Emittance and energy spread measurements of relativistic electrons from laser-driven accelerator

Transcription

1 Emittance and energy spread measurements of relativistic electrons from laser-driven accelerator OUTLINE ALPHA-X Project Introduction on laser wakefield accelerator (LWFA) LWFA as a light source Electron beam characterization: emittance and energy spread DIPAC 2011 Workshop, Hamburg May 2011 Grace G. Manahan

2 Advanced Laser Plasma High-energy Accelerator towards X-rays Collaboration of several universities and research centre in UK (Strathclyde, St. Andrews, Dundee, Abertay-Dundee, Cockroft Institute) Beam line is located at University of Strathclyde (Group Leader: Prof. Dino Jaroszynski) ALPHA-X beamline (~ 10m) ALPHA-X Group in Strathclyde

3 ALPHA-X beamline Laser: 900 mj, 35 fs, 800 nm, initial a o 1.0 Plasma source: 2 mm He gas jet n e cm -3 Electron spectrometer Electron characterization: emittance and energy spread Uses quadrupole lenses to aid focusing Can measure electron energy up to 660 MeV (B=1.65 T) energy resolution is ~ % (energy dependent)

4 Laser wakefield accelerators (LWFA) Requires I laser Wcm -2 (a o > 1 for λ=1μm) Relativistic self focusing leads to plasma guiding Ponderomotive force leads to charge separation forming the plasma wakefield Electrons are self-trapped at the back of the bubble and accelerated to high energy Maximum electron s energy at the dephasing length: Conditions for bubble formation: k R 2 p a o max 2a o ( laser / 3 plasma ) 2 2 P Pcrit ( [ fs]/ [ m]) 30GW

5 Laser wakefield accelerators (LWFA) few mm-cm acceleration length 80 MeV 1GeV electron energy ΔE/E ~ 3% G.R. Geddes et al. ΔE/E ~ 10% ΔE/E ~ 6% J. Faure et al. S.P. D. Mangles et al.

6 LWFA as coherent radiation source? Radiation brightness: I B 4 2 x y Radiation coherence: x, y rad 4

7 Beam emittance Phase space ellipse x' Transformation of phase ellipse for different locations in a drift space x' area = x x z = 0 z = z 1 z = z 2 x2 2xx x Twiss parameters tan The shape and orientation of the ellipse changes but the area is the same (Liouville Theorem)

8 Beam emittance Emittance as the area of trace space: dxdx ellipse Emittance as a beam quality: rms rms beam width x rms beam divergence 2 x 2 x x 2 correlation between x & x (~zero at the beam waist) Other definitions of emittance: norm where: 4 rms z z v z / c z [Lapostelle s definition]

9 Pepper-pot emittance measurement Detector: Ce:YAG screen Coupled with a 14-bit CCD camera 10 μm resolution Ce:YAG mask Transmission light image of the tungsten sheet Pepper-pot mask: tungsten sheet 27 x 27 grid 25 ±5 μm diameter 150 μm separation distance 29.5 cm 60 cm

10 Pepper-pot emittance measurement x m,j L j th rms x 2 x 2 x x 2 mask In all beamlets: Mean position: p x n j x m, j j Mean divergence: p x n j x j j screen In j th beamlet: Total charge: Mean divergence: rms divergence: xj j, x j L n j x j j N p n j j x x xx p 2 1 n ( x x) 2 N 2 j mj j p n j, ( xmj x) N j x j p 2 n j x j x j Nxx N j Advantages of pepper-pot method Single shot measurement Less prone to space charge effect Can obtain x and y emittance at one measurement

11 Stability of the electron beam 20 (a) 15 counts 10 5 x = 1.4 mrad horizontal position (mrad) Electron beam as seen in the Ce:YAG screen The electron beam has a good pointing stability (the deviation is less than one spot size) counts (b) y = 1.3 mrad vertical position (mrad)

12 Transverse emittance measurement n,x = 1.3 -mm-mrad n,y = 1.5 -mm-mrad n,x = 1.9 -mm-mrad n,y = 1.8 -mm-mrad n,x = 2.2 -mm-mrad n,y = 2.5 -mm-mrad

13 Transverse emittance measurement 64 out of 400 consecutive shots were measured 10 (a) 10 (b) Count 5 Count n,x [ mm mrad] [ mm mrad] y n 2.2 ± 0.7 -mm-mrad 2.3 ± 0.6 -mm-mrad E. Brunetti et al. PRL, 105, (2010)

14 2D PIC code OSIRIS simulation n (-mm-mrad) rms beam radius (m) The emittance growth is dictated by the transverse forces acting on the electron before and during the capture in the plasma bubble time [1/ p ] Time evolution of normalised transverse emittance and beam radius of the electron bunch

15 Electron beam s energy spectra Electron s energy (MeV)

16 Electron beam s energy spectra Beam loading effect There seems to be scaling of energy spread and central energy to charge M. Wiggins et al., PPCF 52, (2010).

17 Summary Transverse emittance and energy spread were measured to characterised the electron beam from laser wakefield accelerator. The normalised emittance is n,x,y = 2.2 ± 0.7, 2.3 ± 0.6 -mm-mrad, which is comparable to that of a linear accelerator (measurement was resolution limited). Central energy and energy spread shows scaling with the charge. The brightness for this accelerator is approximately equal to: B Am1 rad1 which is suitable for driving a compact coherent radiation FEL. 17/17