Study of Secondary Emission Enhanced Photoinjector

Xiangyun Chang1, Ilan Ben-Zvi1,2, Andrew Burrill1, Peter D. Johnson2 Jörg Kewisch1 Triveni S. Rao3 and YongXiang

Zhao2

Collider-Accelerator Department1, Physics Department2, Instrumentation Division3

Brookhaven National LaboratoryUpton NY 11973 USA

Introduction

Schematic diagram of a secondary emission enhanced photoinjector

The advantages of the Secondary Emission Enhanced Photoinjector

• 1. Reduction of the number of primary electrons by the large SEY, i.e. a very low laser power requirement in the photocathode producing the primaries.

• 2. Protection of the cathode from possible contamination from the gun, allowing the use of large quantum efficiency but sensitive cathodes.

• 3. Protection of the gun from possible contamination by the cathode, allowing the use of superconducting gun cavities.

• 4. Production of high average currents, up to ampere class.

• 5. Expected long lifetime

Design considerations • The metal layer

– The choice of material.• One would like to use material which has

high electrical conductivity (σ) to reduce the resistance of the film.

• On the other hand one would like use material which big long Continuous Slowing Down Approximation (CSDA) range parameter RCSDA to reduce the energy loss in the metal film

• The Al is the best choice. 24 /105.310 cmgkeVRAl

mkeVRkeVL AlSTP 3.1/1010

• RF penetration of the Al film for 700MHz MW

It is proven that the RF penetration of the metal film is very small for our application.

So, Al film not only carries out the replenishment current but also the large RF current. This makes the thicker Al film more important.

mAl

Al

1.32

AlAlt

• The secondary electron yield (SEY)

SEY measurement for reflection mode

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

J. Appl. Phys., Vol. 82, No. 4, 15 August 1997It is expected that almost all the secondary electrons produced would come out from the diamond for our transmission mode. We will assume a conservative SEY value of 300 at 4keV for our application.

• 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 and the limited electron drifting velocity in diamond.

– The allowed transmission time is 120~200ps for our case.

– The drift velocity is smvD /107.2 5

mvpst DDmd 32120

• Diamond thermal conductivity

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

. Grain size and surface roughness of our 30μm thick diamond sample

• 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: • Free electrons on diamond conduction band (Nitrogen

doping). Under the strong RF field, it will behave the same way like the secondary electrons. So, not only produces extra heat but also the background current and back-bombardment.

• If the holes on the valence band (Boron doping) is big, it will only produce the extra heat.

– Field shielding problem:• If the concentration of free electrons or holes is too

high, it can be thought as a bad quality conductor. So, the RF field attenuates inside diamond surface and then affects the drift velocity of the secondary electron

• Limits on impurities:– The concentration of the free electrons or holes should be

well below the secondary electron density in diamond. For RHIC cooling, charge/bunch=20nC, the density is about:

– The Hydrogen, Phosphorus and other alkali elements impurities form strong chemical bonds with C with no free electron. So, this impurities are not important .

– Nitrogen:1ppm or more ( ) The activation energy can be 1.7eV or more. The portion exited to conduction band by thermal at T=500K:

– Boron:

• Boron doping concentration can be easily less than

313419

9

/101030106.1

1020cmn

317 /1078.11 cmppm

18107.1

exp

Tk

eV

n

n

BN

e

4109.137.0

exp

Tk

eV

n

n

BB

h

314 /10 cm

Diamond temperature • RHIC e-cooling project:Charge=20nC/bunchRepetition frequency=9.4MHzRadius R>10mmPrimary electron energy EPri=10keVDiamond thickness rDmd=30μm

Al thickness tAl=800nm

Peak RF field on cathode E0=15MV/m

SEY=300Temperature on diamond edge Tedge=80K

Primary electron pulse length PlsPri=10deg

Temperature distribution for Cooling

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20

R (mm)

T (

K) R=10mm

R=12.5mm

R=15mm

R 10mm 12.5mm 15mm

Primary power=6.2667(W

)6.2667(W

)6.2667(W

)

Secondary power=7.5536(W

)7.5536(W

)7.5536(W

)

RF power=7.5241(W

)19.9958(

W)48.5851(

W)

Replenishment power=

0.0422(W)

0.0459(W)

0.0538(W)

Total power=21.3866(

W)33.8620(

W)62.4592(

W)

Edge Temperature effectTemperature distribution for Cooling (tAl=800nm)

0

50

100

150

200

250

300

0 5 10 15

R (mm)

T (

K) Tedge=80K

Tedge=140K

Tedge=200K

Tedge 80K 140K 200K

Primary power= 6.2667(W) 6.2667(W)6.2667(W

)

Secondary power= 7.5536(W) 7.5536(W)7.5536(W

)

RF power=19.9958(W

)30.0569(W

)42.0461(

W)

Replenishment power= 0.0459(W) 0.0690(W)

0.0966(W)

Total power=33.8620(W

)43.9461(W

)55.9630(

W)

Al thickness effect

Temperature distribution for Cooling

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15

R (mm)

T (

K) tAl=800nm

tAl=600nm

tAl=400nm

• Energy Recovery Linac (ERL) project in BNLCharge=1.42nC/bunchRepetition frequency=703MHzRadius R~5mmPrimary electron energy EPri=10keVDiamond thickness rDmd=30μmAl thickness tAl=800nmPeak RF field on cathode E0=15MV/mSEY=300Temperature on diamond edge Tedge=80KPrimary electron pulse length PlsPri=10deg

Temperature distribution for ERL

0

50

100

150

200

250

300

0 2 4 6 8

R (mm)

T (

K) R=2.5mm

R=5mm

R=7.5mm

R 2.5mm 5mm 7.5mm

Primary power=33.3333(

W)33.3333(

W)33.3333(

W)

Secondary power=40.1786(

W)40.1786(

W)40.1786(

W)

RF power= 0.0460(W) 0.6691(W) 3.3975(W)

Replenishment power= 0.0250(W) 0.0227(W) 0.0228(W)

Total power=73.5829(

W)74.2037(

W)76.9322(

W)

Secondary electron beam quality

• Bunch broadeningDrift velocity is saturated if E>2MV/m, the broadening is

determined by the straggling distance of primary electron in diamond:

This is equivalent to a 2.6ps long laser pulse

• Limit on charge density

For RHIC cooling example with R=12.5mm, Q<< 65nCFor ERL example with R=5mm, Q<<10nC

mkeVRkeVD CCStrg 41.0/10%2510

24 /1082.210 cmgkeVRC

002/ ERQESPC

• Secondary electron temperature

Under

00

W

LeD

Le

TWTWveE

dt

Wd

dt

Wd

1

2 2538 0.41i r m r mE a E a

mMVE /100

eVTe 4.0

eVmMVmTSurface 1/101.0

Experiments

Conclusion

• The study and calculations show great feasibility of Secondary Emission Enhanced Photoinjector.

• The new approach has many advantages.