Optimal Photon Energy Transfer on Titanium Targets for Laser Thrusters

Using two infrared pulsed lasers systems: a picosecond solid-state Nd:YAG laser with tunable repetition rate (400 kHz - 1MHz) working in a burst mode of multi-pulse train and a femtosecond Ti:Sapphire laser amplifier with tunable pulse duration inthe range of tens of femtoseconds up to tens of picoseconds, working in single-shot mode (TEWALASS facility from CETAL-NILPRP), we have investigated the optimal laser parameters for kinetic energy transfer to a titanium target for laser-thrustapplications. In the single-pulse regime, we controlled the power density by changing both duration and pulse energy. Inthe multi-pulse regime, the train’s number of pulses (burst length), and the pulse energy variation were investigated. Heatpropagation and photon reflection-based models were used to simulate obtained experimental results. In the single-pulseregime, optimal kinetic energy transfer was obtained for power densities of about 500 times the ablation threshold correspondingto the specific laser pulse duration. In multi-pulse regimes, the optimal number of pulses per train increases with the trainfrequency and decreases with the pulse power density. An ideal energy transfer efficiency resulting from our experiments andsimulations is close to around 0.02%.

Energy Flux into Near-Inertial Internal Waves Below the Surface Boundary Layer in the Global Ocean

Near-inertial internal waves (NIWs) are thought to play an important role in powering the turbulent diapycnal mixing in the ocean interior. Nevertheless, the energy flux into NIWs below the surface boundary layer (SBL) in the global ocean is still poorly understood. This key problem is addressed in this study based on a Community Earth System Model (CESM) simulation with a horizontal resolution of ~0.1° for its oceanic component and ~0.25° for its atmospheric component.The CESM shows good skill in simulating NIWs globally, reproducing the observed magnitude and spatial pattern of surface NIW currents and wind power on NIWs (WI). The simulated downward flux of NIW energy (FSBL) at the SBL base is positive everywhere. Its quasi-global integral (excluding the region within 5°S-5°N) is 0.13 TW, about one-third the value of WI. The ratio of local FSBL to WI varies substantially over the space. It exhibits an increasing trend with the enstrophy of balanced motions (BMs) and a decreasing trend with WI.The kinetic energy transfer from model-resolved BMs to NIWs is positive from the SBL base to 600 m but becomes negative further downwards. The quasi-global integral of energy transfer below the SBL base is two orders of magnitude smaller than that of FSBL, suggesting the resolved BMs in the CESM simulations making negligible contributions to power NIWs in the ocean interior.

Development of surface roughness length and drag parameterizations over deep seas

<p>Many formulations to determine the sea surface roughness length (z<sub>0</sub>) have been proposed in the past. The well-known Charnock’s equation is applied in most of the previous research. In this study, a different point of view is adopted to develop a new formulation. The starting point is an alternative method for surface roughness length calculation, i.e., the Lettau’s method. This method has already been validated onshore in the presence of obstacles over a domain; for obstacles with a defined cross-section perpendicular to the wind direction plane. Over deep waters, it is expected to find only one type of obstacle, i.e., consecutive waves forming straight lines. Different wave systems and the presence of swell add complexity to determine the sea surface profile. Hence, the adaptation of Lettau’s method seems reasonable, but the demonstrated dependency of z<sub>0</sub> to wave age cannot be neglected.</p><p>Wind-generated waves result from a kinetic energy transfer between the atmosphere and the sea surface. However this physical process is not represented in the well-known logarithmic law. While this effect can be neglected onshore, in offshore environments it can be significant, as 20% of the time z<sub>0</sub> is found to be over the expected range. Therefore, a kinetic energy transfer correction is included into an offshore logarithmic law. With an aerodynamic z<sub>0</sub>, achieved by the adaptation of the Lettau’s equation, and the new offshore logarithmic law, an empirical method for the kinetic energy transfer correction is proposed.</p>