# Presentation: 062963

## Studies in the Secondary Emission Enhanced Photocathode 

**Xiangyun Chang****1****, Ilan Ben-Zvi****1,2****, Andrew Burrill****1****, Peter D. Johnson****2**** Jörg Kewisch****1**** Triveni S. Rao****3****, Zvi Segalov****2**** and YongXiang Zhao****2**
**Collider-Accelerator Department****1****, Physics Department****2****, Instrumentation Division****3**
**Brookhaven National Laboratory**
**Upton NY 11973 USA**

## Outline

**Introduction**
**Brief introduction of the Secondary Emission Enhanced Photoinjector (or diamond cathode) and its advantages.**
**Details in design considerations**
**Why diamond?**
**Why Al the coated metal material?**
**Hydrogenation surface and the Secondary Electron Yield (SEY).**
**Optimizing the diamond thickness.**
**Thermal consideration.**
**Impurity effects.**
**Mechanical consideration. RF pressure?**
**Temperature? RHIC e-cooling and ERL examples.**
**Electron beam quality? Bunch broadening, Charge limit and electron temperature**
**First step experiment results**
**Future objectives**
**Conclusions**

## Motivation

**The creation of high average-current, high brightness electron beams is a key technology for a large number of accelerator-based systems**
**Electron cooling of hadron accelerators **
**Energy-Recovery Linac (ERL) light sources**
**Ultra-high-power Free-Electron Lasers (FELs)**

## Traditional laser photo-cathodes in RF guns:

- Metallic cathodes.
- Low QE (Cu: 10-4@266nm, Mg: 10-3@266nm ). Low average current
- Semi-conductor photo-cathodes.
- High QE (10-1, 532nm, Green light). Sensitive to any contamination, short lifetimes.

## Schematic diagram of a secondary emission enhanced photoinjector 

## The advantages of the Secondary Emission Enhanced Photoinjector

**Reduction of the laser power requirement, i.e. the number of primary electrons required is reduced by the large secondary electron yield (SEY).**
**Protection of the cathode from possible contamination from the gun, allowing the use of large quantum efficiency but sensitive cathodes. **
**Protection of the gun from possible contamination by the cathode, allowing the use of superconducting gun cavities. **
**Production of high average currents, up to ampere class. **
**Expected long lifetime **

## Design considerations

**The diamond**
**Wide band gap (5.47eV).**** A good insulator with very high breakdown field (1000MV/m). Easy to demonstrate Negative Electron Affinity (NEA). **
**The hardest material****. Free standing for a very thin film.**
**The highest thermal conductivity****. Handle high power with a thin film.**
**The very high mobility and saturation velocity**** (                ). Applicable for high-frequency RF cavities**

**The concept of NEA surface**

## The metal layer 
RF penetration of the metal film of the diamond cathode. 

It is proven that the RF penetration of the metal film is very small for our application.
So, the metal coating not only carries the replenishment current but also the large RF current. This requires the a small resistance metal film. 

- RF penetration of the metal film of the diamond cathode.
**It is proven that the RF penetration of the metal film is very small for our application.**
**So, the metal coating not only carries the replenishment current but also the large RF current. This requires the a small resistance metal film.**

## Schematic explanation of the poor RF transmission  through a thin metal film

## The choice of the metal material.
One would like to use metal of which the electrical conductivity (σ) is as high as possible to reduce the resistance of the film.
On the other hand one would like use metal in which the energy loss of the primary electrons is as small as possible. This is determined by the Continuous Slowing Down Approximation (CSDA) range parameter RCSDA.
The Al is the best choice. 

- One would like to use metal of which the electrical conductivity (σ) is as high as possible to reduce the resistance of the film.
- On the other hand one would like use metal in which the energy loss of the primary electrons is as small as possible. This is determined by the Continuous Slowing Down Approximation (CSDA) range parameter RCSDA.
- The Al is the best choice.

## The hydrogenation surface. 

- SEY measurement for reflection mode. (A) After a saturated atomic-hydrogen exposure. (B) After heating to 900 ̊C

- A. Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, and R. Abrams

- J. Appl. Phys., Vol. 82, No. 4, 15 August 1997

**It is expected that almost all the secondary electrons produced would come out from the diamond for our transmission mode. **
**We will assume SEY to be about 300 at 4keV for our transmission mode.**

## The thickness of diamond window
One would like to use thicker diamond to improve the thermal conduction of the window.
On the other hand, for our RF application the diamond thickness is limited due to the allowed transmission time for the secondary electrons.
The electron drifting velocity in diamond.
Initial RF phase (30~50deg.)
The allowed transmission time is 120~200ps for our case.
The diamond thickness is estimated: 

- One would like to use thicker diamond to improve the thermal conduction of the window.
- On the other hand, for our RF application the diamond thickness is limited due to the allowed transmission time for the secondary electrons.
- The electron drifting velocity in diamond.
- Initial RF phase (30~50deg.)
- The allowed transmission time is 120~200ps for our case.
- The diamond thickness is estimated:

## Thermal properties

- Diamond has the highest thermal conductivity. The thermal conductivity is determined by the grain size, boundary, umklapp process and impurity of the film.

- E. Worner*, E. Pleuler, C. Wild, P. Koidl , Diamond and Related Materials 12 (2003) 744–748

## The impurity problem 
Impurities: Boron (p-type), Nitrogen (n-type), Hydrogen (n-type), Phosphorus (n-type), Lithium (n-type) and Sodium (n-type). 
Heating problem: 
Electrons on diamond conduction band (N type) behave the same way like the secondary electrons. So, generate extra heat and the background current. 
Holes on the valence band (P type) only generate the extra heat.
Charge carrier trapping and field shielding problem:
The impurities and grain boundaries can trap the charge carriers therefore attenuate the RF field inside diamond and finally affects the drifting of the secondary electron.

- Impurities: Boron (p-type), Nitrogen (n-type), Hydrogen (n-type), Phosphorus (n-type), Lithium (n-type) and Sodium (n-type).
- Heating problem:
- Electrons on diamond conduction band (N type) behave the same way like the secondary electrons. So, generate extra heat and the background current.
- Holes on the valence band (P type) only generate the extra heat.
- Charge carrier trapping and field shielding problem:
- The impurities and grain boundaries can trap the charge carriers therefore attenuate the RF field inside diamond and finally affects the drifting of the secondary electron.

## Limits on impurities:
The number of the electrons or holes generated by impurities should be well below the secondary electron generated by the primaries. For RHIC cooling, charge/bunch=20nC, the electron number is: 

The Hydrogen, Phosphorus and other alkali elements impurities form strong chemical bonds with C with ignorable excited electron.
Nitrogen: The activation energy can be 1.7eV or more. The number of electrons exited to conduction band by thermal at concentration of 1ppm and T=500K:

Boron:

Boron doping concentration should be much less than 1ppm 

- The number of the electrons or holes generated by impurities should be well below the secondary electron generated by the primaries. For RHIC cooling, charge/bunch=20nC, the electron number is:
- The Hydrogen, Phosphorus and other alkali elements impurities form strong chemical bonds with C with ignorable excited electron.
- Nitrogen: The activation energy can be 1.7eV or more. The number of electrons exited to conduction band by thermal at concentration of 1ppm and T=500K:
- Boron:
- Boron doping concentration should be much less than 1ppm

##  Mechanical consideration:
30μm thick, 10mm diameter, ~1Atm

RF pressure

- 30μm thick, 10mm diameter, ~1Atm
- RF pressure

-  The pressure due to DC field on backside of diamond and the primary electron generated pressure are ignorable.

## Diamond temperature 

- RHIC e-cooling project

| Charge | 20 nC/bunch |
| --- | --- |
| Repetition frequency | 9.4 MHz |
| Radius | >10 mm |
| Primary electron energy | 10 keV |
| Diamond thickness | 30 μm |
| Al thickness | 800 nm |
| Peak RF field on cathode | 15 MV/m |
| SEY | 300 |
| Temperature on diamond edge | 80 K |
| Primary electron pulse length | 10 deg |

## Slide 18

| R (mm) | 10 | 12.5 | 15 |
| --- | --- | --- | --- |
| Primary power (W) | 6 | 6 | 6 |
| Secondary power (W) | 8 | 8 | 8 |
| RF power (W) | 8 | 20 | 49 |
| Replenishment power (W) | 0.04 | 0.05 | 0.05 |
| Total power (W) | 22 | 34 | 63 |

## Edge Temperature effect

| Tedge (K) | 80 | 140 | 200 |
| --- | --- | --- | --- |
| Primary power (W) | 6 | 6 | 6 |
| Secondary power (W) | 8 | 8 | 8 |
| RF power (W) | 20 | 30 | 42 |
| Replenishment power (W) | 0.05 | 0.07 | 0.1 |
| Total power (W) | 34 | 44 | 56 |

## Al thickness effect

## Energy Recovery Linac (ERL) project in BNL 

| Charge | 1.42 nC/bunch |
| --- | --- |
| Repetition frequency | 703 MHz |
| Radius | ~5 mm |
| Primary electron energy | 10 keV |
| Diamond thickness | 30 μm |
| Al thickness | 800 nm |
| Peak RF field on cathode | 15 MV/m |
| SEY | 300 |
| Temperature on diamond edge | 80 K |
| Primary electron pulse length | 10 deg |

## Slide 22

| R (mm) | 2.5 | 5 | 7.5 |
| --- | --- | --- | --- |
| Primary power (W) | 33 | 33 | 33 |
| Secondary power (W) | 40 | 40 | 40 |
| RF power (W) | 0.05 | 0.7 | 3.4 |
| Replenishment power (W) | 0.03 | 0.03 | 0.03 |
| Total power (W) | 74 | 74 | 77 |

## Secondary electron beam quality

**Bunch broadening**
- Drift velocity is saturated if E>2MV/m, the broadening is mainly determined by  the straggling distance of primary electron in diamond:
- This is equivalent to a 3ps long laser pulse
**Charge limit**
- For RHIC cooling example with R=12.5mm, Q<< 65nC
- For ERL example with R=5mm, Q<<10nC

## Secondary electron temperature 

	If

- 	If

- M. P. Seah and W. A. Dench, *Surf. Interface Anal.* **1**, pg10 (1979)

## Experiments

## As a first step experiment of the Secondary Emission Diamond Cathode we measured the generation of electron-hole pairs and their transmissions through the diamond under high field 

## Main results 

**Polycrystalline diamond.**** **(Φ6mm,200μm thick)
**The primary electron energy ranges from 0eV to 6keV and the current ranges from 0A to 5μA. No electron or hole transmission is measured (              ) **
**Single-crystal diamond**** (Apollo). **(Φ6mm,200μm thick)
- We got gains of electron and hole from a few to a few hundreds.
- The gain is much smaller than expected and is not stable.

## Natural diamond 3x2.64x0.16mm3 

## Measurements at low temperature

## Ionization of impurities?

## Future tasks

- The electron transmission of the better quality diamond
- The electron emission of a hydrogen termination diamond
- The electron temperature
- RF test.

## Conclusion

**The study and calculations show great feasibility of the ****Secondary Emission Enhanced Photoinjector****. It ****has many advantages****.**
**The experiment shows that the transmission of electron through diamond is strongly dependent on the sample quality. ****With better quality diamond, we expect that the electron transmission through diamond will be appropriate for SEEP.**