LASER SURFACE CLEANING

Russia participates in ITER with few areas of expertise, one of which is active beam plasma spectroscopy. One of the key components of spectroscopy diagnostic equipment called "first mirror" is located inside of the reaction chamber. Surfaces of such mirrors are subjected to the strong external influence such as strong irradiation, bombardment by charge exchange atoms along with pollution with ablated material of the first wall, divertor plates, limiters and other elements of plasma chamber. Laser cleaning systems are developed and used for remote removal of thin films of such nature from first mirror surfaces.
Other cleaning methods lead to irreversible negative consequencies such as mirror surface deformations and contamination of the reaction zone with cleaning process fall-out, and that is unacceptable for the normal operation of reactor and its diagnostic systems.
Various methods of film removal are used depending on film type.
Explosive ablation allows to avoid negative deformations of mirror surface. Films fly-off due to the excessive pressure produced between film and underlying surface. Evaporation is typical for metal films, which are prone to gradual removal.

Films mechanical inspection:

  • Scanning nano-hardness tester Nanoscan-3Di (film thickness, hardness and surface adhesion measurements);
  • Custom developed system for spectral reflection coeffecient measurement;
  • Elemental analysis for determination of film material removal rate.

Research demonstrated the possibility of removal of films with thickness up to 400 nm and various composition - organic , AL, W, Be, that leads to complete restoration of mirror reflictivity in broad spectral band. Moreover, in some cases reflectivity is enhanced due to laser polishing effect.
Optical system for the light transport in ITER port-plug geometry was designed and constructed. Created system allows to provide light intensivity necessary for film removal at the distances determined by port-plug depths.

PLASMA DIAGNOSTICS

Interferometric studies of the electron density dynamics at the periphery of a micropinch discharge

The measurements were performed using a newly developed homodyne quadrature interferometer possessing fairly high dynamic range and performance. The experiments demonstrated the presence of a plasma with an electron density of ne ≥ 1018 cm at the periphery of the discharge gap. The radial electron density distribution was shown to have a pronounced tubular structure. The relatively high values of the peripheral electron density indicate that the process of pinching can be substantially affected by shunt currents flowing at the discharge periphery.

A twin-wave quadrature interferometer for diagnostics of pulsed processes in hydrogen and erosion plasma

A two-wave interferometer is described that is based on 0.6328-and 3.3922-μm He-Ne lasers and allows separation of the contributions of free electrons and the neutral component in partially ionized plasma to the phase shift of a probing electromagnetic wave under conditions of possible vibrations of the facility’s optical elements. Using the quadrature method for forming informative signals, phase shifts were measured in a wide range of fractions of an interference fringe to several fringes with a high homogeneous differential sensitivity. The interferometer was used to measure the dynamics of the linear electron density of both an atmospheric-pressure erosion capillary discharge in air and plasma of a hydrogen target in experiments on deceleration of heavy ions in an ionized substance.

A LIRA Twin-Wave Laser Interferometer–Reflector for Plasma Diagnostics at Large-Scale Facilities

A LIRA interferometric system (active laser interferometer–reflector) is designed for measuring the plasma density and controlling the degree of modification of the reflector surface under conditions of intense vibrations of a plasma facility. The operating principle of the system is based on the intralaser (autodyne) reception of the radiation reflected into the laser. An element of the structure of the laser facility, including a diffusely reflecting surface, can be used as a reflector. The interferometer is built on the basis of two Zeeman He–Ne lasers generating at wavelengths of 632.8 and 3392.2 nm. Under conditions of an actual plasma experiment, a sensitivity of 5 10–7 has been achieved, which, when converted into the linear plasma electron density, is n e L = 2.5 1010 cm–2. The highest time resolution is 10 ns. The interferometer can operate using reflectors with a coefficient of power reflection into the laser of up to 10–12.

Visualization of plasma-induced processes by a projection system with a Cu-laser-based brightness amplifier

A novel method for visualization of the process of interaction of high-power energy fluxes with various surfaces is proposed. The possibility of the dynamic visualization of a surface covered with a ∼3-cm-thick plasma layer with a linear density of ∼1016 cm−2 is demonstrated experimentally. A scheme of intracavity shadowgraphy of phase objects with the use of a laser projection microscope is developed. Shadow images illustrating the development of the plasma torch of an erosion capillary discharge in air are presented.

Plasma lens for the heavy ion accelerator at ITEP

The problem of focusing of intense heavy-ion beams is an important issue for investigating high energy densities in matter. Application of a plasma lens to this area of research has a number of essential advantages in comparison with the traditional system on the basis of quadruple lenses. At ITEP the plasma lens has been designed and installed into the exit channel of the TWAC accelerator complex. The description of the plasma lens and the results of the first experiments with this lens are reported.

NANOMETROLOGY

Three dimensional laser heterodyne interferometer for nanoscale displacement measurements
First generation

The structure and operating principle of the NanoSkan-3Di scanning probe microscope are briefly described. An investigation of the metrological characteristics of this measuring system using linear TGZ-type gratings demonstrated the high level of reproducibility of the measurements and showed that the results agree with the data obtained in a calibration at the PTB (Germany).

Three dimensional laser heterodyne interferometer for nanoscale displacement measurements
Second generation

A three-coordinate heterodyne laser interferometer has been developed to measure the displacement of the probe microscope scanner with a subnanometer resolution that provides traceability of measurements to the standard of meter through the wavelength of a stabilized He-Ne laser. Main sources of errors are investigated, and their influence is minimized so that the resulting measurement uncertainty of the system does not exceed 0.2 nm, and the resolution is 0.01 nm. The investigation of metrological characteristics of the three-coordinate interferometer was carried out with a scanning probe microscopy (SPM) NanoScan-3D using TGZ-type calibration gratings. The values measured with SPM fell within the 95% confidence interval given by Physikalisch-Technische Bundesanstalt (PTB) (Germany). SPM equipped with a laser interferometer was used to measure the characteristics of dynamic etalons of geometric dimensions.

Laser interferometerr for probe velocity measurements in dynamic hardness testers

Leeb hardness test is currently one of the prevailing dynamic methods of hardness determination. Leeb method is based on the variation of relationships of the probe pre- and post rebound velocities. Laser interferometers are utilized for the calibration of such systems. They are used for probe velocity measurement at the moment of probe striking studied surface, with the accuracy being more than ~0.001 m/s. However currently used calibaraion devices comrpise large-scale sophisticated open ray laboratory installations. Compact fiber laser interferometer was developed allowing precision measurement of the probe velocities during its movement from beginning to the second rebound. Designed setup allows to conduct accurate measurement of the material hardness, with the velocimeter of some kind (such as inductive sensor) being included in hardness tester proposed interferemoter also allows to perform tester calbration. Developed system is based on PDV (Photonic Doppler Velocimeter) method - direct optical heterodyning with diret transformation of the doppler signals. Main advantages of the method are interference resistivity, high accuracy and ease of assembly.

HIGH ENERGY DENSITY PHYSICS

Mobile laser measurement system based on multichannel unequal path interferometer with quadrature-differential photodeteсtion.

Gathering experimental data on physical processes during intensive loading of the matter is the complicated fundamental task. The reasons of that are small duration of these processes and physical and chemical properties of studied objects. Special character of such experiments and difficulty of the extreme state matter researches set high demands to the research methods and instrumentation. The most informative and convenient observable parameter in the high energy density physics is the particle velocity. Usually it is measured by various laser interferometers.
Presented device belongs to VISARs - Velocity Interferometer System for Any Reflector. Quadrature-Differential Unequal Path Interferometer (QDUPI; КДНИ in russian) designed for remote non-contact measurement of the dependence between part velocity and time during intensive dynamic loads in shock-wave experiments. Structually system consists of individual units interfaced with optical fibers: laser source, light gathering and light transport subsystems, interferometer optical unit and photodetector unit (control unit). The interferometric system is completely remotely controlled from the PC using dedicated software for control unit operation.
Optical design is based on modified Michelson interferometer. Device analyzes doppler shift of the light reflected from the moving object. Multichannel registration system provides high accuracy and low noise by the polarization encoding of the optical signals and multistep algorythm of the experimental data processing. Unit design with fiber connections, remote adjustment, automated setup control, custom made software for data processing - all of these advantages establish developed system as the powerful and convenient tool for the shock-wave experiments in the HED physics experiments.
Single interferometer optical unit can be used for simultaneous analysis of multiple optical signals, so that simultaneous velocities measurement for multiple surface points of the object or even for multiple objects become possible. That concept is physically realized in multichannel system, in which seven parallel optical rays processed independently the same way it happens in single channel systems.

OPTICAL ELEMENTS QUALITY INSPECTION

Automated certification system for optical elements quality

Laboratory setup for the detection, counting and measuring of optical irregularities such as bubbles, impurities and striae.
Currently developed MJ laser facility is expected to comprise 5000 800x400 mm broad area optical amplifiers. These amplifiers (slabs) are under high quality demands, because even the tiniest irregularity of refractivity of the amlifier medium could lead to significant deformations of the laser beam wavefront. Thus optical quality control of the produced elements is essential.
Among most common local optical irregularities are air microbubbles and striae.
Stria is inhomogeneity of optical density of the medium. Difference between the refractivity of the optical glass and striae are of the 10-4 – 10-7 magnitude, so usually they cannot be detected unaided. Striae are unacceptable defect in the glass.
Bubble is the closed cavity filled with gas located inside the medium.
System allows detection and measurng of such defects with one of the dimensions no less than 30 um with less than 10% error. Constructed laboratory setup is capable to process optical elements with up to 810x460x60 mm dimensions.
Working principle is based on the automated dark-field detection of the irregularities with their following measurement with digital optical microscope.