Field enhancement coefficient determination methods: dark current and Schottky enabled photo-emissions

Wei Gai

ANL

CERN RF Breakdown Meeting

May 6, 2010

2

SCHOTTKY-ENABLED PHOTOEMISSION 1

Study a new regime of electron beam generation that has potential impact on the future Linear Collider and Light sources;

High brightness electron beam – minimize emittance; Scheme: Employ Schottky effect in the RF photocathode gun and with

low energy photons; This technique also produces a reasonable estimate of the field

enhancement factor without employing the Fowler-Nordheim model.

“… it now appears that the intrinsic cathode emittance will be the dominant quantity that limits our achieving beams of higher brightness from an rf gun.”

ANL Theory Institute Workshop on Production of Bright Electron Beams Sept. 22-26, 2003

In this presentation, thermal emittance = intrinsic emitance

Yusof et al., Phys. Rev. Lett. 93, 114801 (2004)

3

Emittance from RF photocathode {KJ Kim, NIM A275, 1989}

xscRFscRFthermal J 22222

where RF and sc are the emittances from RF field in the gun and from space charge effect, respectively. Jx is correlation between RF and sc and normally is zero.

There are ways to do emittance compensation to handle RF and sc {B. Carlsten, NIM A285, 313 (1989)}, so in principle, the only limitation left is the thermal emittance thermal:

SCHOTTKY-ENABLED PHOTOEMISSION 2 – Emittance in RF Photoinjector

4

where h : photon energy : material’s bulk work function : a constant : field enhancement factor : RF phase E : Electric field magnitude

Our scheme is to use h< , and then employ the Schottky effect to lower the effective work function eff, where

At the cathode 0 xpx

Therefore,

However, for a cathode in an electric field E: )( EhEkin

cm

Emx

cm

px kin

rmsrms

rmsrmsthermal0

0

0,

2

xxthermal pxpxcm

22

0

1

For a typical cathode: hEkin

)( Eeff

Minimizing Ekin will minimize thermal.

-eff

SCHOTTKY-ENABLED PHOTOEMISSION 3 – Thermal Emittance

5

Metal

Image potentiale2/160z

Electrostatic potential-eEz

Effective potential

z0

eff

EF

0

3

4Ee

eff = -

SCHOTTKY-ENABLED PHOTOEMISSION 4 – The Schottky Effect

6

Beam dump

ICT

Solenoid

Solenoid

Cathode

5 eV 3.3 eV

Mirror

YAG

Camera

Light source: Frequency-doubled Ti:Sapphire laser 372 nm (3.3 eV), 1 – 4 mJ, 8ps.

Photocathode: Mg, = 3.6 eV.

Example of Schottky effect on the cathode: at E() = 60 MV/m, ~ 0.3 eV

SCHOTTKY-ENABLED PHOTOEMISSION 5 – Schematic of Beamline

7

SCHOTTKY-ENABLED PHOTOEMISSION 6 - RF Scans

ee

eE() = - Emax sin()

Photocathode

h

E(

)

Phase (deg)

RF Phase

Laser injection

RF frequency = 1.3 GHz (Period ~ 770 ps)

Laser pulse length = 6 – 8 ps

Metallic photocathode response time ~ fs

We can safely assume that all the photoelectrons emitted in each pulse see the same E-field strength

8

SCHOTTKY-ENABLED PHOTOEMISSION 7 - Charge Obtained From Typical RF Scans

0 20 40 60 80 100 120 140

Cha

rge

RF Phase (deg)

1nC, 100 MV/m

Theoretical RF Scan

Theoretical result from PARMELLA simulation of our RF photoinjector (H. Wang)

We see the “expected” flat-top profile

Full range of charge detected ~ 130 degrees

0 20 40 60 80 100 120 1400.0

0.1

0.2

0.3

0.4

Cha

rge

Q (

nC)

RF Phase (deg)

28 MV/m 17 12

Experimental RF Scan

X.J. Wang et al. Proc. 1998 LINAC

Our scans

9

0 20 40 60 80 100 1200.00

0.05

0.10

0.15

0.20

0=20O

0=30o

Cha

rge

(nC

)

RF Phase (deg)

Emax(MV/m)

92

77

52

28

17

14

0=50O

h = 3.3 eV; = 3.6 eV

Laser beam diameter = 2 cm (0.35 mJ/cm2)

A noticeable shift of the onset of photoelectron production with decreasing RF power.

An RF phase scan allows us to impose different electric field magnitude on the cathode at the instant that a laser pulse impinges on the surface, i.e. E() = Emax sin().

h = 5 eV, = 3.6 eV

No change in the phase range over all RF power.

New observation Typical photoinjector conditions

0 20 40 60 80 100 120 1400.0

0.1

0.2

0.3

0.4

Cha

rge

Q (

nC)

RF Phase (deg)

28 MV/m 17 12

SCHOTTKY-ENABLED PHOTOEMISSION 8 – Experimental Result 1: RF Phase Scans

10

0 (deg) Emax (MV/m) E0 = Emax sin(0) (MV/m)

20 28 9.2 6.8

30 17 8.5 7.3

50 14 11 5.8

0 20 40 60 80 100 1200.00

0.05

0.10

0.15

0.20

0=20O

0=30o

Cha

rge

(nC

)

RF Phase (deg)

Emax(MV/m)

92

77

52

28

17

14

0=50O

2)( EhQ

0

max

4

)sin()(

e

EE

At threshold, Q = 0. This allows us to make a reasonable estimate of the maximum .

h = 3.3 eV; = 3.6 eVThis is a new and viable technique to realistically determine the field enhancement factor of the cathode in a photoinjector

SCHOTTKY-ENABLED PHOTOEMISSION 9 – Determination of Field-Enhancement Factor

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SCHOTTKY-ENABLED PHOTOEMISSION 10 - Experimental Result 2 : Intensity Dependence

Parameters:

h = 3.3 eV

= 3.6 eV

E field on cathode: 80 MV/m

Laser spot diameter: 2 cm

0.4 0.6 0.8 1.0 1.2 1.40.04

0.08

0.12

0.16

Cha

rge

(nC

)

Laser Energy Per Pulse (mJ)

As we increase the laser intensity, we detect more charge. We definitely are detecting photoelectrons and not dark currents!

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SCHOTTKY-ENABLED PHOTOEMISSION 11- Experimental Results 3 : Detection Threshold?

0 20 40 60 80 100 120 140 160 180

0.2

0.4

0.6

0.8

1.00.0

0.2

0.4

0.6

0.8

1.0

Am

plitu

de (

arb.

uni

ts)

(deg)

Am

plitu

de (

arb.

uni

ts)

0 20 40 60 80 100 1200.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Cha

rge

(nC

)

(deg)

Simulated Detection Threshold Using A Sine Function

Experimental Observation

Emax = 28 MV/m

The shift in the photoemisson threshold is not due to the detection threshold.

Two different scans with different amount of charge produced, but with the same RF amplitude, show the same phase angle for the photoemission threshold.

No cutoff

With cutoff

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The Effective Work Function for Cu Under External Electric Field

0 20 40 60 80 1001 .6

1 .8

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0

3 .2

3 .4

3 .6

3 .8

4 .0

4 .2

4 .4

4 .6

ef

f (e

V)

A p p lie d F ie ld (M V /m )

E ffe c tiv e W o rk F u n c tio n fo r C u (0 = 4 .6 e V )

=1

=7

=10

=15

=20

=50

377 nm

400 nm

14

Estimate of the Field-Enhancement Factor in an RF Photoinjector

15

Estimate of the Field-Enhancement Factor in an RF Photoinjector

Measured dark current as a function of forward rf power in the gun (Fig. 8)

To extract the values of (i) current I and (ii) E-field, I used the following:

(i) E-field

P = kE2.Since at P=3 MW, E=120 MV/m, we have k=2.08×10-4 MW/(MV/m)2. From this, E-field can be extracted from the data.

(ii) Current I

Assuming that the dark current pulse length is 2.5 s, then I = Q/t. The values of I can then be extracted from Q.

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Estimate of the Field-Enhancement Factor in an RF Photoinjector

An estimate of the field-enhancement factor is made using the Fowler-Nordheim model.

8 .0x10 -9 1 .0x10 -8 1 .2x10 -8 1 .4x10 -8 1 .6x10 -8-23 .8

-23 .6

-23 .4

-23 .2

-23 .0

-22 .8

-22 .6

-22 .4

-22 .2

-22 .0

Lo

g(I

/E2.

5 ) (A

.V-2

.5. m

2.5 )

1 /E (m /V )

The non-linear data points (red) have been excluded in the fit.

Using Eq. 14 from Wang and Loew (SLAC-Pub-7684), for the case of an RF field, the slope of the graph can be expressed as:

5.195.2

10 1084.2

)1(

)(log

Ed

EId

For Cu, = 4.6 eV. We then obtain the value of from the linear fit to be ~100.

Question: Is this the same β as in the Schottky experiments?A: Don’t know, but underline physics should be the same.

β~100

What next?

17

We are planning to perform further experiments with Cu cathodes in both L and S-band guns.(possible setup are at ANL and Tsinghua University)