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

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: 10 MeV Peak Current: 1 kA  X-ray SASE FEL Emittance: 1  mm mrad Length: 100 fs  injector for LWFA Charge: 100 pC

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

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 ½ 1 Er/Es 0 MeV L/R=0 -L/2 L/2 -½ ½ 1 R 2 MeV L/R=0.3 -L/2 L/2 -½ 1 R -L/2 L/2 Z -½ ½ 10 MeV L/R=1.2 R L r R

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

Transverse phase space px 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)

GPT simulations: 2 MeV DC gun (1) (Van der Geer et al., PRE 65, 046501 (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!

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!

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.

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 0.42-0.52 T

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

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

GPT simulations: DC + RF gun (4)

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]

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!

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

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

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 mrad @ 12 MeV

Experimental status: Pulsed DC acceleration Brookhaven National lab: 1 MV (5 MV?) pulses 1 GV/m @ 1 ns Under construction @ TU/e: 2.5 MV pulses 1 ns

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

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]

Experimental setup RF photogun

First results RF photogun: Beam on phosphorescent screen: Energy : 5..6 MeV Charge : 100 .. 500 pC UV laser power : 50 J 1 mm

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?