Plasma based compact light sources. Amichay Perry

Transcription

1 Plasma based compact light sources Amichay Perry

2 Conventional undulators\wigglers Monochromatic & Tunable XRays High brilliance Good angular collimation [1] [2] The fundamental wavelength is given by: 2 2 u K cm, K 1 u So in order to generate hard xrays we need γ 10 4 Which corresponds to electrons with >5GeV Can we make it smaller?

3 Conventional accelerators Charged particles are accelerated using electric fields [3] Electrostatic accelerators are limited by breakdowns (electric discharge) E If AC fields are used we need something to transform the TEM mode propagating in vacuum to a mode with longitudinal electric field Up: paschen s curve describing the DC breakdown voltage between two electrodes as a function of pressure*distance But these are also limited by breakdowns to energy gains of a few tens MeV/m [4]

4 Plasma oscillations 1. We start with a neutral plasma a fluid composed of massive positive ions and electrons with zero net charge 2. Imagine that the electrons are displaced slightly from their equilibrium positions by δx, while the ions are fixed 3. Then we get a net negative charge (per unit area) n e e δx and a net positive charge (per unit area) n e e δx, separated by δx (polarization) x E 4. The resulting electric field is: E n e x e 0 And the restoring force will cause the electrons to oscillate with a characteristic frequency: 2 p 1 nee khz 9 ne m cm 0 e

5 Plasma Waves But how can we displace the electrons? A propagating EM wave has an associated momentum So we are left with an effective charge separation: When this wave interacts with the plasma it will push the light electrons away, while the heavier ions remain in place In the trailing edge electrons are attracted back to the positive ions which remained behind Electrons are pushed away in the leading edge

6 Plasma Waves The wavefronts generated by the pulse will be synchronized if The condition for resonant excitation is met: In that case a plasma wave is created, with a phase velocity equal to the velocity of the generating pulse (the group velocity of the laser, close to the speed of light) L P 2 p [5] Another scheme to excite plasma waves is by using two lasers with frequencies: ω 1, ω 2 > ω P (Why?) ω 1 ω 2 = ω P (Beatwave excitation) The plasma had converted our TEM mode into a mode with a longitudinal electric field!

7 Plasma Acceleration Because the plasma is not limited by breakdowns it can sustain high electric fields (up to 100GeV/m) in the ion channel created in the wake of the laser pulse If a bunch (pulse) of already relativistic electrons is injected into that channel, they can surf the plasma wave and get accelerated to higher and higher energies [6] [7,8]

8 Plasma Undulators Principle of operation The electric field pattern generated by the plasma wave can also be used to wiggle the electrons and produce xrays The unulator period is equal to the plasma wavelength: [9] [10] In the plasma undulator the plasma wave propagates perpendicular to the electron beam direction

9 Plasma undulators The strength parameter So K is determined by the intensity of the laser pulse and can approach unity (wiggler regime) for intense beams

10 Plasma undulator typical figures of merit Parameter Value 30THz 35fsec 10um 200 (~100MeV) 10keV (on axis) mm for 100 undulations E max 0 0 p

11 Summary Plasma based technology looks like an exciting and promising route towards compact light sources Tunable high brilliance hard xrays seem to be feasible from cm scale devices Simulations indicate that the good properties of undulator radiation should be preserved. However, that strongly depends on the quality of the input electron beam

12 References tesla.desy.de Thank you & Enjoy the break!! 8. V. Malka, J. Faure, Y. Gauduel, E. Lefebvre, Principles and applications of compact laser plasma accelerators, Nature Physics 4, (2008) 9. S. Corde, K. Ta Phuoc, Plasma wave undulator for laseraccelerated electrons, Phys.Plasmas 18:033111, R. Williams, C. Clayton, C. Joshi, T. Katsouleas, Studies of classical radiation emission from plasma wave undulators, IEEE Transactions on Plasma Science, vol. 21, no. 1, p