 kW @ und.

## Electron Microscope Guns

- _Linac Injector Gun_
**1 – 5 MeV (kinetic)**
**0.1 – 1 nC / bunch**
**nA – mA**
**~ 1 ********m norm. emittance**
**~ 1% rms energy spread**
**1****st****-order optics (solenoid)**

- _Electron Microscope_
**10 – 50 keV (kinetic)**
**Bunches?  What bunches?**
**few mA**
**~ 1 nm norm. emittance**
**~ 10****-5**** rms energy spread**
**High-order optical corrections**

## Directions of exploration...

- _Some different requirements_
**“****Big Iron”**
- lower emittance (10x)
- higher peak currents (5x)
- emittance aspect ratio control
**“****Desktop”**
- ultra-low emittance (103x)
- very low energy spreads (10-5)
**“****Mini-Me”**
- (usually) modest beam quality (e.g. we can sometimes get there today)
- high average power (e.g. 1 MW from the gun for FEL)
- at least quasi-CW operation
- beam halo is a critical issue

- _Some common themes_
- Higher performance levels than are routinely achieved today are demanded
- With few exceptions, the injector is a quasi-standalone component
- Injector reliability is key to the uptime of the entire facility
- Backup injector capability would be a large benefit
- “Beyond the beam physics” issues are very important

## An Interim Summary...

***An Interim Summary...***

## What is being modeled?

- Surface heating
- Wakes

- Retarded
-     potentials
- Beam loading
- Beam Emission
- e- secondary
- Halo formation

- Surface gradients
- Shunt impedance

**Notes:**
Note that this says nothing about self-consistency;  it just says what one is attempting to take into account.

## Common Elements

*Performance Figures*
- Better cathodes
- Higher duty factors
- Better beam quality

*Fabrication Issues*
- Improved symmetrization
- Thermal issues (cooling, transients)
- Higher-capacity power couplers
- Routine maintenance

*R&D Requirements*
- Cathode research
- Extended injector theory
- Expanded & improved simulation codes
- cavity / beam interactions
- wakefields
- HOM effects
- Beam halo

## A Word on Photocathodes...

*Drive laser requirements*

| Target emittance | x < ...* |
| --- | --- |
| 5 m (IR, UV FEL) | 1.8 mm |
| 1 m (LCLS) | 0.36 mm |
| 0.1 m (SRR, X-FEL) | 36 m |
| 1 nm (E-microscope) | 0.36 m |

- * for Ek = 1 eV;  √2 th ≤ total

## Injector Development Efforts:  Simulations

**DC guns:**
- 0.1 m @ 0.08 nC
**SRF guns:**
-  n ~ 5 nm, E/E ~ 5x10-5, Ek ~ 1.7 MeV, Iavg ~ 90 A
-  n ~ 0.1 m @ 0.05 nC
-  n ~ < 5 m @ 1 A
**NC guns:**
- needle cathode:  0.05 m @ 0.02 nC
- planar focusing cathode:  0.13 m @ 0.1 nC

***Simulation Results:  ***
***No thermal emittance included!***
***Single-bunch performance only!***

## Injector Development Efforts:  Who & What?

**Cornell**
- DC guns for ERLs
- massive concurrent processing & optimization using ASTRA
**Advanced Energy Systems**
- DC/SRF hybrid with JLab
- NC-CW for IR-FEL with LANL
- SRF CW with BNL and JTO
- High-duty-factor with SLAC
- Polarized source studies
**SPring-8 / Riken / KEK**
- DC gun for FEL
**LBNL**
- High-rep-rate NC guns
**TU-Eindhoven**
- DC/RF hybrid NC guns
**Vanderbilt**
- Needle cathodes
**LANL**
- high-power CW NC guns

**Stanford / SLAC**
- Polarized-beam gun
- High-duty-factor NC operation
- Multifrequency gun designs
**BNL**
- SRF gun with AES & Rossendorf
- Electron cooling injectors
**DESY & PITZ**
- High-rep-rate NC guns
- Next-generation injector research
**FNAL**
- Flat-beam production
- LN2-cooled NC guns
**Rossendorf**
- fully SRF gun development w/ novel focusing
**ANL**
- high-power CW NC & SRF guns
- e-microscope guns (just starting)

## Injector Development Efforts:  Cathodes

**Brookhaven National Laboratory**
- Nb cathodes (“native” SRF gun cathodes)
- Diamond-plate secondary-emission cathode
**U. Maryland & Naval Research Laboratory**
- Thermionic-assisted photocathodes
- General cathode emission theory
**SPRING-8**
- DC gun cathodes
**SLAC**
- Polarized electron cathode for RF guns

## Parting thoughts – Part 1

**Injector development is proceeding in many directions.**
**Many designs begin to approach materials & technological limits (e.g. thermal emittance, rf coupler power handling).**
**Many common themes unite the work, including:**
- need for more cathode research for better cathodes (lifetime, QE, thermal), and
- need for theory & simulations with expanded capabilities to take into account new design features.
**This is an exciting time to be working on injector design**

## Slide 27

## Desktop Accelerator Development

**Interesting things to do with just a gun**
**Choices for cathodes **
**General limitations**
- Duty factor
- Drive laser
**Losing the laser**
**Some performance calculations**
**Concluding remarks and thoughts**

## Fun Gun Things (no linacs required)

**Particle beam & accelerator physics**
- basic beam physics measurements
- beam manipulation
- diagnostics development
**Applications**
- Electron microscopy
- Long-wavelength radiation generation
- E-beam welding
- Cancer therapy
- Etc...
**Teaching**

## Beam Emission Timing

## Cathode Overview

- Emission mechanism:  Use laser pulse to excite electrons off the cathode surface

***Cathode Overview***

_**“**__**Ideal” Cathode Checklist:**_
**Long lifetime**
**Rapidly switchable on/off**
**Damage resistant**
**High charge density**
**Cryogenic compatible**
**Simple operation**

- _Thermionic cathodes_
- Emission mechanism:  heat the cathode to “boil” the electron sea in the metal

- Advantages:
- Long lifetime
- Robust
- Simple to operate

- Disadvantages:
- Not rapidly switchable
- High temperature required

- Advantages:
- Rapidly switchable
- High charge density

- Disadvantages:
- Efficiency-lifetime tradeoffs
- External drive laser required

- _Field-emission cathodes_
- Emission mechanism:  Electric field pulls electrons from cathode surface

- Advantages:
- Very simple
- High charge density
- Rapid turn-on/off

- Disadvantages:
- Problematic gating (for rf app.)
- Damage questions

## Limitations on Electron Gun Performance

**Duty factor**
- Field gradients of 10 – 100 MV/m, over 10s of cm, desired
- Cu cavity guns:  max. duty factor 25% to date
*low-frequency*
*anything but compact*
- SRF gun developments promise true CW operation
**Drive laser**
- Laser rep rates often lower than gun rf frequencies
*higher charge per pulse for same <I>*
*higher laser energies required*
- Unobtainium cathodes needed for true CW & long-life operation

## Field Emission Cathode Gating Scheme

**FE cathodes emit electrons when the field is high enough**
**operate at low temperature (unlike thermionic cathodes)**
**emit only under “internal” influences (unlike photocathodes)**

## Can these fields be realized?

- 1300 MHz

- 3900 MHz

## Will it work?

**Simulation:**
- Incorporated the Shottky emission model directly
- Also includes space-charge effects

## Expected scaling – emittance

- Beam current in mA

## Expected scaling – energy spread

- Beam current in mA

## Sample calculation #1 – e-beam welder

***Sample calculation #1 – e-beam welder***

**Requirements**
- Modest beam quality
- High beam power density
- Long depth-of-focus

- Multifrequency FE-cathode gun

- 3rd-harmonic linearizer

- 1st-harmonic linearizer

- focusing coil

- 2-m drift

- final-focus test spot

- I = 5 mA
- x = 0.45 mm
- n,x = 3.6·10-2 m

- theory ~ 75 m
- sim ~ 94 m

## Scale drawing of welder system

## Sample calculation #2 – microscope front-end

***Sample calculation #2 – microscope front-end***

**Requirements:**
- low energy spread beam
- high transverse quality

- Multifrequency FE-cathode gun

- 1st-stage energy filter

- 3rd-harmonic linearizer

- 2nd-stage energy filter

- 0.5 mA, 10%
- energy spread

- 0.1 mA, 2%
- energy spread

- 0.1 mA, 0.005%
- energy spread

- 50 A 0.001%
- energy spread
- (expected)

- After 3rd-harmonic linearizer:
- - n,x = 3.1·10-3 m
- - n,y = 6.3·10-3 m
- - E = 110 V

## Towards a High-Voltage Electron Microscope?

**Cavity design can be improved**
- “Mark I” design used for example
- Not optimized for lowest possible emittance
**Alternate means of reducing energy spread**
- Add more harmonics to further flatten pulse?
- Different beamline geometries to better preserve emittance
**Focusing system & abberation correction – study needed**
**Present design represents about 6 months of part-time effort from one accelerator physicist and one vacuum engineer.**

## Recap & Concluding Thoughts

**Field emission cathodes hold great promise for future “desktop” injector development**
- lower maintenance & upkeep than drive lasers
- modest bunch charge for low emittance
- high average beam currents for high-power operation
**Several applications immediately suggest themselves**
- microscope gun
- e-beam welder
**Some advanced applications also interesting**
- THz radiation source
- cancer therapy

## Slide 43

## Linac-Based Light Sources

**X-FELs:  Minimize size of linac and undulator**
**lowest possible beam energy for a given wavelength**
**saturation length “balanced” between emittance, energy spread and diffraction**

- 4 GeV for 1.5 Å

## Linac-Based Light Sources - SRRs

**Obtain > 100x peak brightness over 3****rd****-generation facilities**
**Obtain ps-scale or better bunch durations**

| Source | Norm. Transverse Emittance [m] | Bunch Charge
[nC] | Bunch Length [ps] | Peak Current
[A] | Peak Brightness Enhancement | Avg. Brightness Enhancement |
| --- | --- | --- | --- | --- | --- | --- |
| APS | 43 x 0.5 | 3 – 18 | 20 - 40  | 300   | 1 | 1*       |
| S-band Gun | 1 x 1 | 1.0 | 0.33 | 3000†  | 45 | 45*       |
| New Gun (I) | 0.1 x 0.1 | 0.1 | 0.1  | 1000†  | 135 | 135*   |
| New Gun (II) | 0.1 x 0.1 | 0.1 | 0.1  | 1000†  | 135 | 13.5‡    |

**†  ****With linac-based bunch compressor**
**‡  ****Assuming 10 mA average beam current**
***  Assuming 100 mA average beam current**

## Linear Collider Guns

**Q:****	****Why pursue high-brighness electron guns for LCs?**
**A:****	****Damping rings are very expensive;  potential payoff is great**
**What are the basic requirements for a LC gun?**
**Capable of generating polarized electron beams**
**Capable of generating “flat” beams**
- damping ring elimination would be ideal
- reducing damping ring complexity (size, cost) still worthwhile

## Linear Collider Guns – Flat Beam Production

***Linear Collider Guns – Flat Beam Production***

- Emittance Compensation Solenoid

- RF Gun

- Bucking Coil

- Magnetized beam

- Transformation Optics

- Asymmetric Beam Emittance:  “Flat” Beam

- FNAL:  50:1 ratios measured

## Polarized Electron Beam Production

**Method:  Use a “strained” semiconductor cathode with NEA surface to generate polarized electrons**
**Successfully used with DC guns**
**Issues**
- Lifetime
*RF gun vacuum environment*
*back-bombardment ions ****and electrons***
- Dark current
*NEA surface, high gradient fields*

## A Word on Cathodes...

*Drive laser requirements*

| Target emittance | x < ...* |
| --- | --- |
| 5 m (IR, UV FEL) | 1.8 mm |
| 1 m (LCLS) | 0.36 mm |
| 0.1 m (SRR, X-FEL) | 36 m |
| 1 nm (E-microscope) | 0.36 m |

- * for Ek = 1 eV;  √2 th ≤ total