729 research outputs found
First results of the CERN Resonant WISP Search (CROWS)
The CERN Resonant WISP Search (CROWS) probes the existence of Weakly
Interacting Sub-eV Particles (WISPs) like axions or hidden sector photons. It
is based on the principle of an optical light shining through the wall
experiment, adapted to microwaves. Critical aspects of the experiment are
electromagnetic shielding, design and operation of low loss cavity resonators
and the detection of weak sinusoidal microwave signals. Lower bounds were set
on the coupling constant GeV for axion like
particles with a mass of eV. For hidden sector photons, lower
bounds were set for the coupling constant at a mass
of eV. For the latter we were probing a previously
unexplored region in the parameter space
Dielectric response effects in attosecond time-resolved streaked photoelectron spectra of metal surfaces
The release of conduction-band electrons from a metal surface by a
sub-femtosecond extreme ultraviolet (XUV) pulse, and their propagation through
the solid, provokes a dielectric response in the solid that acts back on the
photoelectron wave packet. We calculated the (wake) potential associated with
this photoelectron self-interaction in terms of bulk and surface plasmon
excitations and show that it induces a considerable, XUV-frequency-dependent
temporal shift in laser-streaked XUV photoemission spectra, suggesting the
observation of the ultrafast solid-state dielectric response in contemporary
streaked photoemission experiments.Comment: 4 pages and 4 figures, submitted to PR
Extended Classical Over-Barrier Model for Collisions of Highly Charged Ions with Conducting and Insulating Surfaces
We have extended the classical over-barrier model to simulate the
neutralization dynamics of highly charged ions interacting under grazing
incidence with conducting and insulating surfaces. Our calculations are based
on simple model rates for resonant and Auger transitions. We include effects
caused by the dielectric response of the target and, for insulators, localized
surface charges. Characteristic deviations regarding the charge transfer
processes from conducting and insulating targets to the ion are discussed. We
find good agreement with previously published experimental data for the image
energy gain of a variety of highly charged ions impinging on Au, Al, LiF and KI
crystals.Comment: 32 pages http://pikp28.uni-muenster.de/~ducree
Investigations on RF Behavior of a V-Band Second Harmonic Gyrotron for 100/200 kW Operation
This article presents the investigations on RF-behavioral aspects for the possible operation of a V -band, continuous wave (CW) second harmonic gyrotron for plasma diagnostic application. Keeping in view the design goals and constraints, initial design studies for the mode selection and the computation of starting currents are carried out. From these studies, two possible modes, namely, TE 7,3 and TE 8,3 are considered for the second harmonic operation. Later, the cold cavity design and self-consistent calculations are carried out for the selected operating modes. All the computations are performed using the latest version of our in-house code Gyrotron Design Studio Second Harmonic Version 2020 (GDS2H-2020) with Glidcop as the cavity material. The RF behavior studies confirm the feasible operation of such a second harmonic gyrotron with power levels in excess of 115.52/217.64 kW with the chosen modes of operation
Simulation tools for computer-aided design and numerical investigations of high-power gyrotrons
Modelling and simulation are essential tools for computer-aided design (CAD), analysis and optimization of high-power gyrotrons used as radiation sources for electron cyclotron resonance heating (ECRH) and current drive (ECCD) of magnetically confined plasmas in the thermonuclear reactor ITER. In this communication, we present the current status of our simulation tools and discuss their further development
Realistic Design Studies on a 300-GHz, 1-MW, DEMO-Class Conventional-Cavity Gyrotron
This article presents the realistic initial design studies of a 300-GHz, 1-MW, conventional-cavity gyrotron for its probable application in the next-generation thermonuclear fusion reactors. Keeping the design goals, parameters, and constraints in view, the very high-order TE 49,18 mode is chosen as the operating mode after a careful mode-selection calculation considering realistic ohmic cavity losses. After mode selection and mode competition studies, the cold-cavity design and initial design of a triode-type magnetron injection gun (T-MIG) and a gyrotron magnet are carried out and an electron beam radius of 8.11 mm is obtained with 2.4% velocity spread. Furthermore, investigation on RF behavior of the cavity is performed with the T-MIG beam parameters. By varying the nominal beam parameters, single-mode self-consistent calculations are conducted and achieved the desired output power. Then, multimode time-dependent self-consistent calculations are carried out before and after space-charge neutralization (SCN) with realistic velocity spread (up to 6%) and different beam radii for the assessment of the start-up scenario. Before SCN without velocity spread, the beam voltage is depressed to 70.08 kV and 0.72-MW output power is obtained, whereas with velocity spread (6%), 0.69-MW output power is obtained with 8.11 mm of beam radius. After 60% of SCN in the start-up scenario with velocity spread (6%), the beam voltage increases to 74.83 kV, and thereby, an output power of 0.91 MW is obtained
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