New photocathode materials for electron ion colliders
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New Photocathode Materials for Electron-ion-colliders. Zhaozhu Li, Kaida Yang, Jose M. Riso and R. Ale Lukaszew 1 Department of Physics, College of William and Mary 2 Department of Applied Science, College of William and Mary. Acknowledgements. College of William and Mary

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New photocathode materials for electron ion colliders

New Photocathode Materials for Electron-ion-colliders

Zhaozhu Li, Kaida Yang, Jose M. Riso and R. Ale Lukaszew

1 Department of Physics, College of William and Mary

2 Department of Applied Science, College of William and Mary


Acknowledgements
Acknowledgements

College of William and Mary

Professor R. A. Lukaszew

Dr Jose Riso

Kaida Yang

Doug Berringer

Jefferson Lab

Dr Matt Peolker

Dr Marcy Stuzman

Funding

Department of Energy

Award #

DE-SC0008546

Principal Investigator

R. A. Lukaszew


Outline
Outline

INTRODUCTION:

About the Goal and Photocathodes

APPROACHES:

To Find A Metal-based Photocathodes Able to Sustain High Currents

REALIZATION:

Schematic Design and Experiment Setup

ON THE WAY:

Premilinary Results and Future Plan


Goal

Electron Ion Collider

(EIC)

robust metal-based

photocathode

large

currents

+

eRHIC and MEIC: 100mA unpolarized e-beam

eRHIC: 50mA polarized e-beam

spin-polarized

currents

1 http://www.bnl.gov/cad/eRhic/

2 http://www.jlab.org/conferences/qcd2012/talks/wednesday/Pawel%20Nadel-Turonski.pdf

Fig 2

Fig 1


Semiconductor photocathodes
Semiconductor Photocathodes

Polarized e-beam:

Unpolarized e-beam:

Strained Superlattice

GaAs/GaAsP

Many options

Multi-alkali photocathodes

GaAs, etc

Polarization 90%

Quantum Efficiency 1%

Quantum Efficiency ≦10%

Pressure ~ E-10 torr

Sensitive to contamination

Life time ~ hours or days

Response time ~ 10s picosecs

More stable to environment contamination

Life time ~ years

Response time ~ picosecs


Metal based photocathodes
Metal-based Photocathodes

QE: much lower than that of semiconductor photocathodes

High reflectivity

Short escape

depth

High Work

Function

High number of scattering events

step A

step B

step B


Surface plasmon resonance spr
SurfacePlasmon Resonance(SPR)

A:

  • SPR: Electrons oscillates coherently on a metal boundary

  • Excitation: satisfying dispersion relationship

  • We need to enhance the wave vector to

  • excite the surface plasmon resonance

  • Grating method to excite SPR

Fig 3

1 A. Hibbins, "Grating Coupling of Surface Plasmon Polaritons at Visible and Microwave Frequencies", phd thesis

Fig 4


Additional layer to lower the work function
Additional layer to lower the work function

B:

MgO

Metal

Substrate

Theoretical Prediction


Additional layer to lower the work function1
Additional layer to lower the work function

B:

MgO

Metal

Substrate

Theoretical Prediction

Fig 6

Fig 5

1 L. Giordano et al, Phs Rev B 73, 045414 (2005)

2 T Konig et, al,J. Phys. Chem. C 2009, 113, 11301

Fig 4


Afm characterization a ag mgo sample
AFM characterization a Ag/MgO sample

This sample gives closest SPR measurement to the predicted angle.



SPR angle

The 1st 20s MgO shows two flat dips in SPR figure between 43 to 47 as shown in purple. The 2nd 20sMgO sample also shows two dips but the flat region from 1st sample is more likely to be one time occasion since the other results seem to have the same tendency.

The results for different sputtering time of MgO up to 40s show a very similar SPR angle~ 48.8 degree.(The total internal reflection angle has been adjusted to be the same position for different measurements.) However, the Rpp reaches to a low level region~less than 1.5V from 43.5 to 55.5 degree

MgO/Ag ~ 48.8 degree

Ag ~ 41.5 degree


S chematic design
Schematic Design

2

1 Transport Fork

2 A New Arm: Manipulator

3 Faraday Cup

4 Sample holder and sample

5 Laser light

6 Additional fork to help transport the sample

Sample preparation in-situ under ultra high vacuum ~ E-9 torr

3

1

4

5

6

Loadlock Overview


Simulation
Simulation

  • Under two excitation methods: k vector to excite to be the same

Mathematic program

to simulate SPR

Find resonance angle

Calculate the SPR angle

under grating scheme



Experiment setup1
Experiment Setup

Grounding

Keithley Picoammeter


Experiment setup2
Experiment Setup

Ceramic Isolated with Chamber





Preliminary results
Preliminary results

  • Aspects of our setup have been tested using the photocathode experimental system at JLab

  • Current very small ~ E-2 picoA

  • We just finish setting up this week!


Fine tuning photocurrent measurement

Blocking spurious light: The current increases from 0.083pA to ~0.089pA

~10 degrees

~60 degrees

~80 degrees

Rotating polarization with respect to pattern on sample: The current decreasesto 0.085 pA and again goes upto ~0.087pA


Conclusions and future plans
Conclusions and Future Plans

We use SPR and MgO thin film coating in our experimental approach to achieve suitable metal-based photocathodes.

The resultsare still very preliminary and further improvements and calibration will be conducted.

We will try more energetic photons for efficient photocathode excitation (e.g. blue, at 400nm has an energy of 3.1 eV compared to the ~0.8 eV in IR light). For that we will use a tighter pattern for the diffraction grating (going from CD to bluray DVD). We will update our simulations to this new geometry to establish the thickness so that the SPR can be excited at 45 degrees incidence.


Polarized current
Polarized current?

  • Our ultimate goal is to deposit a magnetic material such as "silmanal", which is a silver alloy with Mn and Al. This belongs to the so-called "Heussler alloys“ known for their high degree of polarization

  • Silmanal is magnetic and therefore it can be used to spin-polarize the photo-electrons. The major constituent of the alloy is silver. Hence our preliminary studies on Ag photocathodes.


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