X-Ray Polarisation of Z-Pinch Plasma
X-Ray Polarisation Researches of the High Current Z-Pinch Plasma Radiation. Theory and Experiment
Tech Area / Field
- PHY-PLS/Plasma Physics/Physics
3 Approved without Funding
TRINITI, Russia, Moscow reg., Troitsk
- Kurchatov Research Center, Russia, Moscow
- Ruhr Universität Bochum / Fakultaet für Physik und Astronomie Experimentalphysik insbe. Gaselektronik, Germany, Bochum\nUniversity of Maryland, USA, MD, College Park\nKyoto University / Graduate School of Engineering, Japan, Kyoto
Project summaryX-ray spectroscopy is the most powerful way of both laboratory and space high temperature plasmas investigation. Analysis of different x-ray lines and their relative intensities may provide information on temperature and density, ionization states, electromagnetic fields, ion velocities, suprathermal electron energy, etc. Classical plasma diagnostics usually assumes, that the x-ray lines, emitted from hot plasmas, are not polarized.
However, recent experiments, performed in laser produced plasma , vacuum sparks , plasma focus device  and solar flares  have shown, that x-ray line emission of He-like ions, widely used for diagnostics purposes, are essentially polarized. Complete interpretation of these measurements is not yet possible because of theoretical difficulties and because several experimental parameters are insufficiently known. Examples are the radiation angular distribution, diagnostics calibration as function of polarization and wavelengths, and how the data are changed over space and time.
Polarization phenomena is needed to be throughly studied for more complete understanding of physical processes in hot plasmas. Intensities of polarized lines may contain information on inner electro-magnetic fields, electron beams, process of plasma x-ray emission spectra formation. The proper accounting of this effect will lead to essential corrections in existing approaches of density and temperature determination.
In this connection, in the leading laboratories of Europe, USA and Japan an active theoretical study of this phenomenon has been started, as well as the development of the diagnostics techniques, based on the measurement of polarization degree in the x-ray radiation (Plasma Polarization Spectroscopy) in spectral lines of multicharged ions and continuos radiation.
A given Project expects a wide range of studies to be done in the x-ray radiation polarization from the Z-pinch plasmas, including:
- development of a theoretical x-ray plasma radiation polarization model;
- development of codes for calculating the plasma parameters on the basis of spectroscopic measurement data;
- development of a new diagnostics technique for investigation of polarization of lines and an instrument for this purpose;
- realization of experimental studies of polarization phenomena at various facilities.
Physical reason for polarization is connected with anisotropy of excitation process. X-ray radiation of atoms in x-ray tube are partially polarized, and x ray radiation, emitted by ions, are also polarized, if ions are excited by an anisotropically distributed electrons velocity function. Experiments, carried out on an ion trap , clearly show polarization for He-like ions. Theory [6, 7], made for this sort of ions, agrees with experiment.
Another reason for polarization might be the splitting of atomic and ionic energy levels in electric and magnetic fields. The resulting spectral lines are also split (Stark and Zeeman splitting), and the different components are polarized.
Likewise for plasma, microscopic electric fields in electron collisions with ions, result in asymmetric excitation of single ions and polarized x-rays in their radiative decay. This takes place, because radiator dipole moment orients along resulting local electric microfields of surrounding ions.
In a z-pinch plasma, which is planned to be investigated, the strength of inner electric fields may reach 108 V/cm. In the case of isotropy for such fields, the polarization is absent, and, vice versa, the presence of polarization confirms the anisotropy of electric fields in the plasma. The maximal polarization degree should be observed, when a field, causing the anisotropy is comparable in it’s magnitude-with an average microfield of the plasma ions. In the case of the presence of strong magnetic fields the picture becomes still more complete due to probable Zeeman splitting of some levels.
Therefore the polarization measurements can be used for production of a qualitative and quantitative information about the average magnitude of electric and magnetic fields, existing in a plasma. Moreover, the produced data on polarization have a practical significance for correct interpretation the plasma emission spectrum, for plasma parameters investigation and for optimizing the x-ray radiation yield. The importance of this phenomena for proper understanding of z-pinch plasma development is mentioned elsewhere [8, 9].
The first measurements of line polarization for a helium-like argon at the Plasma Focus facility, Mather’s type, was realized in [3, 10]. Two similar crystal spectrographs with mutually perpendicular dispersion planes were used for spectra registration. The spectra analysis has shown, that the plasma parameters, estimated from the relative line intensities, registered with various instruments in the same experiment, do not coincide. The most probable reason for such disparity can be different polarization of the x-ray lines, used for determining the plasma parameters.
Indeed, the theory deductions confirm the fact, that the degree of polarization and it’s direction can be different for the resonance and intercombination lines of the helium-like ions. This sort of ions are mainly present in the laboratory plasma and widely-used for estimating it’s density .
The treatment of the produced Plasma Focus spectra confirms a considerable (up to 30%) polarization of the resonance argon line and the presence of an electron beams (a few percents). As known, the presence of electron beam may strongly affect the population of some ionized argon levels. Therefore the similar measurement can provide not only the quantitative information about the magnitudes of electromagnetic fields but that about the electron beam driven in these fields.
However, for the correct interpretation of experimental results one needs an information about the degree of isotropy of radiation, plasma optical depth, reflection coefficients of the crystals, used in the experiments, dependent on polarization and on the wavelength of the incident radiation.
The measurements are also needed to be done with time and space resolutions, because degree of polarization strongly varies in time and because different parts of plasma radiate with different polarization. It is necessary to account of a radiation source size affect, when the plasma is asymmetric and the measurements are done with two instruments, directed differently with respect to plasma.
Polarization measurements, performed at vacuum spark device [2, 12], shows polarization of He-like iron ions only qualitatively, because they were done in 5000 overlapping shots and also without spatial resolution. The use of high spectral, time and spatial resolutions techniques with high sensitivity in such the experiments is obvious to obtain proper information on polarization effect.
In a given Project it is expected to realize polarizational measurements with time (1 nsec) and space (ten microns) resolutions. Spectra will be registered with the same unique instrument, operating in the two crystal circuit-diagram or in the single crystal one, using reflection from various planes. This scheme allows to avoid difficulties connected with the limited source size.
The space resolution will be realized due to the use of a spherical and toroidal crystal with spectral resolution better than 5ґ10-5. This crystal will give the spectral focusing in a meridianal direction and allowing one to produce the source image in the sagittal one at a given wavelength. High quality quartz crystals, connected with substrates by optical contact will be calibrated for different wavelengths and polarizations.
The manufacturing of prototype unit of this instrument was sponsored by US firm “Ecopulse” and first device is planned to be used in the nearest future.
X-ray films and absolutely calibrated charged coupled devices (CCD) with spatial resolution about 10 microns will be used for time integrated measurements. Special CCD control block will provide the following functions:
- synchronization of device operation with installation time cycle;
- conversion of CCD signal to digital code and transport of one to computer;
- set of codes will provide the process and representation of data.
Time resolved measurements will be done with microchannel plates (MCP), providing time resolution better than 1 nsec. The set of MCP sensors will measure x-ray intensity in several spectra intervals at the wide dl ~ 0,1 A. The frame camera on the MCP base (4 frames) will be used for spectral image measurements.
Experiments are planned to be realized on Russian installations Angara-5-1, S-300, vacuum spark. We already have the good experience in doing mutual polarization study with Poland [9, 10]. We will also set up collaborations with US laboratory (SNL Sandia), Japan (Kyoto University), group in France, some non-traditional science powerhouse countries, as Portugal.
Developed technique might be used in mutual experiments on different installations. Both the designed x-ray spectrograph and parts (crystals, detector system) of it will be proposed for commercial spreading through “Ecopulse”.
In the frame of the Project so called post-collisional theoretical model will be developed to describe phenomena under investigation. This model studies the interaction of the radiator and resulting electric fields, arisen due to existence of surrounding ions and exciting electron, after the excitation process. The necessary strong bond approach and the use of rotating coordinate system were suggested in [13, 14, 15]. The model, proposed in the Project, assumes the development of Seaton’s formalism  for the case of a long range post-collisional interaction, not having a spherical symmetry.
Such the improved approach allows one to eliminate the contradiction between the theory and the experiment in the near threshold range , intrinsic to the conclusions by Percival and Seaton, as well as to all the subsequent calculations based on the radiator state quantization in the co-ordinate system, related to the exciting electron velocity up to an instant of collision. This theoretical method will provide analytical dependence of intensity and degree of polarization of x-ray lines on plasma parameters.
2D MHD codes describing z-pinch plasma dynamics will be used to obtain plasma parameters in time. The 2D MHD, presented in  model will be used as the basis. Combination of post - collisional model results and 2D MHD codes will provide soft ware production, describing both the polarization degree of investigated lines and plasma parameters: temperature, density, anomalous resistance, distribution of electromagnetic fields, fast particle generation. These codes, possessing the suitable service, accessible to be used by experimentalists, will be proposed for commercial spreading.
Thus after termination of the Project the following results are expected to be produced:
1. Production of a post-collisional theoretical representation of x-ray plasma radiation polarization from z-pinch, taking account of a dipole radiator moment dynamics in the resulting electric field of exciting electron and that of surrounding ions included.
Such a theoretical approach, accounting the post-collisional interaction between radiator and electric fields of surrounding ions, will allow one to eliminate the existing (at present) contradiction between the theory and the experimental data, produced in the near threshold range.
2. Development of a soft-ware product, including 2D MHD codes, allowing one to determine the plasma temperature and density in time, distribution and magnitudes of electromagnetic fields, the luminosity of the lines, widely-used in the plasma diagnostics, and to make the comparison with the results of the experiment. It is expected to produce a commercial modification of such a soft ware product, possessing of a suitable service accessible to utilization by experimentalists.
3. Development of a new technique for measuring the line spectrum polarization of multicharged ions and of the Ka lines of low ionized ions. The implementation of such technique will also allow one to make conclusions about the inner electromagnetic fields and an electron velocity distribution function.
It is expected, that the comparison of the polarization measurements with the theory deductions will result in the introduction of necessary corrections into some existing spectroscopic techniques for determination the parameters of turbulent z-pinch plasma.
4. Production of a unique double focusing x-ray spectrograph to register the spectra in the range 700 eV — 20 keV. The mica crystals, as well as quartz one, connected by optical contact with spherical and toroidal substrates, provided a high spectral resolution (5ґ10-5) and a high space one (ten microns) will be used as dispersive elements. The calibration studies of dispersive elements for this device will be performed with the two crystal diffractometer, the alignment experiments will be carried out at the vacuum spark facility.
The spectrograph, produced by Kurchatov Institute together with the US firm "Ecopulse", will be used as the instrumentation base. The position sensitive detectors with spatial resolution of 15 mk and microchannel plates with the amplification higher than 105 and with the time resolution of 1 nsec, developed at TRINITI, will be used as detectors. There are some grounds to expect that the similar instrument will be commercially-saleable.
5. The x-ray radiation polarization study, testing of a new diagnostics techniques, including the x-ray measurements with the produced instrument, will be realized at the facilities, vacuum spark (current level is 150 kA), STAND-300 (current level is 1-3 MA) and at ANGARA-5-1 (current level 3-4 MA).
Moreover at ANGARA-5-1 facility the continuos and line radiation measurements will be realized in the range 1-2 keV with the absolutely calibrated x-ray multilayer reflecting mirrors. The polarization measurements at these facilities will be accompanied by other independent techniques for measuring the plasma and electron beam parameters: analysis of x-ray bremsstrahllung spectra, laser shlierien, collector current measurements, etc.
6. An analysis of the experimental data and the comparison with theoretical deductions about the mechanism of emerging the polarization will permit to produce the scaling law for the magnitude of the effect, depending on the z-pinch current level.
This analysis will also provide information on the distributions of electromagnetic fields in plasma and on their origin. An analysis of a bond between the current and suprathermal electrons and the polarization value, as well as that with some x-ray radiation line intensities, will allow one to consider an opportunity to use the new technique for measuring the electron beam driven in high current z-pinch facilities.
Such studies are of importance for comprehending the physical processes in z-pinch plasmas, as well as for solving the applied tasks, related with the production of a maximal x-ray radiation yield at a given power of the facility.
The development of polarization measurements is accompanied by the number of difficulties-experimental and theoretical nature-and need the joint efforts of the scientists from many countries. It is related to the creation of reliable techniques and instruments for registering the polarization of a line x-ray radiation spectrum and that of a continuos one with high spectral, space and time resolution and for developing a good theoretical basis to interpret the produced results.
The role of collaborators can be manifested in various aspects, e.g., consultations in the main scientific problems, joint interpretation of the observed phenomena included, exchange by diagnostics techniques and by technological developments, co-ordination of the studies at various facilities.
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