DISPERSION IN THE WOOD MOUSE, APODEMUS-SYLVATICUS - VARIABLE RESOURCES IN TIME AND SPACE

(1) The abundance and dispersion of a population of Apodemus sylvaticus was investigated with respect to tree seed availability and vegetative structure over three harvests.

The POINT-AGAPE survey - I. The variable stars in M31

For the purposes of identifying microlensing events, the POINT-AGAPE collaboration has been monitoring the Andromeda galaxy (M31) for three seasons (1999-2001) with the Wide Field Camera on the Isaac Newton Telescope. In each season, data are taken for one hour per night for roughly 60 nights during the six months that M31 is visible. The two 33 x 33 arcmin(2) fields of view straddle the central bulge, northwards and southwards. We have calculated the locations, periods and brightness of 35 414 variable stars in M31 as a by-product of the microlensing search. The variables are classified according to their period and brightness. Rough correspondences with classical types of variable star (such as Population I and II Cepheids, Miras and semiregular long-period variables) are established. The spatial distribution of Population I Cepheids is clearly associated with the spiral arms, while the central concentration of the Miras and long-period variables varies noticeably, the brighter and the shorter period Miras being much more centrally concentrated.

EXPERIMENTAL STUDIES OF LINE INTENSITY RATIO AS A DIAGNOSTIC OF RECOMBINING PLASMAS

Double slits have been incorporated in a flat field spectrometer to record spatially resolved and integrated spectra simultaneously. Variation of the absorbed irradiance and ionisation stage along the fibre plasmas has been monitored. By comparison of the spatially resolved and integrated resonance line ratios, it is found that the spatially integrated values deviated significantly from the real experimental circumstances due to nonuniformity along the plasmas.

Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser-solid interactions

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

Third harmonic order imaging as a focal spot diagnostic for high intensity laser-solid interactions

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime ( 10(19) Wcm(-2) is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (> 10(20) Wcm(-2)) an important application for the field of high laser contrast interaction science.

Temporally and spatially resolved measurements of multi-megagauss magnetic fields in high intensity laser-produced plasmas

We report spatially and temporally resolved measurements of self-generated multi-megagauss magnetic fields produced during ultrahigh intensity laser plasma interactions. Spatially resolved measurements of the magnetic fields show an asymmetry in the distribution of field with respect to the angle of laser incidence. Temporally resolved measurements of the self-generated third harmonic suggest that the strength of the magnetic field is proportional to the square root of laser intensity (i.e., the laser B-field) during the rise of the laser pulse. The experimental results are compared with numerical simulations using a particle-in-cell code which also shows clear asymmetry of the field profile and similar magnetic field growth rates and scalings.

Effect of laser intensity on fast-electron-beam divergence in solid-density plasmas

Metal foil targets were irradiated with 1 mu m wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10(19) W cm(-2), giving values of both I lambda(2) and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K-alpha emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with I lambda(2) and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.

Lateral electron transport in high-intensity laser-irradiated foils diagnosed by ion emission

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>= 10(19) W/cm(2)) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields (similar to 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.

Generation of a train of ultrashort pulses from a compact birefringent crystal array

A linear array of n calcite crystals is shown to allow the generation of a high contrast (> 10: 1) train of 2(n) high energy (> 100 mu J) pulses from a single ultrafast laser pulse. Advantage is taken of the pulse-splitting properties of a single birefringent crystal, where an incident laser pulse can be split into two pulses with orthogonal polarizations and equal intensity, separated temporally in proportion to the thickness of the crystal traversed and the difference in refractive indices of the two optic axes. In the work presented here an array of seven calcite crystals of sequentially doubled thickness is used to produce a train of 128 pulses, each of femtosecond duration. Readily versatile properties such as the number of pulses in the train and variable mark-space ratio are realized from such a setup. (c) 2007 Optical Society of America

Reduction of proton acceleration in high-intensity laser interaction with solid two-layer targets

Reduction of proton acceleration in the interaction of a high-intensity, picosecond laser with a 50-mu m aluminum target was observed when 0.1-6 mu m of plastic was deposited on the back surface (opposite side of the laser). The maximum energy and number of energetic protons observed at the back of the target were greatly reduced in comparison to pure aluminum and plastic targets of the same thickness. This is attributed to the effect of the interface between the layers. Modeling of the electron propagation in the targets using a hybrid code showed strong magnetic-field generation at the interface and rapid surface heating of the aluminum layer, which may account for the results. (c) 2006 American Institute of Physics.

Observations of the filamentation of high-intensity laser-produced electron beams

Filamented electron beams have been observed to be emitted from the rear of thin solid targets irradiated by a high-intensity short-pulse laser when there is low-density plasma present at the back of the target. These. observations are consistent with a laser-generated beam of relativistic electrons propagating through the, target. which is subsequently fragmented by a Weibel-like instability in the low-density plasma at the. rear. These, measurements are in agreement with particle-in-cell simulations and theory, since the filamentation instability is predicted to be dramatically enhanced when the electron beam density approaches that of the background plasma.

Laboratory measurements of 0.7 GG magnetic fields generated during high-intensity laser interactions with dense plasmas

We report measurements of ultrahigh magnetic fields produced during intense (similar to10(20) Wcm(-2) mum(2)) laser interaction experiments with solids. We show that polarization measurements of high-order vuv laser harmonics generated during the interaction (up to the 15th order) suggest the existence of magnetic field strengths of 0.7+/-0.1 GG in the overdense plasma. Measurements using higher order harmonics indicate that denser regions of the plasma can be probed. This technique may be useful for measurements of multi-GG level magnetic fields which are predicted to occur at even higher intensities.