Raphael Weingartner

Laser -driven soft-X-ray undulator source

High-intensity X-ray sources such as synchrotrons and free-electron lasers need large particle accelerators to drive them. The demonstration of a synchrotron X-ray source that uses a laser-driven particle accelerator could widen the availability of intense X-rays for research in physics, materials science and biology. Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research1; however, their dependence on large-scale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasma-wave accelerators2,3,4,5,6,7,8,9,10 provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray sources to the university-laboratory scale. Here, we demonstrate the generation of soft-X-ray undulator radiation with laser-plasma-accelerated electron beams. The well-collimated beams deliver soft-X-ray pulses with an expected pulse duration of ∼10 fs (inferred from plasma-accelerator physics). Our source draws on a 30-cm-long undulator11 and a 1.5-cm-long accelerator delivering stable electron beams10 with energies of ∼210 MeV. The spectrum of the generated undulator radiation typically consists of a main peak centred at a wavelength of ∼18 nm (fundamental), a second peak near ∼9 nm (second harmonic) and a high-energy cutoff at ∼7 nm. Magnetic quadrupole lenses11 ensure efficient electron-beam transport and demonstrate an enabling technology for reproducible generation of tunable undulator radiation. The source is scalable to shorter wavelengths by increasing the electron energy. Our results open the prospect of tunable, brilliant, ultrashort-pulsed X-ray sources for small-scale laboratories.

Short-pulse optical parametric chirped-pulse amplification for the generation of high-power few-cycle pulses

We report ultrabroadband optical parametric chirped-pulse amplification (OPCPA) with an output pulse energy of up to 250 μJ from an OPCPA stage pumped by short pulses of ~100 fs duration at 395 nm wavelength. In order to generate ultrahigh-power pulses in the few-cycle regime, such a short-pulse-pumped OPCPA scheme appears to be a promising route, by virtue of its inherently advantageous features. Firstly, the stretching and compression fidelity as well as the pulse contrast are increased due to the short pump- and seed-pulse durations. Additionally, the higher pump powers allow for using thinner OPA crystals, thereby increasing the amplification bandwidth that will support even shorter pulse durations. We present experimental results where the effective bandwidth of the seed pulses was increased in the OPCPA process resulting in a shortened transform-limited pulse duration in addition to the energy gain. The amplified pulses from OPCPA have been compressed to the sub-10-fs, few-cycle range by using chirped mirrors. Scaling of this short-pulse-pumped OPCPA technique for few-cycle-pulse generation to the highest (TW–PW) power levels is also planned (Petawatt Field Synthesizer project at the Max-Planck-Institut fur Quantenoptik).

Characterization and tuning of ultrahigh gradient permanent magnet quadrupoles

The application of quadrupole devices with high field gradients and small apertures requires precise control over higher order multipole field components. We present a new scheme for performance control and tuning, which allows the illumination of most of the quadrupole device aperture because of the reduction of higher order field components. Consequently, the size of the aperture can be minimized to match the beam size achieving field gradients of up to

The Petawatt Field Synthesizer: A New Approach to Ultrahigh Field Generation

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Cryogenic Undulator for a Table Top FEL

at good imaging quality. The characterization method based on a Hall probe measurement and a Fourier analysis was confirmed using the high quality electron beam at the Mainz Microtron MAMI.

OPA development on the Petawatt Field Synthesizer

The Petawatt Field Synthesizer (PFS) at MPQ will deliver few-cycle pulses at Petawatt power. Short-pulse OPCPA and a diode-pumped, CPA Yb:YAG pump laser are key technologies, and results of the ongoing development will be presented.

First milestone on the path toward a table‐top free‐electron laser (FEL)

A laser plasma accelerator is under development at the Max‐Planck‐Institut fur Quantenoptik in Garching. This accelerator will be part of an X‐ray table top FEL in the future. The FEL radiation will be produced with a small period in‐vacuum undulator. The coercivity of the magnetic material has to be sufficiently high in order to avoid demagnetization due to electron losses. The best performance can be achieved with a cryogenic permanent magnet undulator design. (Nd0.2Pr0.8)2Fe14B magnets are suited for low temperatures since they do not suffer from a spin reorientation. A new (Nd0.2Pr0.8)2Fe14B material has been characterized at various temperatures and the results are presented. Based on this material a 20 period prototype with a period length of 9 mm and a magnetic gap of 2.5 mm is currently under construction.

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

We report on recent OPCPA progress at the PFS system.We present a scheme for generating a broadband (700 nm-1400 nm) seed pulse for OPA, and a new preamplifier setup for the CPA pump laser chain.

Basic Concepts and Current Status of the Petawatt Field Synthesizer－A New Approach to Ultrahigh Field Generation

Latest developments in the field of laser‐wakefield accelerators (LWFAs) have led to relatively stable electron beams in terms of peak energy, charge, pointing and divergence from mm‐sized accelerators. Simulations and LWFA theory indicate that these beams have low transverse emittances and ultrashort bunch durations on the order of ∼10 fs. These features make LWFAs perfectly suitable for driving high‐brightness X‐ray undulator sources and free‐electron lasers (FELs) on a university‐laboratory scale. With the detection of soft‐X‐ray radiation from an undulator source driven by laser‐wakefield accelerated electrons, we succeeded in achieving a first milestone on this path. The source delivers remarkably stable photon beams which is mainly due to the stable electron beam and our miniature magnetic quadrupole lenses, which significantly reduce its divergence and angular shot‐to‐shot variation. An increase in electron energy allows for compact, tunable, hard‐X‐ray undulator sources. Improvements of the electr...