384 research outputs found
Beam Dynamics in High Intensity Cyclotrons Including Neighboring Bunch Effects: Model, Implementation and Application
Space charge effects, being one of the most significant collective effects,
play an important role in high intensity cyclotrons. However, for cyclotrons
with small turn separation, other existing effects are of equal importance.
Interactions of radially neighboring bunches are also present, but their
combined effects has not yet been investigated in any great detail. In this
paper, a new particle in cell based self-consistent numerical simulation model
is presented for the first time. The model covers neighboring bunch effects and
is implemented in the three-dimensional object-oriented parallel code
OPAL-cycl, a flavor of the OPAL framework. We discuss this model together with
its implementation and validation. Simulation results are presented from the
PSI 590 MeV Ring Cyclotron in the context of the ongoing high intensity upgrade
program, which aims to provide a beam power of 1.8 MW (CW) at the target
destination
Towards Quantitative Simulations of High Power Proton Cyclotrons
PSI operates a cyclotron based high intensity proton accelerator routinely at
an average beam power of 1.3MW. With this power the facility is at the
worldwide forefront of high intensity proton accelerators. The beam current is
practically limited by losses at extraction and the resulting activation of
accelerator components. Further intensity upgrades and new projects aiming at
an even higher average beam power, are only possible if the relative losses can
be lowered in proportion, thus keeping absolute losses at a constant level.
Maintaining beam losses at levels allowing hands-on maintenance is a primary
challenge in any high power proton machine design and operation. In
consequence, predicting beam halo at these levels is a great challenge and will
be addressed in this paper. High power hadron driver have being used in many
disciplines of science and, a growing interest in the cyclotron technology for
high power hadron drivers are being observed very recently. This report will
briefly introduce OPAL, a tool for precise beam dynamics simulations including
3D space charge. One of OPAL's flavors (OPAL-cycl) is dedicated to high power
cyclotron modeling and is explained in greater detail. We then explain how to
obtain initial conditions for our PSI Ring cyclotron which still delivers the
world record in beam power of 1.3 MW continuous wave (cw). Several crucial
steps are explained necessary to be able to predict tails at the level of
3\sigma ... 4\sigma in the PSI Ring cyclotron. We compare our results at the
extraction with measurements, obtained with a 1.18 MW cw production beam. Based
on measurement data, we develop a simple linear model to predict beam sizes of
the extracted beam as a function of intensities and confirm the model with
simulations.Comment: Corrections and new figur
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
Intensity limits of the PSI Injector II cyclotron
We investigate limits on the current of the PSI Injector II high intensity
separate-sector isochronous cyclotron, in its present configuration and after a
proposed upgrade. Accelerator Driven Subcritical Reactors, neutron and neutrino
experiments, and medical isotope production all benefit from increases in
current, even at the ~ 10% level: the PSI cyclotrons provide relevant
experience. As space charge dominates at low beam energy, the injector is
critical. Understanding space charge effects and halo formation through
detailed numerical modelling gives clues on how to maximise the extracted
current. Simulation of a space-charge dominated low energy high intensity (9.5
mA DC) machine, with a complex collimator set up in the central region shaping
the bunch, is not trivial. We use the OPAL code, a tool for charged-particle
optics calculations in large accelerator structures and beam lines, including
3D space charge. We have a precise model of the present production) Injector
II, operating at 2.2 mA current. A simple model of the proposed future
(upgraded) configuration of the cyclotron is also investigated.
We estimate intensity limits based on the developed models, supported by
fitted scaling laws and measurements. We have been able to perform more
detailed analysis of the bunch parameters and halo development than any
previous study. Optimisation techniques enable better matching of the
simulation set-up with Injector II parameters and measurements. We show that in
the production configuration the beam current scales to the power of three with
the beam size. However, at higher intensities, 4th power scaling is a better
fit, setting the limit of approximately 3 mA. Currents of over 5 mA, higher
than have been achieved to date, can be produced if the collimation scheme is
adjusted
Calorimetric Evidence of Multiband Superconductivity in Ba(Fe0.925Co0.075)2As2
We report on the determination of the electronic heat capacity of a slightly
overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our
analysis of the temperature dependence of the superconducting-state specific
heat provides strong evidence for a two-band s-wave order parameter with gap
amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with
the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev.
Lett. 101, 057003 (2008)].Comment: 4 pages, 3 figure
Magnetisation distribution in the tetragonal phase of BaFe2As2
We have determined the spatial distribution of the magnetisation induced by a
field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron
diffraction. Magnetic structure factors derived from the polarisation
dependence of the intensities of Bragg reflections were used to make a maximum
entropy reconstruction of the distribution projected on the 110 plane. The
reconstruction shows clearly that the magnetisation is confined to the region
around the iron atoms and that there is no significant magnetisation associated
with either the As or Ba atoms. The distribution of magnetisation around the Fe
atom is significantly non-spherical with a shape which is extended in the
directions in the projection. These results show that the electrons which give
rise to the paramagnetic susceptibility are confined to the Fe atoms their
distribution suggests that they occupy 3d t_2g type orbitals with about 60% in
those of xy symmetry
Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion
Epitaxially strained LaCoO3 (LCO) thin films were grown with different film
thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT)
substrates. After initial pseudomorphic growth the films start to relieve their
strain partly by the formation of periodic nano-twins with twin planes
predominantly along the direction. Nano-twinning occurs already at the
initial stage of growth, albeit in a more moderate way. Pseudomorphic grains,
on the other hand, still grow up to a thickness of at least several tenths of
nanometers. The twinning is attributed to the symmetry lowering of the
epitaxially strained pseudo-tetragonal structure towards the relaxed
rhombohedral structure of bulk LCO. However, the unit-cell volume of the
pseudo-tetragonal structure is found to be nearly constant over a very large
range of t. Only films with t > 130 nm show a significant relaxation of the
lattice parameters towards values comparable to those of bulk LCO.Comment: 31 pages, 10 figure
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