Progress on the ALPHA-X X project

Transcription

1 Progress on the ALPHA-X X project Dino Jaroszynski University of Strathclyde evelopment f laser akefield ccelerators nd radiation ources

2 Outline Overview and involvement Plasma wakefield accelerator Plasma channel All-optical and conventional injectors Diagnostics RF Photoinjector Undulator

3 Advanced Laser Plasma High-energy ccelerators towards X-rays: : ALPHA-X ( a-x ) Collaborative project involving groups from the UK, EU and US Strathclyde injector, laser-plasma & FEL: experiments & theory CCLRC RAL theory & exps.: wakefield studies and diagnostics Oxford plasma channels Imperial all-optical injector, laser-plasma acceleration CCLRC Daresbury Injector, undulator & FEL Abertay-Dundee injector, electron diagnostics & FEL St Andrews University theory GOALS: Accelerate to 1 GeV in 1cm using a wakefield accelerator. Demonstrate coherent radiation source: FEL

4 People involved in project Dino Jaroszynski, Klaas Wynne, Bob Bingham, Ken Ledingham, Albert Reitsma, Yuri Saviliev, Slava Pavlov, Riju Issac, David Jones, Bernhard Ersfeld, Steven Jamison, Jordan Gallcher, Andrey Lyachev, Enrico Brunetti, Mark Wigggins Strathclyde Karl Krushelnick, Bucker Dangor, Zulfika Najmudin, Malte Kaluza, Alex Thomson, Stuart Mangles Imperial College Bob Bingham, Henry Hutchinson, Peter Norreys, Raul Trines, Kate Lancaster, Chris Murphy RAL (CCLRC) Simon Hooker, Justin Wark, Keith Burnett, Ian Walmsley, David Spence, Tony Gonsalves Oxford Allan Gillespie, Allan McCloud, Steven Jamison, Abertay-Dundee Alan Cairns St Andrews Mike Poole, Mike Dykes, Jim Clark Daresbu (CCLRC) Gennady Shvets Austin Texas Nicola Piovella Milan Terry Garvey LAL Orsay Antonio Ting NRL Chan Joshi, Warren Mori UCLA Tom Katsouleas USC Padma Shukla Bochum Tito Mendonca, Luis Silver IST Portugal Fred van Goor, Arsen Khachatryan, Kees van der Geer, Marieke Loos, Bas van der Geer Th Netherlands Andrey Savilov, Vladimir Bratman IAP, Nizhniy Novgorod

5 Plasma as accelerator UCLA: Tajima + Dawson 1979 Surfer analogy: electron rides the crest of the plasma wave Challenge: to produce and ride the wave

6 Coherent radiation source: Free-electron electron laser (FEL) Use output of wakefield accelerator to drive FEL Take advantage of electron beam properties Coherent spontaneous emission: prebunched FEL Operate in superradiant regime: FEL amplifier Potential compact future x-ray FEL Need GeV beam with < 50 fs electron beam with I > 1 ka Need to operate in superradiant regime to provide useable beam: SASE alone is not adequate Need to consider injection

7 Wakefield acceleration λ p

8 Advanced accelerator medium: plasma Conventional Accelerator Gradients < 100 3GHz 1 TeV Collider - 50 km long Peak gradients limited by breakdown Plasma Accelerator Ultimate disposable structure High fields: E = 4π en n MAX 1 0 ee MAX ωp k = c n n 0 1 ev / cm Example: n 0 = cm -3 ee MAX = 1 GeV/cm Cavity structure frequency determined by feature size. In plasma by density.

9 Lasers: ASTRA, TOPS and OXFORD TOPS (Strathclyde): 5TW source (800nm, 50fs 10Hz 250mJ) upgrade to 1J (20 TW) ASTRA (RAL): 10 TW source (800nm, 50fs, 10Hz, 500 mj) upgrade to 1J Oxford: 2 TW source (800nm, 50fs, 10Hz, 100 mj) Strathclyde: 6 MeV High-brightness sub-picosecond photoinjector being constructed

10 ALPHA-X X Programme Main areas of research: Injectors (conventional and all-optical) Laser-plasma wake-field acceleration Plasma capillaries Free-electron laser (FEL) Beam transport systems Diagnostics 6.5 MeV photo-injector 1 J 40 fs 800 nm beam transport wakefield accelerator plasma filled capillary GeV beam transport γ = period undulator FEL or synchrotron source IR to VUV SASE or SACSE λ u = γ λ 2 ( 1 a u 2 5 2γ = laser

11 General layout of ALPHA-X Radiation containment chamber ALPHA-X BEAM LINE Steering coils 1 m Main solenoid LASER to photocathode and diagnostics section Gate valve with foil interface LASER to plasma accelerator section PLASMA ACCELERATOR SECTION LASER output section Faraday cup UNDULATOR UNDULATOR Faraday cup DIPOLE F RF photoinjector RF waveguide Focusing magnet (capillary) Gate valve with foil interface Focusing magnet (undulator) DIPOLE (electron energy spectrum)

12 Detailed layout of experiment ALPHA-X BEAM LINE Steering coils 1 m Main solenoid LASER to photocathode and diagnostics section Gate valve with foil interface LASER to plasma accelerator section PLASMA ACCELERATOR SECTION LASER output section Faraday cup RF photoinjector RF waveguide Focusing magnet (capillary) Gate valve with foil interface Focusing magnet (undulator) DIPOLE (electron energy spectrum)

13 Imperial/RAL/Strathclyde All-optical injection experiments on ASTRA 1018 Wcm-2 in 25 µm spot a0 ~ δγ 3% γ ne~ 2 x 1019cm nm mj 40 fs F/16 mirror S. Mangles et al. Nature 2005 TOP Terahertz to Optical Pulse Source CARE 2004 HAMBURG

14 Electron Spectrometer Imperial/RA Spectrometer: B-field: 30 mt T 1keV - 500MeV 2 poles 50 cm detector plate Detector: Fujifilm BAS Spatial resolution: µm Sensitivity: dictated by noise Electrons: Decreasing Energy Central Spot

15 Modelling of Laser Wakefield Acceleration Strathclyd laser pulse envelope electrostatic wakefield bunch density energy density of wakefield laser pulse envelope dynamics: ponderomotive wakefield excitation, electron bunch acceleration, phase slippage, beam loading z-vgt (units of λp)

16 Laser pulse envelope dynamics Strathclyd laser pulse amplitude: a 0 laser pulse energy depletion rate: ω d ~ a 02 ω s laser pulse envelope plasma density modulation Fourier spectrum of laser pulse z-vgt (units of λp) Linear regime: a 0 ²«1, ω d «ω s : pulse energy loss through photon deceleration without envelope modulation, static wakefield, low energy efficiency Nonlinear regime: a 0 ²~ 1, ω d ~ ω s : pulse energy loss through photon deceleration and strong envelope modulation, dynamic wakefield, better energy efficiency k / k0

17 Acceleration Strathclyd energy gain limited by dephasing, caused by difference between velocities of electron and wakefield scaling γ E L deph n n n 1/ 2 3/ 2 1 p p p v el c > v wf favours low plasma density v g separatrix log(γ) electron orbit laser pulse intensity note logarithmic energy scale ζ/c (fs)

18 Photon kinetic theory Strathclyde/RAL describes photon collective behaviour in plasma with ray-tracing equations frequency change due to spatio-temporal refractive index variation valid if refractive index changes slowly compared to optical cycle/wavelength refractive index determined by quasi-static plasma electron density profile frequency goes up/down in accelerating/decelerating part of wakefield ω ω 0 photon orbits separatrix pulse intensity example: photon dynamics in a plasma wave ζ/c (fs)

19 Beam quality Strathclyd Optimised acceleration minimum energy spread occurs at same time as maximum average energy Trade-off between maximising efficiency and minimising energy spread efficiency (%) energy spread (%) bunch length (µm) trapped fraction average energy (GeV) charge (normalised) charge (normalised) charge (normalised)

20 Strathclyde Injection scheme to minimize energy spread, bunch duration short compared to plasma period very sensitive to timing jitter between laser pulse and electron bunch an interesting option: injection in front of laser pulse initial electron energy low bunch slips backward through laser pulse electrons get trapped behind laser pulse wakefield acts as relativistic mirror and accelerates electrons to high energy scheme crucially depends on overlap of focusing and accelerating regions for nonlinear or narrow channel wakefields e: A. Khachatryan, Phys. Rev. E 65, (2002)

21 Beam transport issues Strathclyde laser pulse guided in plasma channel electrons: trapping should occur before divergence long Rayleigh length & small spot size requires low emittance & low energy low energy causes problems with space charge & beam-plasma interactions, as bunch is subject to transverse two-stream instability εl ( σ / γ ) 2

22 Strathclyde Trapping and acceleration dynamics γ [µm] Ln γ example: z-vt (z-vt) 4 MeV beam in 3.5 x cm -3 plasma Laser pulse acts as filter, rejecting electrons outside spot size Initial bunch spot size can be larger than laser spot size (i.e. if we accept losses) Contrast with conventional injection requires small bunch spot to avoid energy sprea Accelerated bunch has low transverse emittance

23 Oxford/Strathclyd Capillary: preformed plasma waveguide electrodes r nr () = n + δ n r w m Plasma capillary waveguide r 0 = πreδn r 0 = 150 µm capillary n (0) = cm -3 I > W/cm 2 Plasma formed by electrical discharge between electrodes 2 5 cm phase-matching: tapered capillary

24 Plasma waveguide formation Oxford After t ~ 80 ns plasma in quasi equilibrium. Ohmic heating of plasma balanced by conduction of heat to wall 1 d rdr 2 rκe + σ E = dt dr 0 Solution of the heat flow equation yields scaling relation for matched spot size: W M [µm] = a[µm] ( -3 N [cm ]) e 1/ 4

25 Abertay/Strathclyde Diagnostics: electro-optic optic detection single shot spectral encoding cross-correlation Electrons from gas-jets (ATLAS) FELIX electron bunch measurements FEL far-infrared measurements

26 Spectral decoding Measure probe intensity I(λ) known (initial) λ(t) infer I(t) Not suitable for ultra short electron bunches (i.e. <500fs FWHM) Abertay/Strathclyde 1.7ps FWHM Wilke et al. Phys. Rev. Lett (2002). TOP Terahertz to Optical Pulse Source CARE 2004 HAMBURG

27 Abertay/Strathclyde ross-correlation temporal decoding Temporal to spatial mapping of optical probe pulse Avoids problems of inseparability of frequency-time Decoding time-resolution ~ 30 fs

28 Abertay Comparison of temporal & spectral decoding simultaneous measurements Confirms time resolution improvements of TD technique Degradation of SD signal in excellent agreement with calculation NO free parameters in SD calculation based on bunch profile inferred from TD and measured laser parameters. (PRL September 2004) FELIX TOP Terahertz to Optical Pulse Source CARE 2004 HAMBURG

29 Abertay Synchronisation: bunch timing jitter timing jitter ~ bunch duration FELIX TOP Terahertz to Optical Pulse Source CARE 2004 HAMBURG

30 Real time monitoring Abertay Bunch profile modified b changing the buncher and accelerator phase. (FELIX - December 200 FELIX

31 Raman scattering as a plasma diagnostic Strathclyde Stokes and anti-stokes via stimulated Raman backscattering measure of plasma density Plasma channel allows interaction with maximum intensity over many Rayleigh lengths. H 2 H 2 electrode Probe beam pump beam Capillary

32 Density measurements from Stokes and anti-stokes postions Strathclyde Colliding pulses

33 RF Photoinjector

34 Flat and curved cathodes

35 Strathclyde/Daresbur Undulator and Free-Electron Laser Focussing undulator (Daresbury) 200 period Undulator period 1.5 cm Minimum gap 5 mm Undulator parameter a u ~ 1

36 cottish Universities ummer School in hysics USSP 2005 th 29 th August th t Andrews 2-week summer school on The interaction of intense laser radiation with plasma TOPICS i) inertial fusion ii) wakefield accelerators iii) advanced radiation sources