Hans Pécseli
Department of Physics, University of Oslo
hans.pecseli@fys.uio.no
Blindern, 11 November 1996
By a plasma we generally understand a fully, or at least significantly, ionized gas with free electrons and ions. Under such conditions even small variations in the charge distribution will give rise to large currents, and the dynamic properties of plasmas are to a large extent controlled by electric and magnetic forces. Plasma is often called «the fourth state of matter» since its dynamic properties in many ways differ from those of ordinary neutral gas. Plasmas appear on small scale in our natural environment in lightnings, fire, auroras etc. From altitudes above, say, 100km the fraction of charged particles is steadily increasing and the neutral component is negligible above 400km altitude. The gas in the upper parts of the ionosphere is completely ionized. On cosmological scales plasma is entirely dominating the universe, with safe estimates giving that at least 99% of all matter is in the plasma state. Its study is thus of basic importance for our understanding of nature.
It may be appropriate to rise the question whether research in plasma physics has been important for the progress of society and of science. This question is addressed in quite detail in several hearings and publications, i.e. Vol. 7 of Physics Through the 1990s, Plasmas and Fluids (National Academy Press, 1986), a report issued under the charter granted to the National Academy of Sciences by the Congress of the United States, and also Research Briefing on Contemporary Problems in Plasma Science (National Academy Press, 1991). The first one of these two references, in particular, explicitly recommends that plasma science becomes an integrated part of university physics curricula thus emphasizing the overall importance of the field. Some of the conclusions of these studies are included in the following.
Plasmas are also important for many technologically significant processes. It is important for welding, plasma etching, in high voltage circuit breakers, light sources etc. The interaction between charged particles and matter is a question of central importance for the semi-conductor industry, and plasma effects have entered this field also. In many chemical processes it is advantageous to rise the temperature in order to get higher reaction rates. The result can be a partial ionization of the gas and so called «plasma chemistry» is becoming an important field, opening possibilities for production of new materials. Advanced methods for waste disposal rely on processing materials in the plasma state. An interesting recent development concerns the study of dust-plasmas, i.e. charged dust (or other macroscopic particles) in a plasma environment. This problem is relevant to astrophysics as well as technological processes. Interplanetary plasmas have imbedded a nontrivial amount of macroscopic dust particles, where a significant fraction is charged. The dynamic properties of the surrounding plasma is modified significantly by these charged particles. Charged dust is present also in a number of industrial plants for ion-implantation etc. where an understanding of the properties of this (unwanted) component is of great importance. This subfield poses an interesting possibility for contact between basic research and immediate practical applications.
Novel principles for particle accelerators are incorporating basic plasma processes and these devices are expected to be important for science and industry in the future by providing a possibility for very compact sources of energetic particles.
The understanding of the interaction between charged particles and matter in processes like ion implantation, sputtering, plasma etching etc. is of utmost importance for central sub-branches in the industrial development. The importance of these processes for the industrialized modern society can hardly be overestimated.
An important reason for the interest in plasma physics today is its relevance for fusion research. The peaceful use of fusion energy is expected to require a control of light elements at temperatures in excess of millions of degrees where all matter is in the plasma state. There are reasons to believe that it will be possible in the future to control and maintain a plasma by externally applied electric and magnetic fields even at these extreme conditions. The most successful experiment today is the «Tokamak», a toroidal device where the plasma can be confined by a combination of externally applied magnetic field in addition to those generated by currents in the plasma itself. In November 1991 a 200 million degrees plasma was generated in such a device, the Joint European Tokamak (JET) in England. It was confined for more than 2seconds and delivered in excess of 2MW power by fusion processes. Similar results were later obtained also in other devices.
The controlled use of fusion energy may solve mankinds energy demands for all foreseeable future. The early expectations were however far too optimistic and the problem proved to be much more complicated than anticipated. Although fusion research has made a significant progress it still has a long way to go. The recent results from the European tokamak device (JET) has made it plausible that no essential problems prohibit the demonstration of scientific break even, i.e. achievement of a thermal energy density where fusion processes give a significant energy output. This result is certainly within reach, and is probably delayed only by practical considerations such as a concern for radioactive contamination of the inner walls of the confining test-device. The step from here to an actual prototype reactor is however very big and this advance is not to be expected in the near future.
A study of the Earths ionosphere and magnetosphere provides an understanding which can be applied to many magnetized planets in or outside our solar system. Although the Earth may be unique in a biological context, it has many features in common with most of the other planets in our solar system. As an example, it was well known that Jupiter was a significant source of radio noise in our near space environment, but is was eventually realized that also the Earth is a very similar emitter of radio noise, the so called auroral kilometric radiation, which has subsequently been studied extensively by instrumented spacecrafts. From a space science point of view, the earth is similar to, rather than distinct from, other planets, and its interaction with the magnetosphere of the Sun can provide information which can be universally applied to many other distant solar systems in the universe. A dedicated effort can take advantage of the relative ease with which the near Earth environment can be probed with scientific satellites and ground based equipment.
Most important may be the contribution of plasma sciences to our basic understanding of nature. Plasma science is particularly rich in the area of non-equilibrium and nonlinear processes. It was one of the first disciplines to grapple with the relationship between nonlinearity and complexity, a problem that is recognized today as a hallmark of late twentieth century science.
The physics of plasmas are studied by theoretical, experimental and numerical methods. The experimental work, in particular, follows several main lines. One is a dedicated fusion oriented effort. These experiments demand such huge investments that international collaboration is mandatory to meet these demands. Individual national laboratories are usually engaged in specific tasks, certain diagnostics etc. The basic laboratory research in the field is maintained by smaller experimental groups. The study of plasma processes in space is on the other hand also a very resource consuming activity, and is again requiring extensive international collaboration, although we have witnessed very successful national programs also. Space and laboratory experiments are to some extent complementary. They explore different ranges of dimensionless parameters. Space plasma configurations usually contain a much larger number of gyro-radii and Coulomb mean free paths than achieved in laboratory plasma experiments. In the laboratory special plasma configurations are set up intentionally, whereas space plasma configurations assume spontaneous forms that are recognized only as a result of many single point measurements. Space plasma are free of boundary effects; laboratory plasma are not, and often suffer severely from surface contamination. Because of differences in scale, probing a laboratory plasma disturbs it; diagnosing a space plasma usually does not. The pursuit of a static equilibrium is central to high-temperature fusion oriented plasma physics, whereas space plasma physics is concerned with large scale, time dependent flows. Certain problems are best studied in space others in the laboratory. The central difference between laboratory and space plasma experiments is the possibility for interaction between the experimentalist and the experiment. In laboratory this interaction is often a trivial matter, parameters can be adjusted and fine tuned and experiments repeated until a desired signal-to-noise ratio is obtained. The same process is possible only very seldom in space experiments. It may sound paradoxical, but a deep theoretical background and understanding is much more important for space plasma research than for laboratory investigations; in the laboratory it is in principle possible (although reality may often be more restrictive) to map out a full parameter space by repeated measurements and thus at least ideally to obtain a rather complete understanding. This is not generally feasible in space experiments where frequently only a limited amount of data are available. The quality of these data is with present standards very good, however. The recent strengthening of theoretical space physics, together with the increasing capability of space plasma instrumentation and the superiority of space plasma environment for certain types of measurements, means that the experimental diagnosis and theoretical interpretation of certain space plasma processes now matches the best of current laboratory practice.
Theoretical and experimental investigation of plasma dynamics encounter serious complications and numerical methods have proved specially important for the field. In particular, actual simulations of the plasma, often including millions of mutually interacting simulation particles, has proved to be of utmost importance. The implementation of such models is however by no means straight forward; although modern computers allow treatments of systems with millions of degrees of freedom, this is still insufficient for a one-to-one simulation of plasmas or fluids. Considering plasmas, in particular, it is necessary to let many «real» particles to be represented by one or just a few «simulation» particles. A representation which leaves basic physical phenomena retained poses a very intricate theoretical problem, which has only recently been solved by the so called «particle-in-cell» methods. In optimum cases some of the simulation results can be tested against certain experimental observations and if agreement with these is verified, one can have confidence in the predictions of the numerical code also for quantities which are not directly accessible for measurements. It is for instance difficult to perform direct measurements of phase space dynamics, while these can be obtained numerically. The electric field variations on the other hand are more easily measurable and can be directly compared with the results of the numerical analysis.
Originally, computers were thought of as mere extensions of the physicists abilities; numerically one could perform the calculations which were too time consuming to be practically realizable, but in principle the computer did what the scientist could have done. Slowly, it was realized that numerical methods are to be considered as an independent and equal discipline. By numerical particle simulations, in particular, we can obtain information which can not conceivably be obtained by any other means. In a computer, a plasma state can be generated where idealized conditions are fulfilled, conditions which in natural plasmas can be considered as approximations only. Theoretical investigations, which are generally based on some simplifications can be tested with the assumed conditions being ideally satisfied and the validity of theoretical models can be assessed. It is thus fair to state that the present approach to numerical methods in plasma physics is founded on a basically new philosophy. Modern plasma physics is based on three disciplines on the same standing; theoretical, experimental and numerical investigations and progress is made by the constant interaction and interplay between these methods of approach.
In spite of the many reasons for interest in plasma physics it is nevertheless a relatively young discipline. Although often spectacular, the early experiments in the 18th and 19th century were the works of gifted amateurs. It seems fair to claim that plasma physics matured in our century. This happened through detailed studies of Langmuir (who actually introduced the word «plasma» in this context) and several others around 1920. The main reason for this late development may be that production of plasma in a laboratory is most simple in electrical discharges, and technology was not sufficiently developed for these in earlier times. Since then, the study of matter in the plasma state has made a very significant progress by the combined efforts of researchers with very different interests, ranging from basic research in astrophysics to direct industrial applications.
Plasma Physics; A General Introduction
This document was generated using the LaTeX2HTML translator Version 96.1 (Feb 5, 1996) Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
The command line arguments were:The translation was initiated by Runar Jørgensen on Tue Jan 7 18:06:02 MET 1997