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Probe Measurements of Plasma Parameters

#3248


Development of Automated Plasma Electrokinetic Measurements Probe System

Tech Area / Field

  • PHY-PLS/Plasma Physics/Physics

Status
3 Approved without Funding

Registration date
08.04.2005

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • LISMA - All-Russian Scientific Research Institute of Light Sources, Russia, Mordoviya, Saransk

Collaborators

  • Universidade da Madeira, Portugal, Funchal\nCNRS / Centre de Physique des Plasmas et de leurs Applications de Toulouse, France, Toulouse\nUniversité de Limoges, France, Limoges\nUniversité d'Orléans / Groupe de Recherches sur l'Energetique des Milieux Ionises, France, Orleans\nUniversity of Sheffield / Department of Chemistry / High Temperature Science Laboratories, UK, Shiffield

Project summary

The goal of the project is to develop the automated complex for probe measurements of electrokinetic characteristics of electronic plasma component, namely the concentration and electron energy distribution function (EEDF), rates of various kinds of electrons inelastic reactions, factors of electrons mobility and diffusion, and etc.

Traditionally special interest to probe techniques is explained not only by capability to measure a great number of the most important parameters of plasma. The method of probes is the unique direct one for obtaining data on local characteristics of plasma under research. Proposed by Langmuir for the first time it was based on supposition that the electron velocity distribution function (EVDF) is maxwellian. Meeting a number of requirements among which the most important is a small probe radius and a small thickness of the pre-probe layer of spatial charge as compared to the free path (so called collisionless mode of the probe operation) the Langmuir theory allowed to define the temperature and concentration of electrons by the probe electron current. Druyvesteyn put forward the probe method in his work, where it was shown, that meeting above-mentioned requirements it is possible to measure arbitrary isotropic EVDF by finding the second derivative of the probe electron current.

By present the major of probe measurements have been performed and are being carried out actually for the collisionless mode (or the Langmuir probe method feasible only for pressures up to 1-3 torr due to difficulties related with fabrication of low radius probes) in the absence of the magnetic field and only for practically isotropic EVDF.

All known modern automated systems for probe measurements produced by commercial firms (SCIENTIFIC SYSTEMS, HIDEN, and others) are also capable to perform measurements only for the Langmuir probe mode. At the same time over the recent 10-15 years considerable progress has been achieved in the theory of probe measurements of medium pressure plasma, plasma with considerable anisotropic EVDF, and plasma in the magnetic field.

In the proposed project there is planned to develop an automated system allowing to carry out probe measurements in a wide range of pressures, magnetic fields and anisotropy of EVDF. The basis for development of such a system will be both analytical researches carried out by the participants of the proposed project and numerical codes developed over the last years and intended for processing of the probe currents for obtaining wide data on electronic component of plasma. In particular, the codes take into account the important for probe diagnostic effects, among which are the “sink” of electron to the probe in medium plasma pressure and in the magnetic field as well as the influence of the degree of electron velocity distribution function (EVDF) anisotropy on generation of the probe electron current under low pressures of gas. Further perfection of the mentioned codes takes into account such important factor as effect of electron collisions in the near-probe layer of the spatial charge on accuracy of electron energy distribution function (EEDF) measurements over the first derivative of the probe electron current. As the preliminary research show without taking into account the mentioned factor considerable errors will appear when measuring a high-energy part of the EEDF.

The participants of the project have gained more than 20 year experience in the field of probe diagnostics of plasma. On the base of the analytical research performed by them a number of new methods for treatment of probe voltage-current characteristics were developed.

As a result of the project fulfilment it is expected to manufacture the automated complex differing principally from the known ones; it will allow to perform probe measurements of electrokinetic characteristics of plasma in a considerably wider spectrum of experimental conditions:

  • In range of pressures up to ~102 torr,
  • In range of magnetic fields up to ~103 Oe,
  • For arbitrary degree of EVDF anisotropy in the range of pressures up to ~ 0.1 torr.

The proposed project is combined of fundamental and applied research. In the course of its fulfillment there will be developed and utilized new techniques for processing the probe voltage-current characteristics. Execution of the project will allow to verify more precisely model imagination about the electron plasma component. As the review of many publications shows an incorrect approach to processing of the probe VAC leads often not only to quantitative, but also to qualitatively wrong conclusions about behavior of the measured plasma parameters. So, neglect of the “drain effect” for measuring of electrons temperature (average energy) in magnetized plasma may lead to its raise with the increase of the longitudinal magnetic field applied to plasma instead of the decrease predicted by the theory of positive column. Failure to take account of the considerable degree of EVDF anisotropy may lead to the wrong conclusion about the difference in the longitudinal and transverse temperatures of electrons in the low pressure plasma, and etc.

According to the scope of activities within the framework of the proposed project cooperation with foreign collaborators will include the following:

  • information exchange in the course of the project fulfillment;
  • comments to technical reports (quarter, annual, summary, and etc.) issued by the project participants;
  • control of experimental data and results obtained while the project fulfillment.

The Project meets completely the goals of the ISTC:
  • it makes possible to the scientists and specialists of VNIIEF engaged in development of nuclear weapons to reorient their activities to the civil goals related with development of new conversable technologies;
  • the project is based on modern achievements of science and technology in the field of plasma physics and, thus, encourages integration of nuclear scientists to the international scientific community;
  • the project possesses some commercial potential since as a result of its completion the automated complex will be manufactured allowing to perform probe measurements of electric and kinetic plasma characteristics in a wide range of the parameters under study.

The basic results of the Project will be published.


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