239 research outputs found
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Plasma crowbars in cylindrical flux compression experiments
We have done a series of one- and two-dimensional calculations of hard-core Z-pinch flux compression experiments in order to study the effect of a plasma on these systems. These calculations show that including a plasma can reduce the amount of flux lost during the compression. Flux losses to the outer wall of such experiments can be greatly reduced by a plasma conducting sheath which forms along the wall. This conducting sheath consists of a cold, dense high ..beta.., unmagnetized plasma which has enough pressure to balance a large field gradient. Flux which is lost into the center conductor is not effectively stopped by this plasma sheath until late in the implosion, at which time a layer similar to the one formed at the outer wall is created. Two-dimensionl simulations show that flux losses due to arching along the sliding contact of the experiment can be effectively stopped by the formation of a plasma conducting sheath
Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR
We present an ultra-low temperature study (down to T = 20 mK) of the nuclear
spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet
Mn12-ac. The nuclear spins act as local probes for the electronic spin
fluctuations, due to thermal excitations and to tunnelling events. In the
quantum regime (below T = 0.75 K), the nuclear SLR becomes
temperature-independent and is driven by fluctuations of the cluster's
electronic spin due to the quantum tunnelling of magnetization in the ground
doublet. The quantitative analysis of the nuclear SLR shows that the presence
of fast-tunnelling molecules, combined with nuclear intercluster spin
diffusion, plays an important role in the relaxation process.Comment: RevTex, 5 pages, 3 eps figures; presented at the Internation
Conference on Molecular Magnets (Valencia, 5 - 10 Oct. 2002); to be published
in Polyhedro
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Recent implosion experiments at Nova
Both electron (direct) and x-ray (indirect) driven implosions of DT targets have been done using approx.20 kJ of 0.35 ..mu..m light from the ten beam Nova laser facility. The direct drive targets (glass microballoons with nominal dimensions of 1000 ..mu..m x 2 ..mu..m and DT pressures of 12 to 14 atm) produced neutron yields in excess of 10/sup 13/ and fusion efficiencies >0.15%. Recent experiments will be described, with particular emphasis on measurements made using neutron diagnostics
Muon-spin rotation and magnetization studies of chemical and hydrostatic pressure effects in EuFe_{2}(As_{1-x}P_{x})_{2}
The magnetic phase diagram of EuFe(AsP) was
investigated by means of magnetization and muon-spin rotation studies as a
function of chemical (isovalent substitution of As by P) and hydrostatic
pressure. The magnetic phase diagrams of the magnetic ordering of the Eu and Fe
spins with respect to P content and hydrostatic pressure are determined and
discussed. The present investigations reveal that the magnetic coupling between
the Eu and the Fe sublattices strongly depends on chemical and hydrostatic
pressure. It is found that chemical and hydrostatic pressure have a similar
effect on the Eu and Fe magnetic order.Comment: 11 pages, 10 figure
Measurement of W Polarisation at LEP
The three different helicity states of W bosons produced in the reaction e+
e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W
decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to
measure the polarisation of W bosons, and its dependence on the W boson
production angle. The fraction of longitudinally polarised W bosons is measured
to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and
the second systematic, in agreement with the Standard Model expectation
Search for Anomalous Couplings in the Higgs Sector at LEP
Anomalous couplings of the Higgs boson are searched for through the processes
e^+ e^- -> H gamma, e^+ e^- -> e^+ e^- H and e^+ e^- -> HZ. The mass range 70
GeV < m_H < 190 GeV is explored using 602 pb^-1 of integrated luminosity
collected with the L3 detector at LEP at centre-of-mass energies
sqrt(s)=189-209 GeV. The Higgs decay channels H -> ffbar, H -> gamma gamma, H
-> Z\gamma and H -> WW^(*) are considered and no evidence is found for
anomalous Higgs production or decay. Limits on the anomalous couplings d, db,
Delta(g1z), Delta(kappa_gamma) and xi^2 are derived as well as limits on the H
-> gamma gamma and H -> Z gamma decay rates
Measurement of W Polarisation at LEP
The three different helicity states of W bosons produced in the reaction e+
e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W
decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to
measure the polarisation of W bosons, and its dependence on the W boson
production angle. The fraction of longitudinally polarised W bosons is measured
to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and
the second systematic, in agreement with the Standard Model expectation
Neutral-Current Four-Fermion Production in e+e- Interactions at LEP
Neutral-current four-fermion production, e+e- -> ffff is studied in 0.7/fb of
data collected with the L3 detector at LEP at centre-of-mass energies
root(s)=183-209GeV. Four final states are considered: qqvv, qqll, llll and
llvv, where l denotes either an electron or a muon. Their cross sections are
measured and found to agree with the Standard Model predictions. In addition,
the e+e- -> Zgamma* -> ffff process is studied and its total cross section at
the average centre-of-mass energy 196.6GeV is found to be 0.29 +/- 0.05 +/-
0.03 pb, where the first uncertainty is statistical and the second systematic,
in agreement with the Standard Model prediction of 0.22 pb. Finally, the mass
spectra of the qqll final states are analysed to search for the possible
production of a new neutral heavy particle, for which no evidence is found
Measurement of Exclusive rho+rho- Production in Mid-Virtuality Two-Photon Interactions and Study of the gamma gamma* -> rho rho Process at LEP
Exclusive rho+rho- production in two-photon collisions between a quasi-real
photon, gamma, and a mid-virtuality photon, gamma*, is studied with data
collected at LEP at centre-of-mass energies root(s)=183-209GeV with a total
integrated luminosity of 684.8pb^-1. The cross section of the gamma gamma* ->
rho+ rho- process is determined as a function of the photon virtuality, Q^2,
and the two-photon centre-of-mass energy, W_gg, in the kinematic region:
0.2GeV^2 < Q^2 <0.85GeV^2 and 1.1GeV < W_gg < 3GeV. These results, together
with previous L3 measurements of rho0 rho0 and rho+ rho- production, allow a
study of the gamma gamma* -> rho rho process over the Q^2-region 0.2GeV^2 < Q^2
< 30 GeV^2
Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays
Bose-Einstein correlations of both neutral and like-sign charged pion pairs
are measured in a sample of 2 million hadronic Z decays collected with the L3
detector at LEP. The analysis is performed in the four-momentum difference
range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be
smaller than that of charged pions. This result is in qualitative agreement
with the string fragmentation model
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