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Brilliant, ultra-short vuv/xuv/x-ray photon beams
- Fig. A.2.2.1: Schematic setup for table-top X-FEL: A Petawatt laser pulse is focused on a capillary, in which “bubble” acceleration takes place; within the undulator the electrons produce ultra-brilliant X-ray pulses.
The overall goal of this subproject is to provide brilliant, ultra-short vuv/x-ray photon beams for a wide range of fundamental studies. Two approaches will be drawn upon, offering complementary source characteristics: a free-electron laser (<acronym title="table-top X-ray Free Electron Laser">TT-XFEL</acronym>) and vuv/xuv high-harmonics from laser-induced surface oscillations. In our pusuit of the TT-XFEL, whose parameters are expected - in the long run - to rival those of their large-scale, accelerator-based counterparts, we wish - until 2008 - to demonstrate the feasibility of its laser-driven vuv predecessors. With this first step successfully taken, we will pursue scaling the seed electron energy towards the GeV frontier, which is a prerequisite for realising TT-XFEL. The success of this enterprise will critically depend on the feasibility of building a (surely table-top) laser-plasma accelerator delivering ultra-short (τe≤10 fs), mono-energetic (ΔE/E<0.1 % at 1 GeV), low-emittance (≤1 mm⋅mrad), ultra-high-current (~200 kA) electron bunches.
With beam transport simulations taking into account space-charge effects as well as resistive wall wakefields we have studied the feasibility of table-top FELs1). This design study is based upon the same FEL-simulation tool as used for designing large-scale FELs2).
At present, we have already experimentally verified the feasibility of our miniature focusing and undulator system. This undulator has the world's smallest period of only 5 mm. The PFS-driven compact accelerator and the ultra-high-current electron beam emerging from it promise to reduce the size of the entire FEL source from the km-scale of accelerator-based X-FELs down to a few metres (Fig. A.2.2.1). Once demonstrated, we shall use this source - in collaboration with other MAP groups - for proof-of-principle experiments towards single-molecule imaging (
C.3), medical phase-contrast x-ray imaging, and small-angle x-ray scattering (D.1). If successfully demonstrated, the PFS-driven TT-XFEL will be compact enough to be considered for proliferation into hospitals.
- Fig. A.2.2.1: High harmonics from a few-cycle driven surface are predicted to result in an ultra-intense attosecond xuv/sxr pulse.
In order to produce brilliant photon beams with only few-atosecond duration, high harmonics will be generated by exposing surfaces to a relativistic few-cycle laser pulse from LWS-10 and later from PFS (Fig. A.2.2.2). For efficient generation of higher-order harmonics a laser beam is focused at oblique incidence onto a planar solid target. It instantly forms an overdense plasma, which reflects the laser pulse. Driven by the laser field, the target electron density starts to execute a non-linear excursion at the vacuum-plasma interface. This anharmonic oscillation, in turn, radiates an electromagnetic wave comprising higher-order harmonics of the incident driving laser field. The long-term goal here is to generate single attosecond xuv/sxr pulses with peak intensities ultimately exceeding the terawatt level and pulse durations approaching the few-attosecond regime for exciting applications in ultrafast (C.1.2) and high-field science (B.1.1).
Achieving these goals will critically rely on concomitant advances in “numerical experiments”. To this end, we shall pursue — in parallel to the experimental work outlined above — comprehensive numerical investigations by drawing on MPQ’s world-class expertise in this area and the supercomputing facilities available at the research campus at Garching. Our comprehensive preliminary studies resulted in the following predictions with regard to the achievable parameters of the pursued sources (HH = High Harmonics), revealing their complementarity in terms of pulse duration, brilliance, and wavelength:
| production mechanism | photons/shot (in 0.1% BW) | wavelength (in nm) | pulse duration | divergence (in mrad) | peak brilliance in photons/(smm2 mrad2 0.1% BW) |
|---|---|---|---|---|---|
| HH | ~2⋅109 | 2* | 5 as | 11 | ~1029 |
| TT-XFEL | ~2⋅1012 | 0.25 | 10 fs | 0.03 | ~1033 |
* 2 nm was the shortest wavelength resolved in our PIC simulations.
1) F. Grüner et al., “Design considerations for table-top, laser-based VUV and X-ray free electron lasers”, accepted for publication in Appl. Phys. B (http://arxiv.org/abs/physics/0612125).
2) S. Reiche, “GENESIS 1.3: a fully 3D time-dependent FEL simulation code”, Nucl. Instr. Meth. A 429, 243 (1999).