926 research outputs found
Direct Photons at RHIC
The PHENIX experiment has measured direct photons in
GeV Au+Au collisions and p+p collisions. The fraction of photons due to direct
production in Au+Au collisions is shown as a function of and centrality.
This measurement is compared with expectation from pQCD calculations. Other
possible sources of direct photons are discussed.Comment: 7 pages, 5 figures, presented at Hot Quarks 2004, Taos, N
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Practical sublimation source for large-scale chromium gettering in fusion devices
This paper describes the technique of chromium gettering with a large-scale sublimation source which resembles in its design the VARIAN Ti-Ball. It consists of a hollow chromium sphere with a diameter of approximately 3 cm and an incandescent filament for radiation heating from inside the ball. While the fabrication of the source is described in a companion paper, we discuss here the gettering technique. The experimental arrangement consists of an UHV system instrumented for total- and partial-pressure measurements, a film-thickness monitor, thermocouples, an optical pyrometer, and appropriate instrumentation to measure the heating power. The results show the temperature and corresponding sublimation rate of the Cr-Ball as function of input power. In addition, an example of the total pumping speed of a gettered surface is shown
The Distance to Nova V959 Mon from VLA Imaging
Determining reliable distances to classical novae is a challenging but
crucial step in deriving their ejected masses and explosion energetics. Here we
combine radio expansion measurements from the Karl G. Jansky Very Large Array
with velocities derived from optical spectra to estimate an expansion parallax
for nova V959 Mon, the first nova discovered through its gamma-ray emission. We
spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different
imaging epochs. The first five epochs cover the expansion of the ejecta from
2012 October to 2013 January, while the final four epochs span 2014 February to
2014 May. These observations correspond to days 126 through 199 and days 615
through 703 after the first detection of the nova. The images clearly show a
non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D
modelling of optical spectroscopy, the radio expansion implies a distance
between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4
+/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the
prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red
giant companion in the V959 Mon system. V959 Mon also has a much lower
gamma-ray luminosity than other classical novae detected in gamma-rays to date,
indicating a range of at least a factor of 10 in the gamma-ray luminosities for
these explosions.Comment: 11 pages, 8 figures, 3 tables, submitted to ApJ 2015-01-21, under
revie
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Experiments on steady state particle control in Tore Supra and DIII-D
Particle control is playing an increasingly important role in tokamak plasma performance. The present paper discusses particle control of hydrogen/deuterium by wall pumping on graphite or carbonized surfaces, as well as by external exhaust with pumped limiters and pumped divertors. Wall pumping is ultimately a transient effect and by itself not suitable for steady state particle exhaust. Therefore, external exhaust techniques with pumped divertors and limiters are being developed. How wall pumping phenomena interact and correlate with these inherently steady state, external exhaust techniques, is not well known to date. In the present paper, the processes involved in wall pumping and in external pumping are investigated in an attempt to evaluate the effect of external exhaust on wall pumping. Some of the key elements of this analysis are: (1) charge-exchange fluxes to the wall play a crucial role in the core-wall particle dynamics, (2) the recycling fluxes of thermal molecules have a high probability of ionization in the scrape-off layer, (3) thermal particles originating from the wall, which are ionized within the scrape-off layer, can be directly exhausted, thus providing a direct path between wall and exhaust which can be used to control the wall inventory. This way, the wall can be kept in a continuous pumping state in the sense that it continuously absorbs energetic particles and releases thermal molecules which are then removed by the external exhaust mechanism. While most of the ingredients of this analysis have been observed individually before, the present evaluation is an attempt to correlate effects of wall recycling and external exhaust
The Peculiar Multi-Wavelength Evolution Of V1535 Sco
We present multi-wavelength observations of the unusual nova V1535 Sco
throughout its outburst in 2015. Early radio observations were consistent with
synchrotron emission, and early X-ray observations revealed the presence of
high-energy (>1 keV) photons. These indicated that strong shocks were present
during the first ~2 weeks of the nova's evolution. The radio spectral energy
distribution was consistent with thermal emission from week 2 to week 6.
Starting in week 7, the radio emission again showed evidence of synchrotron
emission and there was an increase in X-ray emission, indicating a second shock
event. The optical spectra show evidence for at least two separate outflows,
with the faster outflow possibly having a bipolar morphology. The optical and
near infrared light curves and the X-ray measurements of the hydrogen column
density indicated that the companion star is likely a K giant.Comment: 20 pages, 13 figures, under review at ApJ, updated to match the most
recent version submitted to the refere
Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare
Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a
major radio outburst of Cygnus X-3 in 2001 September are presented, measuring
the evolution of the spectrum of the source over three decades in frequency,
over a period of six days. Following the peak of the flare, as the intensity
declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to
nu^{-0.6}, after which the spectral index remains at this latter terminal
value; a trend previously observed but hitherto not satisfactorily explained.
VLBA observations, for the first time, track over several days the expansion of
a sequence of knots whose initial diameters are approximately 8
milliarcseconds. The light-crossing time within these plasmons is of the same
order as the time-scale over which the spectrum is observed to evolve. We
contend that properly accounting for light-travel time effects in and between
plasmons which are initially optically thick, but which after expansion become
optically thin, explains the key features of the spectral evolution, for
example the observed timescale. Using the VLBA images, we have directly
measured for the first time the proper motions of individual knots, analysis of
which shows a two-sided jet whose axis is precessing. The best-fit jet speed is
roughly beta = 0.63 and the precession period is about 5 days, significantly
lower than fitted for a previous flare. Extrapolation of the positions of the
knots measured by the VLBA back to zero-separation shows this to occur
approximately 2.5 days after the detection of the rise in flux density of
Cygnus X-3.Comment: 23 pages, 10 figures, accepted by Ap
A One-sided, Highly Relativistic Jet from Cygnus X-3
Very Long Baseline Array images of the X-ray binary, Cygnus X-3, were
obtained 2, 4 and 7 days after the peak of a 10 Jy flare on 4 February 1997.
The first two images show a curved one-sided jet, the third a scatter-broadened
disc, presumably at the position of the core. The jet curvature changes from
the first to the second epoch, which strongly suggests a precessing jet. The
ratio of the flux density in the approaching to that in the (undetected)
receding jet is > 330; if this asymmetry is due to Doppler boosting, the
implied jet speed is > 0.81c. Precessing jet model fits, together with the
assumptions that the jet is intrinsically symmetric and was ejected during or
after the major flare, yield the following constraints: the jet inclination to
the line of sight must be < 14 degrees; the cone opening angle must be < 12
degrees; and the precession period must be > 60 days.Comment: 12 pages 7 figures, accepted by Ap
Elliptic flow at large transverse momenta from quark coalescence
We show that hadronization via quark coalescence enhances hadron elliptic
flow at large pT relative to that of partons at the same transverse momentum.
Therefore, compared to earlier results based on covariant parton transport
theory, more moderate initial parton densities dN/d\eta(b=0) ~ 1500-3000 can
explain the differential elliptic flow v_2(pT) data for Au+Au reactions at
s^1/2=130 and 200 AGeV from RHIC. In addition, v2(pT) could saturate at about
50% higher values for baryons than for mesons. If strange quarks have weaker
flow than light quarks, hadron v_2 at high pT decreases with relative
strangeness content.Comment: Minor changes, extended discussion. To appear in PR
Hadron production in heavy ion collisions: Fragmentation and recombination from a dense parton phase
We discuss hadron production in heavy ion collisions at RHIC. We argue that
hadrons at transverse momenta P_T < 5 GeV are formed by recombination of
partons from the dense parton phase created in central collisions at RHIC. We
provide a theoretical description of the recombination process for P_T > 2 GeV.
Below P_T = 2 GeV our results smoothly match a purely statistical description.
At high transverse momentum hadron production is well described in the language
of perturbative QCD by the fragmentation of partons. We give numerical results
for a variety of hadron spectra, ratios and nuclear suppression factors. We
also discuss the anisotropic flow v_2 and give results based on a flow in the
parton phase. Our results are consistent with the existence of a parton phase
at RHIC hadronizing at a temperature of 175 MeV and a radial flow velocity of
0.55c.Comment: 25 pages LaTeX, 18 figures; v2: some references updated; v3: some
typos fixe
Meson and baryon elliptic flow at high pT from parton coalescence
The large and saturating differential elliptic flow v2(pT) observed in Au+Au
reactions at RHIC so far could only be explained assuming an order of magnitude
denser initial parton system than estimated from perturbative QCD.
Hadronization via parton coalescence can resolve this ``opacity puzzle''
because it enhances hadron elliptic flow at large pT relative to that of
partons at the same transverse momentum. An experimentally testable consequence
of the coalescence scenario is that v2(pT) saturates at about 50% higher values
for baryons than for mesons. In addition, if strange quarks have weaker flow
than light quarks, hadron v2 at high pT decreases with relative strangeness
content.Comment: Talk at SQM2003 [7th Int. Conf. on Strangeness in Quark Matter
(Atlantic Beach, NC, USA, Mar 12-17, 2003)] - 6 pages, 5 eps figs, IOP style
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