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Register a new userRF plasma cleaning of optical surfaces: A study of cleaning rates on different carbon allotropes as a function of RF powers and distances
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An extended study on an advanced method for the cleaning of carbon contaminations on large optical surfaces using a remote inductively coupled low pressure RF plasma source (GV10x downstream asher) is reported in this work. Technical as well as scientific features of this scaled up cleaning process are analyzed, such as the cleaning efficiency for different carbon allotropes (amorphous and diamond-like carbon) as a function of feedstock gas composition, RF power (ranging from 30 to 300W), and source-object distances (415 to 840 mm). The underlying physical phenomena for these functional dependences are discussed.
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To avoid reflectivity losses in ITER optical diagnostic systems, plasma sputtering of metallic First Mirrors is foreseen in order to remove deposits coming from the main wall (mainly beryllium and tungsten). Therefore plasma cleaning has to work on large mirrors (up to a size of 200*300 mm) and under the influence of strong magnetic fields (several Tesla). This work presents the results of plasma cleaning of aluminium and aluminium oxide (used as beryllium proxy) deposited on molybdenum mirrors. Using radio frequency (13.56 MHz) argon plasma, the removal of a 260 nm mixed aluminium/aluminium oxide film deposited by magnetron sputtering on a mirror (98 mm diameter) was demonstrated. 50 nm of pure aluminium oxide were removed from test mirrors (25 mm diameter) in a magnetic field of 0.35 T for various angles between the field lines and the mirrors surfaces. The cleaning efficiency was evaluated by performing reflectivity measurements, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy.
Superconducting Radio Frequency (SRF) cavities performance preservation is crucial, from vertical test to accelerator operation. Field Emission (FE) is still one of the performance limiting factors to overcome and plasma cleaning has been proven successful by the Spallation Neutron Source (SNS), in cleaning field emitters and increasing the work function of Nb. A collaboration has been established between Fermi National Accelerator Laboratory (FNAL), SLAC National Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) with the purpose of applying plasma processing to the Linac Coherent Light Source-II (LCLS-II) cavities, in order to minimize and overcome field emission without affecting the high Q of nitrogen-doped cavities. The cleaning for LCLS-II will follow the same plasma composition adopted at SNS, which allows in-situ processing of cavities installed in cryomodules from hydrocarbon contaminants. A novel method for plasma ignition has been developed at FNAL: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler, allowing in-situ cleaning for cavities in cryomodule. The plasma can be easily ignited and tuned in each of the cavity cells using low RF power. A method for plasma detection has been developed as well, which allows the detection of the plasma location in the cavity without the need of cameras at both cavity ends. The presented method can be applied to other multi-cell cavity designs, even for accelerators where the coupling for the fundamental modes at room temperature is very weak.
A laser ablation system has been constructed and used to determine the damage threshold of stainless steel, rhodium and single-, poly- and nanocrystalline molybdenum in vacuum, at a number of wavelengths between 220 and 1064 nm using 5 ns pulses. All materials show an increase of the damage threshold with decreasing wavelength below 400 nm. Tests in a nitrogen atmosphere showed a decrease of the damage threshold by a factor of two to three. Cleaning tests have been performed in vacuum on stainless steel samples after applying mixed Al/W/C/D coatings using magnetron sputtering. In situ XPS analysis during the cleaning process as well ex situ reflectivity measurements demonstrate near complete removal of the coating and a substantial recovery of the reflectivity. The first results also show that the reflectivity obtained through cleaning at 532 nm may be further increased by additional exposure to UV light, in this case 230 nm, an effect which is attributed to the removal of tungsten dust from the surface.
The phonon properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were explored by means of first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. In addition, the bulk T12 phase is an indirect-gap semiconductor having a bandgap of ~4.89 eV. Whereas the bulk material transforms to a 2D phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller band gaps of ~2.64 eV and ~3.27eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy loss spectrum were also computed and discussed. The structural estimation obtained as well as other findings are in agreement with existing theoretical data. The calculated results are beneficial to the practical applications of these exotic carbon allotropes in optoelectronics and electronics.
A novel photonics-based RF reception approach is proposed as a competitive solution to meet the current challenges of photonic-based approaches and to realize high performances at the same time. The proposed approach adopts the superheterodyne configuration by a combination manner of electronic techniques and photonic techniques, including the ultrawideband generation of optical LO, the two-stage photonic superheterodyne frequency conversion and the real-time IF compensation. An engineering prototype has been developed and its performance has been evaluated in the laboratory environment. The experiment results preliminarily verify the feasibility of the proposed approach and its engineering potential. The typical performances are as follows: 0.1 GHz~ 45GHz operation spectrum range (>40 GHz), 900 MHz instantaneous bandwidth, 101 dBHz2/3 SFDR and 130 dBHz LDR, image rejections of ~80 dB for 1st frequency conversion and >90 dB for 2nd frequency conversion.
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