12

Beyond the RF photogunJom Luiten

Seth Brussaard Fred Kiewiet - RF photogun

Benjamin CanuelDimitri Vyuga - DC photogun

Marieke de Loos - GPTBas van der Geer - GPT

Jan BotmanMarnix van der Wiel

Eindhoven University of Technology

Netherlands

12

Target bunch:

Energy: 10 MeVPeak Current: 1 kA X-ray SASE FELEmittance: 1 mm mrad Length: 100 fs injector for LWFACharge: 100 pC

Pancake bunches & Extreme fields

high pressure SF6

Laser-triggered spark gap:

1 GV/m during 1 ns (BNL)

50 fs UV laser pulse

30 m

1 mm

Keep bunches short during acceleration - no magnetic compression!

12Outline

• Pancake bunches accelerated in uniform fields– Space charge fields in pancake bunches– Longitudinal phase space– Transverse phase space

• GPT simulations– 2 MeV DC gun– DC + RF gun (2+8 MeV)– 10 MeV 1 GV/m DC gun?

• Experimental progress– 2 MeV DC gun– 8 MeV, 2½ cell RF photogun

12Space charge fields in pancake bunches

Lab frame

0 MeV L/R=0

2 MeV L/R=0.3

10 MeV L/R=1.2R

L

bunch=100 fs, R=0.5 mm, Q=100 pC

Rest frame

Rr

Er/Es

0

0R

R

0 L/2-L/2

1

1

1

Long. field Radial field

-½

Ez/Es

0 L/2-L/2

½

½

-½

-L/2 L/2

Z

0

-½ 0

½

12Longitudinal phase space

z0

Ez

E0

Ez

z0

E0

E0-Es

z0

Ez

E0

E0-Es

Peak current in surface charge regime:

E0=1 GV/m, R=0.5 mm I0=4 kA

(cf. Serafini et al., NIMA 387, 305 (1997))

+ -

++ -

-

12Transverse phase space

x

px• Worst case: L/R=0 geometry

• Negligible radial motion

RMS normalized emittance:

1γγlogπREε

Q0.038ε 2

00xn,

0.5 1 1.5 2

0.1

0.2

0.3

0.4

0.5

0.6

Z (mm)

n,x

( mm mrad)E0=1 GV/m R=0.5 mm Q=100 pC

Incl. thermal emittance

Excl. thermal emittance

12GPT simulations: 2 MeV DC gun (1)

(Van der Geer et al., PRE 65, 046501 (2002))

Q=100 pC, R=0.5 mm, E0=1 GV/m

Evaluate bunch at z=4.5 mm

Highly nonlinear radial fields at iris!

12GPT simulations: 2 MeV DC gun (2)

Longitudinal phase space at z=4.5 mm:

•FWHM bunch length 73 fs

•Energy spread ~ 2%

•Peak current 1.2 kA

In agreement with simple model!

100 fs

2%

1.2 kA

12GPT simulations: 2 MeV DC gun (3)

RMS normalized transverse emittance at z=4.5 mm:

n=0.4 mm mrad

• nonlinear electrostatic emittance compensation! (spherical aberration cancels nonlinear space charge fields)

• uniform field results in agreement with simple model.

12GPT simulations: DC + RF gun (1)

2 MV across 2 mm

RF

Cylindrically symmetric

2½ cell 8 MeV S-band RF booster

Solenoid field strength 0.42-0.52 T

12GPT simulations: DC + RF gun (2)

Hollow cathode surface (radius of curvature 3 mm) to minimize beam divergence.

12GPT simulations: DC + RF gun (3)

0.0 0.1 0.2 0.3 0.4 0.5

z [m]

0

1

2

3

4

5

Rad

ius

[mm

]

Particle trajectories for B=0.46 T

At z = 0.2 m:

• n = 1.0 mm mrad

• zFWHM/c = 250 fs

• I = 400 A

12GPT simulations: DC + RF gun (4)

12GPT simulations : DC + RF gun (5)

En

ergy

[M

eV]

Position [fs]

Cu

rren

t [A]

9.4

9.6

-200 0 2000

200

400

Longitudinal phasespace

At z=200 mm.

12

10 MeV 1GV/m DC gun?

Requirement: pulses with picosecond rise time!

t0t1t2t3

electrons

trigger laser

Electrons accelerated by transverse E-fields in coaxial lines

12

Spark-gap plasma column

3 mm

4 m

m

2 MV

1 ns

Near threshold / Tunneling ionization:

Laser intensity > 1018 W/m2

High power Ti:Sapphire laser:

50 mJ / 50 fs = 1 TW

plasma diameter = 0.3 mm

1.5 MV in less than 1 picosecond

E < 1GV/m

12GPT simulations: 12 MeV pulsed DC gun (1)

2 MV, 1 ns pulse

Laser trigger

Accelerator structure

Electron bunch

Spark-gaps

25 mm

EM field calculations:

CST Microwave Studio

12GPT simulations: 12 MeV pulsed DC gun (2)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

z [mm]

0.0

0.2

0.4

0.6

0.8

1.0

Em

itta

nce

[p

i mm

mra

d]

Em

itta

nce

[p

i mm

mra

d]

z [mm]

30.52 30.54 30.56 30.58

z [mm]

0

200

400

600

800

1000

Cu

rre

nt

[A]

Cu

rre

nt

[A]

z [mm]I = 0.7 kA; n=0.6 mm mrad @ 12 MeV

12Experimental status: Pulsed DC acceleration

Brookhaven National lab:

1 MV (5 MV?) pulses

1 GV/m @ 1 ns

1 ns

Under construction @ TU/e:

2.5 MV pulses

12Experimental status: RF Photogun

TE10 modeTEM mode

Movable short

Doorknob(DESY)

10 MW2.998 GHz2½ cell8 MeV

12f0=2998.0 MHz

Q=6500

Reflection < 1%

Superfish:

Measurement:

0-mode: -mode:Intermediate mode:

-40

-30

-20

-10

0

Ref

lect

ion

[dB

]

2.992 2.994 2.996 2.998 3.000

Frequency [GHz]

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

|E/E

max

|

Position [mm]

Superfish Measurement

12Experimental setup RF photogun

12First results RF photogun:

Beam on phosphorescent screen:

Energy : 5..6 MeV

Charge : 100 .. 500 pC

UV laser power : 50 J

1 mm

12Summary

RF photogun DC/RF hybrid

multi stage DC?Operational

Single stage DC DC + RF Multi stage DC

energy 2 MeV 10 MeV 12 MeV

current 1.2 kA 0.4 kA 0.7 kA

n0.4 mm mrad 1.0 mm mrad 0.6 mm mrad