1. Beyond the RF photogun Jom Luiten Seth Brussaard
Fred Kiewiet - RF photogun
Benjamin Canuel
Dimitri Vyuga - DC photogun
Marieke de Loos - GPT
Bas van der Geer - GPT
Jan Botman
Marnix van der Wiel
Eindhoven University of Technology
Netherlands

2. Pancake bunches & Extreme fields
1 mm
high pressure SF6
50 fs UV laser pulse
Laser-triggered spark gap:
1 GV/m during 1 ns (BNL)
Keep bunches short during acceleration - no magnetic compression!
Target bunch:
Energy: MeV
Peak Current: 1 kA  X-ray SASE FEL
Emittance:  mm mrad
Length: fs  injector for LWFA
Charge: pC

3. Outline 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
8 MeV, 2½ cell RF photogun

4. Space charge fields in pancake bunches
bunch=100 fs, R=0.5 mm, Q=100 pC
Long. field
Radial field
Lab frame
Rest frame
Ez/Es 1
Er/Es
0 MeV
L/R=0
-L/2
L/2 -½ 1 R
2 MeV
L/R=0.3
-L/2
L/2 -½ 1 R
-L/2
L/2 Z -½
10 MeV
L/R=1.2 R L r R

5. Longitudinal phase space
(cf. Serafini et al., NIMA 387, 305 (1997)) + - - + - + z Ez E0 z Ez E0
E0-Es Ez z E0
E0-Es
Peak current in surface charge regime:
E0=1 GV/m, R=0.5 mm  I0=4 kA

6. Transverse phase spacepx
Worst case: L/R=0 geometry
Negligible radial motion
RMS normalized emittance: x
Incl. thermal emittance
0.6
n,x
E0=1 GV/m R=0.5 mm Q=100 pC 
0.5
( mm mrad)
0.4
0.3
Excl. thermal emittance
0.2
0.1
0.5 1
1.5 2
Z (mm)

7. GPT simulations: 2 MeV DC gun (1)
(Van der Geer et al., PRE 65, (2002))
Evaluate bunch at z=4.5 mm
Q=100 pC, R=0.5 mm, E0=1 GV/m
Highly nonlinear radial fields at iris!

8. GPT simulations: 2 MeV DC gun (2)
100 fs
Longitudinal phase space at z=4.5 mm:
FWHM bunch length 73 fs
Energy spread ~ 2%
Peak current 1.2 kA 2%
1.2 kA
In agreement with simple model!

9. GPT 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.

10. GPT simulations: DC + RF gun (1)
2 MV across 2 mm
Cylindrically symmetric
2½ cell 8 MeV S-band RF booster RF
Solenoid field strength T

11. GPT simulations: DC + RF gun (2)
Hollow cathode surface (radius of curvature 3 mm) to minimize beam divergence.

12. GPT simulations: DC + RF gun (3)
At z = 0.2 m:
n = 1.0  mm mrad
zFWHM/c = 250 fs
I = 400 A
Particle trajectories for B=0.46 T

13. GPT simulations: DC + RF gun (4)

14. GPT simulations : DC + RF gun (5)
9.4
9.6
-200
200
400
Energy [MeV]
Longitudinal phasespace
At z=200 mm.
Current [A]
Position [fs]

15. Electrons accelerated by transverse E-fields in coaxial lines
10 MeV 1GV/m DC gun?
Electrons accelerated by transverse E-fields in coaxial lines t0 t1 t2 t3
electrons
trigger laser
Requirement: pulses with picosecond rise time!

16. Spark-gap plasma column
E < 1GV/m
3 mm
4 mm
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

17. GPT 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

18. GPT simulations: 12 MeV pulsed DC gun (2)
Emittance [pi mm mrad]
z [mm]
Current [A]
z [mm]
I = 0.7 kA; n=0.6  mm 12 MeV

19. Experimental status: Pulsed DC acceleration
Brookhaven National lab:
1 MV (5 MV?) pulses
1 1 ns
Under TU/e:
2.5 MV pulses
1 ns

20. Experimental status: RF Photogun
TE10 mode
TEM mode
10 MW
2.998 GHz
2½ cell
8 MeV
Doorknob
(DESY)
Movable short

21. Superfish: f0=2998.0 MHz Q=6500 Reflection < 1% 0-mode:
Intermediate mode:
-mode:
Measurement:
Superfish
1.0
Measurement
-10
0.8
Reflection [dB]
|E/Emax|
0.6
-20
0.4
-30
0.2
-40
0.0
2.992
2.994
2.996
2.998
3.000 20 40 60 80
100
120
140
160
Frequency [GHz]
Position [mm]

22. Experimental setup RF photogun

23. First results RF photogun:
Beam on phosphorescent screen:
Energy : MeV
Charge : pC
UV laser power : 50 J
1 mm

24. Summary DC/RF hybrid RF photogun Operational multi stage DC? n
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 n
0.4  mm mrad
1.0  mm mrad
0.6  mm mrad
DC/RF hybrid
RF photogun
Operational
multi stage DC?