Advanced Energy Systems Inc. P.O. Box 7455, Princeton, NJ 08543-7455 Phone:(609) 514-0319

Advanced Energy Systems Inc.P.O. Box 7455, Princeton, NJ 08543-7455

Phone: (609) 514-0319Fax: (609) 514-0318

E-mail: [email protected] Jangho Park Jangho Park

Ultrafast High-brightness Electron Source

mailto:[email protected]

ATF USER MEETING April 26-27, 2012

Ultrafast High-Brightness Electron Source

2 Putting Accelerator Technology to Work

Introduction

• Generation and preservation of ultrafast high-brightness electron beams is one of the major challenges in accelerator R&D.

• In order to generate and preserve the ultrafast high-brightness electron beam, transverse and longitudinal space charge effects have to be considered.

• Several approaches to achieving ultra-short bunches have been explored such as velocity bunching and magnetic compression.

• However, each option suffers drawbacks (extra RF and bulky) in achieving a compact ultrafast high-brightness source.

• A compact ultrafast high-brightness electron source is demanded for Ultrafast/Femtosecond Electron Diffraction (UED/FED) and advanced accelerator experiment.




Bunch Lengthening

• For a high-aspect-ratio (short bunch length) electron bunch, the asymptotic bunch length due to the space charge forces of a uniformly accelerated bunch, ignoring the drive laser duration and assuming prompt response from a copper cathode where the laser spot radius is kept constant, is expressed by:

• Bunch lengthening due to the space charge is inversely proportion to the square of the bunch radius and the square of the accelerating field.

• Bunch length stretches due to the longer path lengths of the outer particles compared with electrons closer to the axis.

• Bunch lengthening due to the geometrical effects is proportional to the square of the beam radius.

.)( 20

2

2

cERQ

emctsc




R=2.55mmR=3.35mm

R=4.25mm

Rc=30mm

Gun Cavity Design

• Gun Cavity Design Aspects– Higher Acceleration Field with x-band RF– Utilizing a curved cathode– Axisymmetry coupling– Copper cathode




Cavity Field Configuration

Axial Field

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-200

-100

0

100

200

Elec

tric

Fiel

d [M

V/m

]

Position from Cathode [cm]

Cathode A Cathode C

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-40

-20

0

20

40

60

Elec

tric

Fiel

d [M

V/m

]

Position from Cathode [cm]

Cathode A, r = 2mm axis Cathode C, r = 2mm axis

Radial Field

• Cathode A: Curved, Rc=30mm• Cathode C: Flat cathode




Transverse Bunch Size Evolution

• Cathode A: Curved, Rc=30mm• Cathode C: Flat cathode




Beam Dynamics Results

0 20 40 60 80 10040

80

120

160

200

240

280

320

360

Bun

ch L

engt

h [f

s]

Bunch Charge [pC]

Cathode A, RB=1.0mm

Cathode A, RB=1.3mm

Cathode B, RB=0.9mm Cathode B, R

B=1.3mm

Cathode C, RB=1.0mm

Cathode C, RB=1.3mm

0 10 20 30 40 500

20

40

60

80

100

120

140

160

180

Bunc

h Le

ngth

[fs]

Beam Position from Cathode [cm]

Cathode A, 50 pC Cathode B, 50 pC Cathode C, 50 pC

• Cathode A: Curved, Rc=30mm• Cathode B: Curved, Rc=40 mm, laser pulse shaping• Cathode C: Flat cathode

25fs

UV




Beam Dynamics Results

Beam dynamics simulation results at the z = 50 cm target location.

Parameter UnitValue by Cathode Type

A B C A BPeak Accelerating Gradient [MV/m] 230 230 230 230 230

Beam Energy [MeV] 2.7 2.7 2.7 2.7 2.7

Bunch Charge [pC] 50 50 50 100 100

Bunch Length [femtosec] 99 89 169 182 166Beam Current [A] 506 562 296 550 602

Energy Spread [keV] 14.8 13 11.7 26.2 21.2

Target Beam Size [mm] 1.36 1.54 1.64 1.72 1.92

Transverse rms Emittance [mm-mrad] 0.69 0.67 1.16 1.94 1.69Brightness [1014 A/m2] 10.6 12.6 2.2 1.5 2.1

• Cathode A: Curved, Rc=30mm• Cathode B: Curved, Rc=40 mm, laser pulse shaping• Cathode C: Flat cathode




Temperature Results

20.00 20.51 21.01 21.52 22.03 22.53 23.04 23.55 24.05 24.56

Temperature K

Mean Temperature of Gun21.94 C

• 3-D ANSYS• 4.0x10-5 duty factor

Cooling channels

20C cooling water




Displacement Results

Vector Sum Displacements inches

.189e-5 .143e-4 .266e-4 .390e-4 .514e-4 .637e-4 .761e-4 .884e-4 .101e-3 .113e-3

Constrained axiallyAxial = X

Held in y and Z at cavity center

Vector Sum Displacements inches

.189e-5 .143e-4 .266e-4 .390e-4 .514e-4 .637e-4 .761e-4 .884e-4 .101e-3 .113e-3

Constrained axiallyAxial = X

Held in y and Z at cavity center




Freq Shift

•Frequency Shift determined from 3-D ANSYS original geometry eigenvalue solution and new geometry from displacements resulting from RF/Thermal loads.

•Frequency Shift from Displacements: -159 kHz

•Frequency shift from rf losses can be offset with cooling temperature




Gun Assembly

Ding HolesFor Tuning(4 per Cell)

Cathode Cell

Full Cell

Vacuum Cross2.75 CF Flanges

Vacuum Grille

RF Input

Waveguide toCoax Transition

Beamline Flange1.33 Conflat




FEED (2)

RETURN

RETURN

INTERNAL PLENUMS

Cooling Arrangement




12.5

RF In

0 30 50 Z [cm]

Laser

Bucking Coil

Solenoid

RF Gun

Laser Mirror

1.Beam Viewer with Faraday Cup2.Pin-hole

3.CTR Target

E-beam

Ion Pump

RF Window

Linear motionIon Pump

UV View Port

Linear motion Camera

InterferometerIR View port

Ion pump

150

1.Beam Viewer

Camera

SteererSteerer

Gate Valve

Proposed Experimental Setup




Proposed Experimental Setup




Conclusions

• AES is developing a ultrafast high-brightness electron source for FED experiments and advanced accelerator applications.

• The ultrafast high-brightness electron beam from X-band curved cathode RF gun is feasible.

• Numerical calculations are improved 5 to 10 times brighter than S-band PC gun.

• Thermal-structural and RF frequency shift analysis shows that the design of this electron gun running at 4.0x10-5 duty factor is robust.

• Planning installation and tests at ATF would start July 2013.• The ATF beam tests will include;

– validate the source,– provide a useful data point on the bunch length and emittance.

• This work was supported by DOE SBIR Phase I grant No. DE-SC0006210. The Phase II grant is anticipating in July 2012.

• Thank You!!!

Documents

Advanced Energy Systems Inc. P.O. Box 7455, Princeton, NJ 08543-7455 Phone:(609) 514-0319