Optical Diagnostic Systems Research Articles

First tests of diagnostic mirrors in a tokamak divertor: An overview of experiments in DIII-D

Mirrors will be used in ITER in all optical diagnostic systems observing the plasma radiation in the ultraviolet, visible and infrared ranges. Diagnostic mirrors in ITER will suffer from electromagnetic radiation, energetic particles and neutron irradiation. Erosion due to impact of fast neutrals from plasma and deposition of plasma impurities may significantly degrade optical and polarization characteristics of mirrors influencing the overall performance of the respective diagnostics. Therefore, maintaining the best possible performance of mirrors is of the crucial importance for the ITER optical diagnostics. Mirrors in ITER divertor are expected to suffer from deposition of impurities. The dedicated experiment in a tokamak divertor was needed to address this issue. Investigations with molybdenum diagnostic mirrors were made in DIII-D divertor. Mirror samples were exposed at different temperatures in the private flux region to a series of ELMy H-mode discharges with partially detached divertor plasmas. An increase of temperature of mirrors during the exposure generally led to the mitigation of carbon deposition, primarily due to temperature-enhanced chemical erosion of carbon layers by D atoms. Finally, for the mirrors exposed at the temperature of ∼160 °C neither carbon deposition nor degradation of optical properties was detected.

Investigations of single crystal and polycrystalline metal mirrors under erosion conditions in TEXTOR

Metal mirrors are planned for optical diagnostic systems of ITER. However, erosion, deposition and particle implantation can change the performance of mirrors. Mirrors made from the single crystal (SC) materials are among the main candidates for use in ITER diagnostic systems operating under erosion-dominated conditions. Laboratory tests have confirmed good optical performance of SC mirrors under erosion, but the dedicated direct comparative test in tokamak environment was missing. Such a direct test was performed in TEXTOR. Single crystal molybdenum, tungsten and polycrystalline (PC) molybdenum mirrors were exposed under the same conditions in the SOL plasma of TEXTOR. Surface and optical properties of mirrors were characterized before and after exposure. Before exposure glow discharge cleaning in hydrogen restored the reflectivity of mirrors oxidized during storage on air. No significant changes in total reflectivity were observed for all mirrors after exposure. Drastic increase of diffuse reflectivity was measured for PC Mo mirror, no change for a SC one. Thus, specular reflectivity of single crystal is higher than of polycrystalline one. The most affected wavelength range is 250–1000 nm, no significant change of reflectivity was noticed in the range 1000–2000 nm. Negligible effect of the exposure on polarization characteristics was observed.

Light scattering properties of natural and artificially demineralized dental enamel at 1310 nm

A fundamental understanding of how near-IR light propagates through sound and carious dental hard tissues is essential for the development of clinically useful optical diagnostic systems, since image contrast is based on changes in the optical properties of these tissues on demineralization. During the caries (decay) process, micropores are formed in the lesion due to partial dissolution of the individual mineral crystals. Such small pores behave as scattering centers, strongly scattering visible and near-IR light. The optical properties of enamel can be quantitatively described by the absorption and scattering coefficients, and the scattering phase function. Our aim is to measure the optical scattering behavior of natural and artificial enamel caries. Near-IR attenuation measurements and angular-resolved goniometer measurements coupled with Monte Carlo simulations are used to determine changes in the scattering coefficient and the scattering anisotropy on demineralization at 1310 nm. An ultra-high resolution digital microradiography system is used to quantify the lesion severity by measurement of the relative mineral loss for comparison with optical scattering measurements. The scattering coefficient increases exponentially with increasing mineral loss. Natural and artificial demineralization increases the scattering coefficient more than two orders of magnitude at 1310 nm, and the scattering is highly forward directed.

Observations and modeling of the coupled latitude‐altitude patterns of equatorial plasma depletions

The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh‐Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low‐latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high‐to‐low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north‐south direction spanning the geomagnetic equator. Ground‐based all‐sky camera systems used to capture the two‐dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L ∼ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube–integrated mean density values versus L shell apex heights show that instability‐induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.

Calculations of neutron and gamma transport in ITER H-alpha and CXRS diagnostic channels

Calculations of neutron and gamma transport were carried out with the MCNP-4A code for some optical diagnostic systems located in the upper and equatorial ports of the reactor ITER. The main emphasis was placed on gamma-neutron fluxes and energy deposition in mirrors, quartz windows and optical fibres. Neutron and gamma spectra at primary vacuum windows were calculated for the purpose of determining accurately their possible transparency degradation in ITER on the basis of irradiation test data. The effectiveness of neutron shielding was considered from a viewpoint of radiation safety. A similar analysis was done for the CXRS diagnostic system: the nuclear energy deposition in mirrors and the dose rate responses at toroidal field coils were calculated.

KEKB beam instrumentation systems

For the stable high-luminosity operation and luminosity increase, the electron and positron storage rings of the KEK B-Factory (KEKB) is equipped with various beam instrumentations, which have been working well since the start of the commissioning in December, 1998. Details and performance of the beam-position monitor system based on the spectrum analysis using DSPs, the turn-by-turn BPM with four-dimensional function available for measurements of the individual bunch position, phase and intensity, the parametric beam-DCCTs designed so as to avoid the magnetic-core-selection problems for the parametric flux modulation, the bunch-by-bunch feedback system indispensable to suppress the strong multibunch instabilities in KEKB, the various optical beam diagnostic systems, such as synchrotron radiation interferometers for precise beam-size measurement, the tune meters, the bunch length monitors and the beam-loss monitors are described. Delicate machine tuning of KEKB is strongly supported by these instrumentations.

Round-robin evaluation of optical fibres for plasma diagnostics

Optical plasma diagnostic systems often require a complex optical path involving mirrors, lenses and windows. The use of optical fibres could significantly simplify the design of such a system. Radiation-induced effects, namely induced absorption and luminescence, may compromise the integration of optical fibres in the diagnostic system. However, recent developments of new fused silica glass types open perspectives to use optical fibres as a wave-guide device. In this paper, we present the common European, Japanese and Russian efforts in setting up a standardized round-robin procedure to test the optical fibres under relevant ITER conditions. The round-robin procedure should allow to select the most appropriate optical fibre. We briefly illustrate the discussion with results of an in-core reactor irradiation of pure silica optical fibres in the BR2 material testing reactor of the Belgian Nuclear Energy Research Centre.

Effects of 14-MeV Neutron Irradiation on Optical Components for Fusion Diagnostics

Various kinds of optical fibers and related optical components for fusion diagnostics were irradiated with 14-MeV neutrons from OKTAVIAN or RTNS-II. This report shows typical fusion neutron irradiation effects on them and their hardness levels to mean 14-MeV neutron fluences at which their important performance degrades 10%. Also experimental formulas for fusion neutron induced backgrounds of scintillators and a photomultiplier tube are given by fitting their pulse height distributions for 14-MeV neutrons with a summation of exponential functions. These data and formulas are useful for the estimation of lifetimes and background levels of optical fusion diagnostic systems which may be located in the 14-MeV neutron environment of a fusion reactor.

Data acquisition and optical diagnostic systems for the UK FEL

This paper describes the data acquisition provisions and associated optical diagnostic procedures and control systems which have been developed for the UK free electron laser. The flexibility of the overall system is illustrated by reference to selected examples of experimental data.

Optical measurements on the UK free-electron laser

We describe the data acquisition and optical diagnostic systems of the UK free-electron laser. Measurements of optical gain are reported and the first attempt at oscillation with laser injection is described.