Martin Laux

Lorentz meets Fano in spectral line shapes: a universal phase and its laser control.

A Phase for Fano In spectroscopy, samples placed between a steady light source and a detector are characterized based on the relative intensities of light absorbed at different frequencies. Temporal behavior—the relaxation of a photoexcited state—can be indirectly inferred from the absorption band shapes. The advent of ultrafast laser technology has enabled increasingly sophisticated measurements directly in the time domain. Ott et al. (p. 716; see the Perspective by Lin and Chu) present an analytical framework to account for asymmetric band shapes, termed Fano profiles, on the basis of a phase shift in the temporal dipole response. An analytical framework bolstered by attosecond spectroscopy conveys a clear understanding of asymmetric spectral line shapes. [Also see Perspective by Lin and Chu] Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase ϕ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.

Reconstruction and control of a time-dependent two-electron wave packet

The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantum three-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect. However, although the motion of single active electrons and holes has been observed with attosecond time resolution, comparable experiments on two-electron motion have so far remained out of reach. Here we show that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium. The beat appears in attosecond transient-absorption spectra measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field. We tune the coupling between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scale quantum-mechanical calculations for the helium atom, we anticipate that multidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems. They might also provide a route to the site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactions.

Fractional high-order harmonic combs and energy tuning by attosecond-precision split-spectrum pulse control

We experimentally control high-order harmonic generation by applying a versatile few-cycle pulse-shape control method: splitting up a single broadband continuous laser spectrum into two sections and applying sub-femtosecond relative time delays. For certain time delays, fractional high-harmonic combs (noninteger harmonics) are generated which we find to result from the controlled interference of two attosecond pulse trains. We also observe time-delay-dependent energy-tunability of the high-order harmonics for an asymmetrically split spectrum consisting of a strong and a weak component. The tuning mechanism is quantitatively understood by the controlled modulation of the instantaneous driver frequency at the peak of the shaped laser pulse.

Characteristics and scaling of energy and particle losses during Type I ELMs in JET H-modes

Recent experiments on the Type I ELMy H-mode regime performed at JET with improved diagnostics have expanded the range of parameters for the study of Type I ELM energy and particle losses. Deviations from the standard behaviour of such losses in some areas of the Type I ELMy H-mode operating space have revealed that the ELM losses are correlated with the parameters (density and temperature) of the pedestal plasma before the ELM crash, while other global ELM characteristics (such as ELM frequency) are a consequence of the ELM-driven energy and particle flux and of the in-between ELM energy and particle confinement. The relative Type I ELM plasma energy loss (to the pedestal energy) is found to correlate well with the collisionality of the pedestal plasma, showing a weak dependence on the method used to achieve those pedestal plasma parameters: plasma shaping, heating, pellet injection and impurity seeding. Effects of edge plasma collisionality and transport along the magnetic field on the Type I ELM particle and energy fluxes onto the divertor target have also been observed. Two possible physical mechanisms that may give rise to the observed collisionality dependence of ELM energy losses are proposed and their consistency with the experimental measurements investigated: collisionality dependence of the edge bootstrap current with its associated influence on the ELM MHD origin and the limitation of the ELM energy loss by the impedance of the divertor target sheath to energy flow during the ELM event.

Impurity behaviour in the ASDEX Upgrade divertor tokamak with large area tungsten walls

At the central column of ASDEX Upgrade, an area of 5.5 m2 of graphite tiles was replaced by tungsten-coated tiles representing about two-thirds of the total area of the central column. No negative influence on the plasma performance was found, except for internal transport barrier limiter discharges. The tungsten influx ΓW stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for ΓW could be found. From these observations a penetration factor of the order of 1% and effective sputtering yields of about 10-3 could be derived, pointing to a strong contribution by light intrinsic impurities to the total \mbox{W-sputtering}. The tungsten concentrations ranged from below 10-6 up to a few times 10-5. Generally, in discharges with increased density peaking, a tendency for increased central tungsten concentrations or even accumulation was observed. Central heating (mostly) by ECRH led to a strong reduction of the central impurity content, accompanied by a very benign reduction of the energy confinement. The observations suggest that the W-source strength plays only an inferior role for the central W-content compared to the transport, since in the discharges with increased W-concentration neither an increase in the W-influx nor a change in the edge parameters was observed. In contrast, there is strong experimental evidence, that the central impurity concentration can be controlled externally by central heating.

Transport into and across the scrape-off layer in the ASDEX Upgrade divertor tokamak

The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the high-resolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η e = d(ln T)/d(ln n) ∼ 2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvature-driven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.

Properties of the new divertor IIb in ASDEX Upgrade

A new divertor configuration (DIV IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δ bot ) up to 0.48, the outer strike point region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV II). The inner part of the divertor strike point module remains unchanged, but at the divertor entrance a smooth transition to the central column is provided to minimize local hydrogen recycling. According to experiences with power handling in DIV II, ordinary fine grain graphite has been chosen for the outer strike point and, as before the tiles are slightly tilted in toroidal direction to hide the leading edges. A first characterization of DIV IIb reveals that the beneficial behaviour of DIV II is essentially maintained. There is an increase of the power density due to geometrical reasons at the outer target, whereas the divertor radiation for similar magnetic configurations is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains unchanged. A significant reduction (20%) of the L-H threshold is observed consistent with larger temperature gradients inside the separatrix just before the transition.

In situ characterization of few-cycle laser pulses in transient absorption spectroscopy

Attosecond transient absorption spectroscopy has thus far been lacking the capability to simultaneously characterize the intense laser pulses at work within a time-resolved quantum-dynamics experiment. However, precise knowledge of these pulses is key to extracting quantitative information in strong-field highly nonlinear light-matter interactions. Here, we introduce and experimentally demonstrate an ultrafast metrology tool based on the time-delay-dependent phase shift imprinted on a strong-field-driven resonance. Since we analyze the signature of the laser pulse interacting with the absorbing spectroscopy target, the laser pulse duration and intensity are determined in situ. As we also show, this approach allows for the quantification of time-dependent bound-state dynamics in one and the same experiment. In the future, such experimental data will facilitate more precise tests of strong-field dynamics theories.

The ASDEX Upgrade divertor IIb—a closed divertor for strongly shaped plasmas

A new divertor configuration (DIV-IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δbot) up to 0.48, the outer strikepoint region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV-II). The inner part of the divertor strikepoint module remains unchanged, but a smooth transition to the central column is provided at the divertor entrance to minimize local hydrogen recycling. An increase in power density is observed due to geometrical reasons at the outer target, whereas the divertor radiation for similar configurations and discharge conditions is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains as before. Detachment in L-mode discharges fits well into a scaling deduced from JET data and earlier ASDEX Upgrade data. A significant reduction (20%) of the L–H threshold is observed compared with DIV-II. Its density dependence is weaker than in the previous DIV-II configuration and there are hints for an influence of triangularity on power threshold. Finally, clear evidence for a parasitic plasma below the divertor roof baffle is found.

Experimental Evidence for Wigner’s Tunneling Time

Figure 1: The difference between the most probable photo-electron emission angle for argon and krypton: experiment and theories (with and without initial momentum and tunneling delay time) Tunneling of a particle through a barrier is one of the counter-intuitive properties of quantum mechanical motion. Thanks to advances in laser technology and generation of electric fields comparable to those electrons experience in atoms, new opportunities to dynamically investigate this process have been developed. For example, in the so-called attoclock measurements [1] the properties of the electron after tunneling are mapped on its emission direction after its interaction with the laser pulse. In this work we investigate the first hundred attoseconds of the electron dynamics during strong field tunneling ionization. We achieve a high sensitivity on the tunneling barrier thanks to two ameliorations to the attoclock principle. Using near-IR wavelength (1300 nm) we place firmly the ionization process in the tunneling regime and limit non-adiabatic effects. Furthermore, we compare the momentum distributions of two atomic species of slightly different atomic potentials (argon and krypton) being ionized under absolutely identical conditions. Experimentally, using a reaction microscope, we apply coincident electron-ion detection in combination with a gas-target that contains a mixture of the two species and succeed in measuring the 3D electronmomentum distributions for both targets simultaneously. Theoretically, the time resolved description of tunneling in strong-field ionization is studied using the leading quantum mechanical Wigner treatment. A detailed analysis of the most probable photoelectron emission for Ar and Kr (Fig. 1) allows testing the theoretical models and a sensitive check of the electron initial conditions at the tunnel exit. The agreement between experiment and theory provides a clear evidence for a non-zero tunneling time delay and a non-vanishing longitudinal momentum at this point [2].