2006.10.04 –10.06 Milan Italy

LUCX system and dark current

• (1) LUCX project

• (2) PhaseⅠand results

• (3) Phase Ⅱ and dark current

Liu shengguang and LUCX group

2006.10.04 –10.06 Milan Italy

(1) Purpose of LUCX

• LUCX ( Laser Undulator Compton X ray source) is for generation x-ray photon by inverse Compton scattering.

• X-ray is for medical application ----K-edge digital subtraction angiography. In this application, 33KeV X ray photons are required.

• Collision angle between electron beams and the laser beams is 20 degree.

• The detector size is 4cm×4cm, 2m away from the collision point. Acceptance angle of the detector is 10mrad.

2006.10.04 –10.06 Milan Italy

(1) Main parameters of LUCX

Electron beam Laser pulse

Energy 43MeV 1064nm (1.17eV)

Intensity 2nC/bunch (5nC/bunch)

1.25*10^10electron/bunch

17µJ/pulse

9.1*10^13photon/pulse

Width (FWHM) 10ps 10ps

Bunch number 100bunches/train

Beam size σx =64um σy =32um σ= 100um

Repetition rate 12.5Hz

Expected X ray photon number on the detector :

21024102

5105.2

/bunch

/train/sec

2006.10.04 –10.06 Milan Italy

(2) Phase Ⅰ

Until the end of 2005, RF gun study had been finished and multi bunch beams had been generated.

2006.10.04 –10.06 Milan Italy

(2) Cs2Te cathode system

MoverMover

Preparation system

RF gunCathode load system

Transport system

Oct. 2005, we prepared a cathode and loaded it.

At the beginning, QE is 3.7% offline by Xe light

One years later, QE become 0.7% online by laser in gun

2006.10.04 –10.06 Milan Italy

(2) RF-gun Cavity

Cu

Beam

Mo Cathode plug

RF gun cavity (1.6cells)

UV laser

For LUCX gun, space charge effect is severe. In order to decrease it, field distribution be tuned into not balanced distribution. Eh/Ef =1.3

2006.10.04 –10.06 Milan Italy

(2) Chicane

A chicane composed of 4 dipole magnets kicks the beam off the main beam line in order to inject laser on the cathode perpendicularly

2006.10.04 –10.06 Milan Italy

(2) Laser system

266nm 11µJ/pulse 13ps(FWHM) Gaussian

Now we have installed a shaper to convert gaussian laser---> Uniform laser (transverse distribution)

2006.10.04 –10.06 Milan Italy

(2) Experimental results

Because of high charge and multi bunch, we must consider beam loading effect and try to compensate it. We apply ΔT method.

Injection time is 1.703µs

with 2.05µs RF pulse

Injection time is 0.908µs

with 2.05µs RF pulse

2006.10.04 –10.06 Milan Italy

(2) Experimental results

Momentum and momentum spread:

220nC/100bunch : 4.65MeV/c 1.3% bunch to bunch

one bunch : 0.9%

Normalized emittance: 49nC/100 bunch 15πmm.mrad

220nC/100bunch 48 π mm.mrad

High charge: 300nC/100bunch has been generated

2006.10.04 –10.06 Milan Italy

(3) Phase Ⅱ

2006.10.04 –10.06 Milan Italy

(3) RF pulse compressor

3dB CouplerKlystron

GUN

ACC

Dummy Load

RRCS

Phase Shifter

0.7

0.3

1.5dB Coupler

Traveling wave type RF compressor

3.7µs flat ---> 1.0µs

2006.10.04 –10.06 Milan Italy

(3) Dark current

Dark current comes from field emission.

In a RF period, dark current occurs at phase region nearby 90 degree (red shadow) .

During a RF pulse, dark current occurs at the time region that blue shadow shows. If RF pulse width increase, the blue region expands lots, dark current increase much .

2006.10.04 –10.06 Milan Italy

(3) Dark current data 1

1917.4

735.5

116.90

200400600800

100012001400160018002000

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Forward RF power to Gun (MW)

dark

cur

rent

in a

RF

puls

e (p

C)

2us1.5us1us

RF pulse Forward power Field at cathode surface

1.0us 8.3MW 81.8MV/m

1.5us 7.0MW 83.8MV/m

2.0us 6.1MW 81.54MV/m

If we regard the value of field gradient at cathode surface as a marker. For LUCX gun, if it is less than 80MV/m, no dark current occurs

2006.10.04 –10.06 Milan Italy

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60

solenoid current (A)

dark

cur

rent

(pC

)

(3) Dark current data 2

Dark current is maximum at solenoid current 50A, corresponding maximum magnetic field is about 1.90KGauss

1.0µs 13.8MW

2006.10.04 –10.06 Milan Italy

(3) Dark current data 3

dark current (1.5us)

735.5

20833.3

0

5000

10000

15000

20000

25000

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0forward power to Gun (MW)

dark

cur

rent

(pC

)/RF

pulse 1.5us(now)

1.5us(2 years ago)

20833.3pC / 735.5pC = 28

2006.10.04 –10.06 Milan Italy

Summary

• LUCX has generated 300nC/100bunch electrons• Comparing to dark current from the RF gun two

years ago, dark current level decrease greatly, gun condition improves greatly.

• Now the dark current is not big problem on our operation conditions.

• QE of the cathode decrease from 3.7% to 0.7% during one year

2006.10.04 –10.06 Milan Italy

END

- 10 10 30 50 70 90 110 130 150

2006.10.04 –10.06 Milan Italy

3.5 10-63.6 10-63.7 10-63.8 10-63.9 10-6410-64.1 10-6

20

30

40

50

60