1,464 research outputs found
Implementation of Design Changes Towards a More Reliable, Hands-off Magnetron Ion Source
As the main ion source for the accelerator complex, magnetron ion
sources have been used at Fermilab since the 1970s. At the offline test stand,
new R&D is carried out to develop and upgrade the present magnetron-type
sources of ions of up to 80 mA and 35 keV beam energy in the context of
the Proton Improvement Plan. The aim of this plan is to provide high-power
proton beams for the experiments at FNAL. In order to reduce the amount of
tuning and monitoring of these ion sources, a new electronic system consisting
of a current-regulated arc discharge modulator allow the ion source to run at a
constant arc current for improved beam output and operation. A solenoid-type
gas valve feeds gas into the source precisely and independently of
ambient temperature. This summary will cover several studies and design changes
that have been tested and will eventually be implemented on the operational
magnetron sources at Fermilab. Innovative results for this type of ion source
include cathode geometries, solenoid gas valves, current controlled arc pulser,
cesium boiler redesign, gas mixtures of hydrogen and nitrogen, and duty factor
reduction, with the aim to improve source lifetime, stability, and reducing the
amount of tuning needed. In this summary, I will highlight the advances made in
ion sources at Fermilab and will outline the directions of the continuing R&D
effort.Comment: 4 pp. arXiv admin note: substantial text overlap with
arXiv:1701.0175
Improvements on the Stability and Operation of a Magnetron H- Ion Source
The magnetron H- ion sources developed in the 1970s currently in operation at
Fermilab provide beam to the rest of the accelerator complex. A series of
modifications to these sources have been tested in a dedicated offline test
stand with the aim of improving different operational issues. The solenoid type
gas valve was tested as an alternative to the piezoelectric gas valve in order
to avoid its temperature dependence. A new cesium oven was designed and tested
in order to avoid glass pieces that were present with the previous oven,
improve thermal insulation and fine tune its temperature. A current-regulated
arc modulator was developed to run the ion source at a constant arc current,
providing very stable beam outputs during operations. In order to reduce beam
noise, the addition of small amounts of N2 gas was explored, as well as testing
different cathode shapes with increasing plasma volume. This paper summarizes
the studies and modifications done in the source over the last three years with
the aim of improving its stability, reliability and overall performance.Comment: 8 pages, 19 figure
Observation of a parity oscillation in the conductance of atomic wires
Using a scanning tunnel microscope or mechanically controlled break
junctions, atomic contacts of Au, Pt and Ir are pulled to form chains of atoms.
We have recorded traces of conductance during the pulling process and averaged
these for a large amount of contacts. An oscillatory evolution of conductance
is observed during the formation of the monoatomic chain suggesting a
dependence on even or odd numbers of atoms forming the chain. This behaviour is
not only present in the monovalent metal Au, as it has been previously
predicted, but is also found in the other metals which form chains suggesting
it to be a universal feature of atomic wires
Determination of the Superconductor-Insulator Phase Diagram for One-Dimensional Wires
We establish the superconductor-insulator phase diagram for quasi-one
dimensional wires by measuring a large set of MoGe nanowires. This diagram is
consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely with
the critical resistance being equal to R_Q = h/4e^2. We find that transport
properties of insulating nanowires exhibit a weak Coulomb blockade behavior.Comment: 5 pages, 4 figure
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Northwest Africa 5790. Top Sequence of the Nakhlite Pile
NWA 5790 is a recently discovered nakhlite. Its mineralogy, petrology and geochemistry suggest that it is the topmost sequence of the nakhlite lava pile
Decoherence due to elastic Rayleigh scattering
We present theoretical and experimental studies of the decoherence of
hyperfine ground-state superpositions due to elastic Rayleigh scattering of
light off-resonant with higher lying excited states. We demonstrate that under
appropriate conditions, elastic Rayleigh scattering can be the dominant source
of decoherence, contrary to previous discussions in the literature. We show
that the elastic-scattering decoherence rate of a two-level system is given by
the square of the difference between the elastic-scattering \textit{amplitudes}
for the two levels, and that for certain detunings of the light, the amplitudes
can interfere constructively even when the elastic scattering \textit{rates}
from the two levels are equal. We confirm this prediction through calculations
and measurements of the total decoherence rate for a superposition of the
valence electron spin levels in the ground state of Be in a 4.5 T
magnetic field.Comment: 5 pages, 3 figure
Dynamics of axialized laser-cooled ions in a Penning trap
We report the experimental characterization of axialization - a method of
reducing the magnetron motion of a small number of ions stored in a Penning
trap. This is an important step in the investigation of the suitability of
Penning traps for quantum information processing. The magnetron motion was
coupled to the laser-cooled modified cyclotron motion by the application of a
near-resonant oscillating quadrupole potential (the "axialization drive").
Measurement of cooling rates of the radial motions of the ions showed an
order-of-magnitude increase in the damping rate of the magnetron motion with
the axialization drive applied. The experimental results are in good
qualitative agreement with a recent theoretical study. In particular, a
classical avoided crossing was observed in the motional frequencies as the
axialization drive frequency was swept through the optimum value, proving that
axialization is indeed a resonant effect.Comment: 8 pages, 9 figure
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