Optical design for divertor Thomson scattering system for JT-60SA

Optical design for divertor Thomson scattering system in JT-60SA has been conducted. The measurement system will use a Nd:YAG laser at 1064 nm, and scattered photons are collected by a collection optical system. The collection optics consists of primary mirror, secondary mirror, relay optics, and fiber collection optics. The laser transmission mirror and collection optics were designed to be installed in a slender lower port of JT-60SA. The assessment of the measurement errors in temperature was conducted for the designed collection optical system. Because of spatial limitation, the solid angle from the measurement points would be small especially for the measurement points in high field side, and consequently, the temperature errors in the high field side would be considerably large. The effects of several improvements on the error are discussed. Moreover, an assessment for the in-vessel laser transmission metallic mirrors is conducted for the present design.     

Zeff first measurements in EAST with a multi-channel visible bremsstrahlung new system

A multi-channel visible bremsstrahlung measurement system was developed to measure the ion effective charge (Z_eff) in EAST tokamak. The system has a temporal resolution of 0.05 ms and spatial resolution of 3 cm. The measurement principle and the design of the 8-channel fiber-photomultiplier tubes (PMTs) coupled system are described, including the calibration process of the measurement system with an integrating sphere. Preliminary experimental results of line integrated bremsstrahlung profile and Z_eff derived from the system are reported.     

2011 Status of the automatic alignment system for the National Ignition Facility

Automated alignment for the National Ignition Facility (NIF) is accomplished using a large-scale parallel control system that directs 192 laser beams along the 300 m optical path. The beams are then focused down to a 50 μ spot in the middle of the target chamber. The entire process is completed in less than 50 min. The alignment system commands 9000 stepping motors for extremely precise adjustment of mirrors and other optics. 41 control loops per beamline perform parallel processing services running on a LINUX cluster to analyze high-resolution images of the beams and their references. This paper describes the status the NIF automatic alignment system and the challenges encountered as NIF development has transitioned from building the laser, to becoming a research project supporting a 24 h, 7 days/week laser facility. NIF is now a continuously operated system where performance monitoring is increasingly more critical for operation, maintenance, and commissioning tasks. Equipment wear and the effects of high energy neutrons from fusion experiments are issues which affect alignment efficiency and accuracy. New sensors needing automatic alignment assistance are common. System modifications to improve efficiency and accuracy are prevalent. Handling these evolving alignment and maintenance needs while minimizing the impact on NIF experiment schedule is expected to be an on-going challenge for the planned 30 year operational life of NIF.     

RF calculation and thermal-stress analysis of the ECRH launcher on EAST tokamak

EAST is a medium sized superconducting tokamak with major radius R = 1.8 m, minor radius a = 0.45 m, plasma current I_p ≤ 1 MA, toroidal field B_T ≤ 3.5 T and expected plasma pulse length up to 1000 s. An electron cyclotron resonance heating (ECRH) launcher for four-beam injection is being installed on EAST tokamak. Four electron cyclotron wave beams which are generated from four sets of 140 GHz/1 MW/1000 s gyrotrons will be injected into the plasma by the spherical focusing mirrors and plane mobile mirrors. The focusing mirrors are spherical to focus Gaussian beams after reflection. Four plane mobile mirrors independently steer continuously in the poloidal and toroidal direction controlled by motors. With the suitable distance between mirrors and appropriate focal length of focusing mirror, the beam radius in the resonance layer of plasma is 31.145 mm. The heat from plasma radiation and metal losses is loaded on the mobile mirror. In order to decrease the temperature and thermal stress, the inner equivalent diameter of water channels is 8 mm and the suggested water velocity is 4 m/s.     

Target area structural design of ShenGuangIII

This paper will present a summary of the target area structural design results of ShenGuangIII (SGIII), which includes target bay and switchyard support structures, target chamber, and beam transport system. The target area building is rectangular and includes a target bay and a switchyard. The outer wall of the target area building is based outside the vibration isolation trenches and separates the diurnal temperature loads and the wind loads that occur outside the target area building during the alignment process prior to a shot. The target bay and switchyard support structures are mainly comprised of the steel reinforced concrete building and the steel space frame. The target chamber is a welded sphere made of aluminum alloy 5052 and has an inner diameter of 6 m and a nominal wall thickness of 8 cm. The beam transport system brought the 48 laser beams to the final optics assemblies (FOAs) and includes 276 transport mirrors. Sources that influence stability are mainly ambient random vibration, ambient wind induced vibrations, diurnal temperature changes, and HVAC. A FEM is used to calculate the responses of the target area structure under broadband ambient random vibration. The position deviations of 48 beam lines are calculated and used to evaluate the stability of the target area structure. The analyses demonstrated that the positioning deviations of 48 beam lines satisfy the deviation budget requirements.     

Theoretical and experimental studies on molybdenum and stainless steel mirrors cleaning by high repetition rate laser beam

Our studies were aimed to determine the damage threshold of molybdenum (Mo) and stainless steel (SS) mirrors to provide the maximum fluence which the mirror surfaces could withstand without affecting their reflectivity properties. A high repetition rate ytterbium fiber laser (20 kHz, 1.06 μm, 120 ns) was applied. The experimental single-pulse and multiple-pulse damage thresholds were obtained. To calculate damage thresholds, a 1D analytical model which takes into account the temperature dependent absorptance and multiple-pulse damage based on plastic deformations accumulation was applied. The experimental damage thresholds and the theoretical ones are in a good agreement. Cleaning tests with the contaminated mirrors exposed in JET have been performed.     

Studies for the design of the Wendelstein 7-X Thomson Scattering polychromators

The aim of the Thomson scattering system is the measurement of electron temperature and density profiles with high time and spatial resolution. The whole Thomson scattering optical system is optimized to minimize losses of scattered light with wavelengths from 700 nm up to 1064 nm. A five-channel polychromator serves as spectral analyzer of the scattered light. The light is carried to the polychromator input via fiber bundles of 3 mm diameter. High performance dielectric interference filters will be used for spectral analyzing. Their transmission curves will be chosen according to the range of electron temperature that is supposed during W7-X operation (from 10 eV up to 10 keV). The design of the polychromator must be optimized for a high throughput and size compactness. The article describes three possible polychromator setups. The main difference between these designs is the usage of relay and field lenses. The optical properties of the designs will be derived both by measurements and by simulations. Limitations considering high Numerical Aperture (N.A.) and fiber bundle size are discussed.     

Pulsed repetition rate nanosecond laser heating and ablation of the tokamak graphite tile deposited layers

Laser heating and ablation of the plasma-facing surface of a graphite tile from TEXTOR tokamak that was covered by a deposited carbon layer has been studied. Laser heating measurements were performed with a pulsed nanosecond Nd-YAG laser (2nd harmonic, 10 kHz repetition rate, 100 ns pulse duration). Surface temperature measurements were recorded with a home-made pyrometer having a response time of 15 μs (t_99%). The experimental results are simulated with an analytical model of laser heating of a surface covered by a deposited layer and heated repeatedly by laser pulses. The comparison between experimental and theoretical data of the observed temperature excursions enables us to assess the deposited carbon layer physical parameters (thermal conductivity, porosity, etc.) if the thermal and optical properties of the graphite substrate are known. Laser ablation measurements were performed with two pulsed nanosecond Nd-YAG lasers (20 Hz and 10 kHz repetition rate with 5 ns and 100 ns pulse duration, respectively). For a plasma-facing graphite surface covered by a thick (～30–50 μm) deposited carbon layer, the ablation threshold is 4.5 ± 1 kJ/m² regardless of the pulse duration. The obtained ablation threshold is significantly lower than the one measured for a virgin tokamak graphite sample. The comparison of the experimental results and theoretical data demonstrated that the laser ablation mechanisms for tokamak graphite and thick carbon layers deposited on plasma-facing surface are different.     

Development of in situ cleaning techniques for diagnostic mirrors in ITER

Mirrors will be used in all optical and laser-based diagnostic systems of ITER. In the severe environment, the optical characteristics of mirrors will be degraded, hampering the entire performance of the respective diagnostics. A minute impurity deposition of 20 nm of carbon on the mirror is sufficient to decrease the mirror reflectivity by tens of percent outlining the necessity of the mirror cleaning in ITER. The results of R&D on plasma cleaning of molybdenum diagnostic mirrors are reported. The mirrors contaminated with amorphous carbon films in the laboratory conditions and in the tokamaks were cleaned in steady-state hydrogenic plasmas. The maximum cleaning efficiency of 4.2 nm/min was reached for the laboratory and soft tokamak hydrocarbon films, whereas for the hard tokamak films the carbidization of mirrors drastically decreased the cleaning efficiency down to 0.016 nm/min. This implies the necessity of sputtering cleaning of contaminated mirrors as the only reliable tool to remove the deposits by plasma cleaning. An overview of R&D program on mirror cleaning is provided along with plans for further studies and the recommendations for ITER mirror-based diagnostics.     

Optical contrast formation in amorphous silicon carbide with high-energy focused ion beams

Thin films (d ～ 1 μm) of hydrogenated amorphous silicon carbide (a-Si_1?xC_x:H), deposited by RF reactive magnetron sputtering with different carbon content x, have been implanted with high fluences (Φ = 1016–1017 cm^?2) of high-energy (E = 0.2–1 MeV) He⁺ ions as the implant species. The induced structural modification of the implanted material results in a considerable change of its optical properties, best manifested by a significant shift of the optical absorption edge to lower photon energies as obtained from photo-thermal-deflection spectroscopy (PDS) data. This shift is accompanied by a remarkable increase of the absorption coefficient over one order of magnitude (photo-darkening effect) in the measured photon energy range (0.6–3.8 eV), depending on the ion fluence, energy and carbon content of the films. These effects could be attributed both to additional defect introduction and increased graphitization, as confirmed by Raman spectroscopy and infra-red (IR) optical transmission measurements. The optical contrast thus obtained (between implanted and unimplanted film material) could be made use of in the area of high-density optical data storage using focused high-energy He⁺ ion beams.     

CTOF measurements and Monte Carlo analyses of neutron spectra for the backward direction from an iron target irradiated with 400–1200 MeV protons

A calorimetric-time-of-flight (CTOF) technique was used for real-time, high-precision measurement of the neutron spectrum at an angle of 175° from the initial proton beam direction, which hits a face plane of a cylindrical iron target of 20 cm in diameter and 25 cm thick. A comparison was performed between the neutron spectra predicted by the MARS and the MCNPX codes and that measured for 400, 600, 800, 1000 and 1200 MeV protons.     

Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering

Vanadium oxide (VO_x) thin films find extensive use in room-temperature bolometers for IR imaging. It is desirable to control and modify the electronic properties of this temperature-sensitive material with treatments such as ion implantation and thermal annealing. In this work, we report on the modification of structural and electrical properties of VO_x thin films of varying compositions, deposited by pulsed dc reactive sputtering using a vanadium target under different oxygen flow rates. The as-deposited resistivities of the films ranged from 0.1 Ω cm to 100 Ω cm and the temperature coefficient of resistance (TCR) values varied from ?1.1% to ?2.7%. VO_x films used in microbolometers need to have a high TCR (>2%) and low resistivity values (1–10 Ω cm) in order to maximize sensitivity in conjunction with the read-out integrated circuit (ROIC). However, one usually finds a high TCR associated with high resistivity. Hence ion implantation followed by annealing was performed with the goal of improving the trade-off between TCR and resistivity. Two species – hydrogen (active) and helium (inert) – were chosen for implantation. Hydrogen is strongly electroactive and is well known for passivating defect states in a wide variety of electronic materials. As inert species, helium was chosen mainly to study the effects of bombardment on the film. The implanted films were annealed in an inert atmosphere to allow defect control and redistribution of atoms, and then characterized by current–voltage measurements over a wide temperature range. An order of magnitude change in resistance, and significant variations in TCR were observed. Further characterization has been done by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) to correlate these resistivity changes with the structure of the films.     

A new linear array detector for high resolution and low dose digital radiography

At the Department of Physics of the University of Bologna a new intensified linear array detector is under development. The core of the system is a digital intensified CCD camera, the electron bombarded charge coupled device (EBCCD). The main innovation is a coherent rectangular-to-linear fiber optics adapter coupling the 1 in. diameter photocathode of the camera with a linear 129 mm × 1.45 mm strip of Gd₂O₂S:Tb. In this way a high spatial resolution over an extended length is obtained. The detector works as an X-ray scanner by means of a high-precision translation mechanical device to inspect a 13 cm × 18 cm area. A complete characterisation of the system has been made in terms of linearity, dynamic range, modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE). At last, radiographic tests on a set of samples have been made and will be presented.     

Development of a TOF-ERDA measurement system for analysis of light elements using a He beam

A time-of-flight ERDA (TOF-ERDA) measurement system has been developed for the analysis of light elements. He ions are used for the incident beam, and recoil light ions are detected with the system. The system consists of a time detector and a silicon detector, and energy and velocity of recoil ion are measured simultaneously. The depth resolution of 21.6 ± 2.2 nm (FWHM) has been obtained by an ERDA measurement of a thin carbon layer onto a silicon wafer using a 5.7 MeV He beam. The mass resolution is better than 1 for elements up to oxygen. Maximum detectable depth of carbon in a PET film is about 650 nm. An ERDA measurement of implanted carbon in a silicon wafer has been demonstrated.     

Performance of the Sumy nuclear microprobe with the integrated probe-forming system

The integrated probe-forming system for the Sumy nuclear microprobe comprises two doublets of magnetic quadrupole lenses of new design. Each doublet has been made from a single piece of soft iron by electro-discharge machining. This paper describes the performance tests of the microprobe including the beam scanning control and data acquisition systems. In the first runs a spot size of about 2 μm (FWHM) was obtained for the high beam current mode (up to 200 pA) by scanning a conventional copper grid with 1000 meshes per inch. Simulated beam optics parameters are compared with the experimental data.     

Deep defect density imaging

Contour and surface plots of S and T parameters were generated for a 2D slice of four 304L steel specimens inserted in a 5 cm × 5 cm plastic holder. This was achieved by performing Doppler broadening spectroscopy (DOBS) coupled with in situ pair-production using a 1 kHz repetition rate linear accelerator (LINAC) at the physics department of Idaho State University. The results confirmed the imaging potential of this technique and also revealed its limitations.     

The chemical durability of glass and graphite–glass composite doped with cesium oxide

The role of temperature in determining the chemical stability of a waste form, as well as its leach rate, is very complex. This is because the dissolution kinetics is dependent both on temperature and possibility of different rate-controlling mechanisms that appear at different temperature regions. The chemical durability of Alumina-Borosilicate Glass (ABG) and Glass–Graphite Composite (GGC), bearing Tristructural Isotropic (TRISO) fuel particles impregnated with cesium oxide, were compared using a static leach test. The purpose of this study is to examine the chemical durability of glass–graphite composite to encapsulate coated fuel particles, and as a possible alternative for recycling of irradiated graphite. The test was based on the ASTM C1220-98 methodology, where the leaching condition was set at a temperature varying from 298 K to 363 K for 28 days. The release of cesium from ABG was in the permissible limit and followed the Arrhenius’s law of a surface controlled reaction; its activation energy (E_a) was 65.6 ± 0.5 kJ/mol. Similar values of Ea were obtained for Boron (64.3 ± 0.5) and Silicon (69.6 ± 0.5 kJ/mol) as the main glass network formers. In contrast, the dissolution mechanism of cesium from GGC was a rapid release, with increasing temperature, and the activation energy of Cs (91.0 ± 5 kJ/mol) did not follow any model related to carbon kinetic dissolution in water. Microstructure analysis confirmed the formation of Crystobalite SiO₂ as a gel layer and Cs⁺¹ valence state on the ABG surface.     

Calculation of maximum release rates in alternative design changes in the thickness of the buffer for the engineered barrier system (EBS) in deep repository by using Amber code

The multibarrier concept forms the basis for geological disposal concepts in most countries and the guideline states that research and development should aim to demonstrate the feasibility of constructing an engineered barrier system (EBS) which is appropriate for the range of relevant geological conditions. The multibarrier system (EBS) and its functions consisting of the glass waste form, overpack and buffer material was located in a sufficiently stable geological environment. When an overpack comes into contact with groundwater it will start to corrode. The wall thickness will then gradually reduce, and the overpack will eventually fail mechanically when its structural strength can no longer support the stress imposed by the surrounding environment. The requirements that influence the thickness of buffer include nuclide migration retardation and heat conductivity, as well as stress buffering capability, self-sealing ability and workability. The migration retardation function is assumed to be the most important of all these requirements with respect to setting the appropriate thickness of buffer. A consideration of these effects and relationship between buffer thicknesses has determined that a reasonable thickness for the buffer is between 400 mm and 700 mm [AECL, H12]. Therefore, the design thickness of buffer material can range from 0.4 m to 0.7 m to account for manufacturing and stress buffering. In this alternative design case, the thickness of the buffer material is set to 0.4 m, 0.5 m, 0.6 m and 0.7 m. The nuclide migration properties of the buffer material are assumed to be the same (PNC, Development and Management of the Technical Knowledge Base for the Geological Disposal of HLW, Supporting Report 2: “Repository” Engineering Technology). The results of calculation are presented that some nuclides such as Se-79, Tc-99, Pd-107, Th-233, U-236, Pb-210, Ra-226 and Np-237 virtually unchanged in case the maximum release rate from EBS corresponding to change thickness of buffer material. Some nuclides such as Cs-135, Nb-94, Nb-93 m, Zr-93, Sn-126, Th-230, Ph-240, Pu-242, U-233, Ac-227, Pa-231 and Th-229 are very little greater for 40 cm, 50 cm and 60 cm in the maximum release rate compared with 70 cm. Maximum release of nuclides U-235, U-234 and U-238 increases in case of 50 cm and 60 cm thickness of buffer and in case 40 cm are the same as 70 cm thickness because the amount of their parents in case 40 cm will decrease before decay and in case 70 cm amount of these nuclides will decrease due to decayed to other nuclides before release from the buffer, then maximum decay happened in case 50 cm. The maximum release rates of short-lived nuclides such as Cm-245, Am-243, Cm-245, Am-241, Pu-241 and Pu-239 increase significantly due to less decay occurring during the reduced buffer transit time.     

Carbon film growth and hydrogenic retention of tungsten exposed to carbon-seeded high density deuterium plasmas

Tungsten (W) targets have been exposed to high density (n_e ? 4 × 10¹⁹ m^?3), low temperature (T_e ? 3 eV) CH₄-seeded deuterium (D) plasma in Pilot-PSI. The surface temperature of the target was ～1220 K at the center and decreased radially to ～650 K at the edges. Carbon film growth was found to only occur in regions where there was a clear CII emission line, corresponding to regions in the plasma with T_e ? 2 eV. The maximum film thickness was ～2.1 μm after a plasma exposure time of 120 s. ³He nuclear reaction (NRA) analysis and thermal desorption spectroscopy (TDS) determine that the presence of a thin carbon film dominates the hydrogenic retention properties of the W substrate. Thermal desorption spectroscopy analysis shows retention increasing roughly linearly with incident plasma fluence. NRA measures a C/D ratio of ～0.002 in these films deposited at high surface temperatures.