Dynamics of relativistic transparency and optical shuttering in expanding overdense plasmas

Overdense plasmas are usually opaque to laser light. However, when the light is of sufficient intensity to drive electrons in the plasma to near light speeds, the plasma becomes transparent. This process—known as relativistic transparency—takes just a tenth of a picosecond. Yet all studies of relativistic transparency so far have been restricted to measurements collected over timescales much longer than this, limiting our understanding of the dynamics of this process. Here we present time-resolved electric field measurements (with a temporal resolution of ~ 50 fs) of the light, initially reflected from, and subsequently transmitted through, an expanding overdense plasma. Our result provides insight into the dynamics of the transparent-overdense regime of relativistic plasmas, which should be useful in the development of laser-driven particle accelerators, X-ray sources and techniques for controlling the shape and contrast of intense laser pulses.

Pulse shape measurements using single shot-frequency resolved optical gating for high energy (80 J) short pulse (600 fs) laser

Relevant to laser based electron/ion accelerations, a single shot second harmonic generation frequency resolved optical gating (FROG) system has been developed to characterize laser pulses (80 J, ∼600 fs) incident on and transmitted through nanofoil targets, employing relay imaging, spatial filter, and partially coated glass substrates to reduce spatial nonuniformity and B-integral. The device can be completely aligned without using a pulsed laser source. Variations of incident pulse shape were measured from durations of 613 fs (nearly symmetric shape) to 571 fs (asymmetric shape with pre- or postpulse). The FROG measurements are consistent with independent spectral and autocorrelation measurements. © 2010 American Institute of Physics.

Bright Laser-Driven Neutron Source Based on the Relativistic Transparency of Solids

Neutrons are unique particles to probe samples in many ?elds of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently
limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to
produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers suf?cient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to
produce a forward peaked neutron ?ux with a record yield, on the order of 1010 n=sr. We present results comparing the two acceleration mechanisms and the ?rst short pulse laser generated neutron radiograph.

Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments. © 2011 American Institute of Physics.

Coherent synchrotron emission from electron nanobunches formed in relativistic laser-plasma interactions

Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense laser-solid interactions have, so far, been consistent with Doppler upshifted reflection from collective relativistic plasma oscillations-the relativistically oscillating mirror mechanism(1-6). Recent theoretical work, however, has identified a new interaction regime in which dense electron nanobunches are formed at the plasma-vacuum boundary resulting in coherent XUV radiation by coherent synchrotron emission(7,8) (CSE). Our experiments enable the isolation of CSE from competing processes, demonstrating that electron nanobunch formation does indeed occur. We observe spectra with the characteristic spectral signature of CSE-a slow decay of intensity, I, with high-harmonic order, n, as I(n) proportional to n(-1.62) before a rapid efficiency rollover. Particle-in-cell code simulations reveal how dense nanobunches of electrons are periodically formed and accelerated during normal-incidence interactions with ultrathin foils and result in CSE in the transmitted direction. This observation of CSE presents a route to high-energy XUV pulses(7,8) and offers a new window on understanding ultrafast energy coupling during intense laser-solid density interactions.

Molecular diagnosis of the myeloproliferative neoplasms: UK guidelines for the detection of JAK2 V617F and other relevant mutations

Molecular genetic assays for the detection of the JAK2 V617F (c.1849G>T) and other pathogenetic mutations within JAK2 exon 12 and MPL exon 10 are part of the routine diagnostic workup for patients presenting with erythrocytosis, thrombocytosis or otherwise suspected to have a myeloproliferative neoplasm. A wide choice of techniques are available for the detection of these mutations, leading to potential difficulties for clinical laboratories in deciding upon the most appropriate assay, which can lead to problems with inter-laboratory standardization. Here, we discuss the most important issues for a clinical diagnostic laboratory in choosing a technique, particularly for detection of the JAK2 V617F mutation at diagnosis. The JAK2 V617F detection assay should be both specific and sensitive enough to detect a mutant allele burden as low as 13%. Indeed, the use of sensitive assays increases the detection rate of the JAK2 V617F mutation within myeloproliferative neoplasms. Given their diagnostic relevance, it is also beneficial and relatively straightforward to screen JAK2 V617F negative patients for JAK2 exon 12 mutations (in the case of erythrocytosis) or MPL exon 10 mutations (thrombocytosis or myelofibrosis) using appropriate assays. Molecular results should be considered in the context of clinical findings and other haematological or laboratory results.

Temporal and geographical distribution of measles virus genotypes

The nucleotide sequence encoding the C terminus of the nucleocapsid protein of measles virus (MV) is the most variable in the genome. The sequence of this region is reported for 21 new MV strains and for virus RNA obtained from cases of subacute panencephalitis (SSPE) tissue. The nucleotide sequence of a total of 65 MV strains has been analysed using the CLUSTAL program to determine the relationships between the strains. An unrooted tree shows that eight different genotypes can be discerned amongst the sequences analysed so far. The data show that the C-terminal coding sequence of the nucleocapsid gene, although highly variable between strains, is stable in a given strain and does not appear to diverge in tissue culture. It therefore provides a good 'signature' sequence for specific genotypes. The sequence of this region can be used to discriminate new imported viruses from old 'endemic' strains of MV in a geographical area. The different genotypes are not geographically restricted although some appear to be the mainly 'endemic' types in large areas of the world. In global terms there appears to be at least four co-circulating genotypes of MV. The low level of divergence in the Edmonston lineage group isolated before 1970 indicates that some isolates are probably laboratory contaminants. This applies to some SSPE isolates such as the Halle, Mantooth and Horta-Barbosa strains as well as some wild-type isolates from that period.

Sequence divergence of measles virus haemagglutinin during natural evolution and adaptation to cell culture

Phylogenetic analysis of the sequence of the H gene of 75 measles virus (MV) strains (32 published and 43 new sequences) was carried out. The lineage groups described from comparison of the nucleotide sequences encoding the C-terminal regions of the N protein of MV were the same as those derived from the H gene sequences in almost all cases. The databases document a number of distinct genotype switches that have occurred in Madrid (Spain). Well-documented is the complete replacement of lineage group C2, the common European genotype at that time, with that of group D3 around the autumn of 1993. No further isolations of group C2 took place in Madrid after this time. The rate of mutation of the H gene sequences of MV genotype D3 circulating in Madrid from 1993 to 1996 was very low (5 x 10(-4) per annum for a given nucleotide position). This is an order of magnitude lower than the rates of mutation observed in the HN genes of human influenza A viruses. The ratio of expressed over silent mutations indicated that the divergence was not driven by immune selection in this gene. Variations in amino acid 117 of the H protein (F or L) may be related to the ability of some strains to haemagglutinate only in the presence of salt. Adaptation of MV to different primate cell types was associated with very small numbers of mutations in the H gene. The changes could not be predicted when virus previously grown in human B cell lines was adapted to monkey Vero cells. In contrast, rodent brain-adapted viruses displayed a lot of amino acid sequence variation from normal MV strains. There was no convincing evidence for recombination between MV genotypes.

Modified Thomson spectrometer design for high energy, multi-species ion sources

A modification to the standard Thomson parabola spectrometer is discussed, which is designed to measure high energy (tens of MeV/nucleon), broad bandwidth spectra of multi-species ions accelerated by intense laser plasma interactions. It is proposed to implement a pair of extended, trapezoidal shaped electric plates, which will not only resolve ion traces at high energies, but will also retain the lower energy part of the spectrum. While a longer (along the axis of the undeflected ion beam direction) electric plate design provides effective charge state separation at the high energy end of the spectrum, the proposed new trapezoidal shape will enable the low energy ions to reach the detector, which would have been clipped or blocked by simply extending the rectangular plates to enhance the electrostatic deflection.

Predictors and dynamics of postpartum relapses in women with multiple sclerosis

Several studies have shown that pregnancy reduces multiple sclerosis (MS) relapses, which increase in the early postpartum period. Postpartum relapse risk has been predicted by pre-pregnancy disease activity in some studies.