High-energy acceleration phenomena in extreme radiation-plasma interactions

We simulate, using a particle-in-cell code, the chain of acceleration processes at work during the Compton-based interaction of a dilute electron-ion plasma with an extreme-intensity, incoherent gamma-ray flux with a photon density several orders of magnitude above the particle density. The plasma electrons are initially accelerated in the radiative flux direction through Compton scattering. In turn, the charge-separation field from the induced current drives forward the plasma ions to near-relativistic speed and accelerates backwards the non-scattered electrons to energies easily exceeding those of the driving photons. The dynamics of those energized electrons is determined by the interplay of electrostatic acceleration, bulk plasma motion, inverse Compton scattering and deflections off the mobile magnetic fluctuations generated by a Weibel-type instability. The latter Fermi-like effect notably gives rise to a forward-directed suprathermal electron tail. We provide simple analytical descriptions for most of those phenomena and examine numerically their sensitivity to the parameters of the problem

Depletion forces near a soft surface

We investigate excluded-volume effects in a bidisperse colloidal suspension near a flexible interface. Inspired by a recent experiment by Dinsmore et al. (Phys. Rev, Lett. 80, 409 (1998)), we study the adsorption of a mesoscopic bead on the surface and show that depletion forces could in principle lead to particle encapsulation. We then consider the effect of surface fluctuations on the depletion potential itself and construct the density profile of a polymer solution near a soft interface. Surprisingly we find that the chains accumulate at the wall, whereas the density displays a deficit of particles at distances larger than the surface roughness. This non-monotonic behavior demonstrates that surface fluctuations can have major repercusions on the properties of a colloidal solution. On average, the additional contribution to the Gibbs adsorbance is negative. The amplitude of the depletion potential between a mesoscopic bead and the surface increases accordingly.Comment: 10 pages, 5 figure

Geometry of lipid vesicle adhesion

The adhesion of a lipid membrane vesicle to a fixed substrate is examined from a geometrical point of view. This vesicle is described by the Helfrich hamiltonian quadratic in mean curvature; it interacts by contact with the substrate, with an interaction energy proportional to the area of contact. We identify the constraints on the geometry at the boundary of the shared surface. The result is interpreted in terms of the balance of the force normal to this boundary. No assumptions are made either on the symmetry of the vesicle or on that of the substrate. The strong bonding limit as well as the effect of curvature asymmetry on the boundary are discussed.Comment: 7 pages, some major changes in sections III and IV, version published in Physical Review

Beam based calibration of X-ray pinhole camera in SSRF

The Shanghai Synchrotron Radiation Facility (SSRF) contains a 3.5-GeV storage ring serving as a national X-ray synchrotron radiation user facility characterized by a low emittance and a low coupling. The stability and quality of the electron beams are monitored continuously by an array of diagnostics. In particular, an X-ray pinhole camera is employed in the diagnostics beamline of the ring to characterize the position, size, and emittance of the beam. The performance of the measurement of the transverse electron beam size is given by the width of the point spread function (PSF) of the X-ray pinhole camera. Typically the point spread function of the X-ray pinhole camera is calculated via analytical or numerical method. In this paper we will introduce a new beam based calibration method to derive the width of the PSF online

A NEW SCHEME FOR ELECTRO-OPTIC SAMPLING AT RECORD REPETITION RATES: PRINCIPLE AND APPLICATION TO THE FIRST (TURN-BY-TURN) RECORDINGS OF THz CSR BURSTS AT SOLEIL

Abstract The microbunching instability is an ubiquitous problem in storage rings at high current density. However, the involved fast time-scales hampered the possibility to make direct real-time recordings of theses structures. When the structures occur at a cm scale, recent works at UVSOR [1], revealed that direct recording of the coherent synchrotron radiation (CSR) electric field with ultra-high speed electronics (17 ps) provides extremely precious informations on the microbunching dynamics. However, when CSR occurs at THz frequencies (and is thus out of reach of electronics), the problem remained largely open. Here we present a new opto-electronic strategy that enabled to record series of successive electric field pulses shapes with picosecond resolution (including carrier and envelope), every 12 ns, over a total duration of several milliseconds. We also present the first experimental results obtained with this method at Synchrotron SOLEIL, above the microbunching instability threshold. The method can be applied to the detection of ps electric fields in other situations where high repetition rate is also an issue

High repetition-rate electro-optic sampling: Recent studies using photonic time-stretch

Single-shot electro-optic sampling (EOS) is a powerful characterization tool for monitoring the shape of electron bunches, and coherent synchrotron radiation pulses. For reaching high acquisition rates, an efficient possibility consists to associate classic EOS systems with the so-called photonic time-stretch technique [1]. We present recent results obtained at SOLEIL and ANKA using this strategy. In particular, we show how a high sensitivity variant of photonic time stretch [2] EOS enabled to monitor the CSR pulses emitted by short electron bunches at SOLEIL [3]. We could thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses stemming from the shortness of the electron bunches. Above this threshold, CSR emission is dominated by drifting structures appearing through spontaneous self-organization. We also consider the association of time-stretch and EOS for recording electron bunch near fields at high repetition rate. We present preliminary results obtained at ANKA, aiming at recording the electron bunch shape evolution during the microbunching instability

Extravasation of adhering vesicles

We study how the passage of lipid vesicles through a small pore can be induced by the difference in non-specific adhesion energy between the two sides of the substrate bearing the pore. This process is inspired from the extravasation of cells or liposomes from blood vessels, which involves adhesion binders. We study the adhesion-dominated regime and we show that the passage of a vesicle of volume V and area A is selective in terms of the reduced volume $v\sim V/A^{3/2}$. Extravasation occurs for adhesion ratios of order unity. We also consider the possibility of pressure-induced extravasation in the presence of adhesion. Finally, we propose a micro-device based on adhesion-induced extravasation, which is designed to sort vesicles according to their deflatedness

Rough wetting

If a rough surface is put in contact with a wetting liquid, the roughness may be spontaneously invaded depending on the surface pattern and the wetting properties of the liquid. Here, we study the conditions for observing such an imbibition and present practical achievements where the wetting properties of the surface can be predicted and tuned by the design of a solid texture. The contact angle of a drop on such a surface (where solid and liquid coexist) is discussed. Finally, the dynamics of the liquid film is found to obey a diffusive-type law, as in the case of porous wicking