[en] Self-induced laser line sweeping of various regimes of sweep direction is reported for an experimental ytterbium fiber laser. The regimes involve sweeping from shorter to longer wavelengths ($1076\text{nm}\to <!--\; ???\; -->1083$ nm)—so-called normal self-sweeping; from longer to shorter wavelengths ($1079\text{nm}\to <!--\; ???\; -->1073$ nm)—so-called reverse self-sweeping; and a mixed regime in which a precarious balance of the normal and reverse sweeping exists and the sweep direction can change between consecutive sweeps. The regimes of sweeping were selected by changing the pump wavelength only. A detailed explanation of this sweep direction dynamics is presented based on a semi-empirical model. This model also provides a way to predict the sweep direction of fiber lasers based on other rare-earth-doped laser media. (letter)

[en] The amplified spontaneous emission from a superluminescent diode was frequency doubled in a periodically poled lithium niobate waveguide crystal. The temporally incoherent radiation of such a superluminescent diode is characterized by a relatively broad spectral bandwidth and thermal-like photon statistics, as the measured degree of second order coherence, $g^{(2)}(0)=1.9\pm <!--\; ??\; -->0.1$, indicates. Despite the non-optimized scenario in the spectral domain, we achieve six orders of magnitude higher conversion efficiency than previously reported with truly incoherent light. This is possible by using single spatial mode radiation and quasi phase matched material with a waveguide architecture. This work is a principle step towards efficient frequency conversion of temporally incoherent radiation in one spatial mode to access wavelengths where no radiation from superluminescent diodes is available, especially with tailored quasi phase matched crystals. The frequency doubled light might find application in imaging, metrology and quantum optics experiments. (letter)

[en] Molecular high-order harmonic generation of H${}_2^{+}$ and its isotopes is investigated by numerical simulations of the non-Born–Oppenheimer time-dependent Schrödinger equations. The general characteristic of the typical high-order harmonic generation (HHG) spectra for the H${}_2^{+}$ molecule indicates that only the odd harmonics can be generated. Here we show that how the initial vibrational states and nuclear dynamics break down this standard characteristic, i.e. a redshift or blueshift of the harmonics appears. We investigate the effect of the initial vibrational states on the redshift or blueshift of the HHG spectrum under trapezoidal laser pulses. The ionization probability and time-frequency analysis are used to illustrate the physical mechanism of the shift of the harmonics. We also show the HHG spectra from the different isotopes of H${}_2^{+}$ molecule with different initial vibrational states. (letter)

[en] The hydrodynamic flow generated by rippled shocks and rarefactions (Richtmyer–Meshkov like flows) is presented. When a corrugated shock travels inside an homogeneous fluid, it leaves pressure, density and velocity perturbations in the compressed fluid. The velocity perturbations generated in the composed fluid are inherently rotational. Vorticity is an important quantity in order to determine the asymptotic rate of growth in the linear stage. The size of the strongest vortices generated by the rippled shocks is analyzed as a function of the shock Mach number for different boundary conditions downstream. Comparison to experiments and simulations is provided for the RMI in the shock and rarefaction reflected cases and the validity of the growth law ${\mathrm{\psi <!--\; ??\; -->}}_{\mathrm{\infty <!--\; ???\; -->}}+\mathrm{\delta <!--\; ??\; -->}{v}_i^{\mathrm{\infty <!--\; ???\; -->}}t$ is emphasized. (paper)

[en] The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma operation was started with 5 MW electron cyclotron resonance heating (ECRH) power and five inboard limiters. Core plasma values of $T_{\text{e}}>8$ keV, $T_{\text{i}}>2$ keV at line-integrated densities $n\approx <!--\; ???\; -->3\cdot <!--\; ???\; -->{10}^{19}{\text{m}}^{-<!--\; ???\; -->2}$ were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found. The energy confinement times are in line with the international stellarator scaling, despite unfavourable wall conditions, i.e. large areas of metal surfaces and particle sources from the limiter close to the plasma volume. Well controlled shorter hydrogen discharges at higher power (4 MW ECRH power for 1 s) and longer discharges at lower power (0.7 MW ECRH power for 6 s) could be routinely established after proper wall conditioning. The fairly large set of diagnostic systems running in the end of the 10 weeks operation campaign provided first insights into expected and unexpected physics of optimized stellarators. (paper)

[en] Polydimethylsiloxane (PDMS) elastomers are used for many applications, such as microfluidics and micro-engineering. This paper presents a new process of integrating soft elastomers into a silicon structure without any assembly steps. The novelty of this process is the use of only one deep reactive ion etch (DRIE) instead of two or more as developed in previous works. Thus, this fabrication process allows the use of elastomers that are usually not compatible with some fabrication processes. Compliant flexures with different interference shapes have been designed, simulated, fabricated, and characterized for generic use and notably for micro-robot joints and compliant micro-systems. The experimental results show that the 400 μm  ×  400 μm cross-sectional area samples can be bended more than ${60}^{\circ <!--\; ???\; -->}$ without delamination. (technical note)

[en] Recent upgrades in H-1 power supplies have enabled the operation of the H-1 experiment at higher heating powers than previously attainable. A heating power scan in mixed hydrogen/helium plasmas reveals a change in mode activity with increasing heating power. At low power ($<<!--\; <\; -->50$ kW) modes with beta-induced Alfvén eigenmode frequency scaling are observed. At higher power modes consistent with an analysis of nonconventional global Alfvén eigenmodes (GAEs) are observed, the subject of this work. We have computed the mode continuum, and identified GAE structures using the ideal MHD solver CKA and the gyrokinetic code EUTERPE. An analytic model for ICRH-heated minority ions is used to estimate the fast ion temperature from the hydrogen species. Linear growth rate scans using a local flux surface stability calculation, LGRO, are performed. These studies demonstrate drive from the radial spatial gradient of circulating particles whose speed is significantly less than the Alfvén speed, and are resonant with the mode through harmonics of the Fourier decomposition of the strongly shaped heliac magnetic field. They reveal drive is possible with a small $(n_f/n_0<<!--\; <\; -->0.2)$ hot energetic tail of the hydrogen species, for which $T_f><!--\; >\; -->300\mathrm{e}\mathrm{V}$. Local linear growth rate scans are also complemented with global calculations from CKA and EUTERPE. These qualitatively confirm the findings from the LGRO study, and show that the inclusion of finite Larmor radius effects can reduce the growth rate by a factor of up to ten, and increases the marginal stability fast ion temperature by a factor of two. Finally, a study of damping of the global mode with the thermal plasma is conducted, computing continuum damping , and the damping arising from finite Larmor radius and parallel electric fields (via resistivity). We find that continuum damping is of order 0.1% for the configuration studied. A similar calculation in the cylindrical plasma model produces a frequency 35% higher and a damping 30% of the three-dimensional result: this confirms the importance of strong magnetic shaping to the frequency and damping. The inclusion of resistivity lifts the damping to $\mathrm{\gamma <!--\; ??\; -->}/\mathrm{\omega <!--\; ??\; -->}=-<!--\; ???\; -->0.189$. Such large damping is consistent with experimental observations that in absence of drive the mode decays rapidly (∼0.1 ms). (paper)

[en] Experiments with the new neutral beam injection source of TCV have been performed with high fast-ion fractions (>20%) that exhibit a clear reduction of the loop voltage and a clear increase of the plasma pressure in on- and off-axis heating configurations. However, good quantitative agreement between the experimental data and TRANSP predictions is only found when including strong additional fast-ion losses. These losses could in part be caused by turbulence or MHD activity as, e.g. high frequency modes near the frequency of toroidicity induced Alfvén eignmodes are observed. In addition, a newly installed fast-ion D-alpha (FIDA) spectroscopy system measures strong passive radiation and, hence, indicates the presence of high background neutral densities such that charge-exchange losses are substantial. Also the active radiation measured with the FIDA diagnostic, as well as data from a neutral particle analyzer, suggest strong fast-ion losses and large neutral densities. The large neutral densities can be justified since high electron temperatures (3–4 keV), combined with low electron densities (about $2\times <!--\; ??\; -->{10}^{19}$ m⁻³) yield long mean free paths of the neutrals which are penetrating from the walls. (paper)

[en] The recent determination of the β-function of the quantum chromodynamics (QCD) running coupling ${\mathrm{\alpha <!--\; ??\; -->}}_{\stackrel{¯<!--\; ??\; -->}{\mathrm{M}\mathrm{S}}}(Q^2)$ to five-loops, provides a verification of the convergence of a novel method for determining the fundamental QCD parameter ${\mathrm{\Lambda <!--\; ??\; -->}}_s$ based on the light-front holographic approach to nonperturbative QCD. The new five-loop analysis, together with improvements in determining the holographic QCD nonperturbative scale parameter κ from hadronic spectroscopy, leads to an improved precision of the value of ${\mathrm{\Lambda <!--\; ??\; -->}}_s$ in the $\stackrel{¯<!--\; ??\; -->}{\mathrm{M}\mathrm{S}}$ scheme close to a factor of two; we find ${\mathrm{\Lambda <!--\; ??\; -->}}_{\stackrel{¯<!--\; ??\; -->}{\mathrm{M}\mathrm{S}}}^{(3)}=0.339\pm <!--\; ??\; -->0.019\mathrm{G}\mathrm{e}\mathrm{V}$ for $n_f=3$, in excellent agreement with the world average, ${\mathrm{\Lambda <!--\; ??\; -->}}_{\stackrel{¯<!--\; ??\; -->}{\mathrm{M}\mathrm{S}}}^{(3)}=0.332\pm <!--\; ??\; -->0.017\mathrm{G}\mathrm{e}\mathrm{V}$. We also discuss the constraints imposed on the scale dependence of the strong coupling in the nonperturbative domain by superconformal quantum mechanics and its holographic embedding in anti-de Sitter space. (paper)

[en] This paper presents the design, analysis and testing of a novel decoupled 2-DOF flexure-based micropositioning stage driven by piezoelectric-actuators (PZTs). In order to enlarge the travel range, a Scott-Russell mechanism and leverage mechanism are arranged in series, constituting a two-grade displacement amplifier to conquer the small displacement of the PZT. The design micropositioning stage is composed of symmetrically distributed flexure modules and each flexure module comprises compound parallelogram flexure beams serving as input decoupling, which allows the output decoupling by employing the tridimensional double compound parallelogram flexure mechanism. Based on the analytical model of both the amplifier and the XY stage established in static and dynamic analysis, the dimensions and performance of the stage has been conducted, which are verified by finite element analysis with ANSYS Workbench and prototype experiment with the fabricated prototype of the designed stage. It can be seen that the workspace of the developed stage is $148.11\mathrm{\mu <!--\; ??\; -->}\text{m}\times <!--\; ??\; -->149.73\mathrm{\mu <!--\; ??\; -->}$m with the maximum output coupling errors of 0.693% and 0.637% in the y and x directions. The experimental results demonstrate that the proposed micropositioning stage possesses good performance in trajectory tracking and can achieve a wide range of precise positioning. (paper)