Showing papers in "Methods in Experimental Physics in 1971"

15. Dense Plasma Focus

Abstract: Publisher Summary This chapter describes the dense plasma focus. Dense plasma focus (DPF) is a plasma discharge with plasma densities n > l0 l9 /cm 3 and temperatures of a few kilo electron volts lasting 100 to 150 nsec. The focus is conjectured to be a short but finite two-dimensional z -pinch forming near or at the end of a coaxial plasma accelerator. The dense plasma focus accelerator requires, for optimum operation, simultaneous application of multiple condenser modules. Inclusion of other plasma heating mechanisms and real boundary conditions at the electrodes is needed for a closer fit to the experimental results. The inclusion of simple magnetic fields does inhibit plasma heating but leads to a more stable focused column. The intense neutron production can also be used to supplement the nuclear fission reactor neutron spectrum for medical research. More recently, the application of intense pulsed neutron sources to the assay of fissionable materials in the nuclear safeguards program has seemed possible.

Deep space plasma measurements

Abstract: Publisher Summary This chapter describes experimental methods used for the study of plasma in extraterrestrial space by means of instruments carried on earth satellites and space probes. The techniques described in the chapter were developed primarily for observations within the interplanetary medium (the solar wind), but have also been used for the study of plasma within the magnetosheath, the magnetotail, and the outer magnetosphere. This chapter focuses on the measurements beyond the plasmasphere. The emphasis is on the physical principles underlying the instrumentation and on the methods for analyzing the observations. The chapter discusses the general properties of various types of detectors. Most of the effort in data analysis, particularly in the case of the solar wind, has gone into estimating the hydrodynamic parameters of the plasma (moments of the distribution function).

14. Radio Wave Scattering from the Ionosphere

Abstract: Publisher Summary This chapter describes the radio wave scattering from the ionosphere. The chapter focuses on the outdoor plasmas. Radar scattering measurements made from the ground at frequencies of 50 MHz or higher, frequencies far above the maximum value of the plasma frequency found in the ionosphere are discussed. At such frequencies most of the transmitted energy passes through the ionosphere and is lost; only a small fraction is scattered back to the receiver from small fluctuations in the electron density. As a result, high power transmitters and sensitive receiving equipment and sophisticated data analysis techniques must be used. The chapter discuses incoherent (or Thomson) scatter; it involves scattering from random thermal fluctuations in the plasma density. The chapter also describes scattering from ion-acoustic waves generated by a type of two-stream instability that is encountered in certain regions of the ionosphere. Some general features of all scatter measurements are discussed. Incoherent scatter and the much stronger scattering from unstable regions are examined. The incoherent scatter experiments have shown remarkable quantitative agreement with theory. A very wide range of observations are explained by the linearized kinetic theory.

11. Optical Refractivity of Plasmas

Abstract: Publisher Summary This chapter describes the optical refractivity of plasmas. The chapter focuses on the propagation through plasmas of electromagnetic waves whose frequencies (optical and near-optical spectral band) are larger than any characteristic plasma frequency. The refractive index is related to the dispersion relation, ω = ω( k ) , and the dispersion relation for free electrons is derived for the case of small amplitude waves in nearly collisionless, cold plasma with or without a stationary magnetic field applied parallel to the direction of wave propagation. The chapter discusses various standard measurements of refractive effects such as interferograms, schlieren pictures, shadowgrams, and Faraday rotation measurements. Measurements of refraction phenomena by techniques that depend specifically on the long coherence lengths of laser light are also discussed. These are (1) the coupled resonator gas laser interferometer technique for time resolved interferometry in which the laser is often both the source and phase sensitive detector and (2) holography, a new technique that can be adapted to make some of the standard measurements.

3. Plasma Diagnostics by Light Scattering

Abstract: Publisher Summary This chapter discusses plasma diagnostics by light scattering. In plasmas, such scattering is mainly due to electron density fluctuations. The details of the scattering depend on the spectrum of these fluctuations which can be related to plasma parameters such as temperature and density. Light scattering therefore provides a means of making a localized, minimal perturbing measurement of these quantities. The relevance of the fluctuation spectrum can be seen from a macroscopic, dielectric view of the scattering process. The scattered wave depends on the spectral component of the dielectric coefficient with wave vector. Studies of fluctuations in unmagnetized plasmas have shown that the density fluctuations may be separated into two categories depending on the relative magnitude of the fluctuation wavelength and the Debye length, λ D . In selecting a detector for scattered radiation, one is primarily guided by the consideration of sensitivity and frequency response.

Whistlers - Diagnostic tools in space plasma

Abstract: Whistlers as diagnostic tools in space plasma, measuring electron densities at large distances in earth outer atmosphere within magnetosphere

16. Plasma Problems in Electrical Propulsion

Abstract: Publisher Summary This chapter discusses the plasma problems in electrical propulsion. The involvement of plasma physics and technology in the area of space propulsion has occurred because (1) with the exception of nuclear explosions, the electrical or electromagnetic acceleration of conducting matter is the only feasible way of bringing rocket exhausts to the very high velocities necessary for deep space flight with payloads of men and their support equipment; (2) Ionized gas, or plasma, is the most convenient conducting material to use, as by virtue of its usually low density, it can be brought to the requisite velocities without the expenditure of prohibitive increments of energy. Several excellent developments of the rationale for electrical propulsion are available. The field of electric propulsion is concerned with plasma of medium to heavy elements, fairly well ionized, and having electron and ion densities in the range of 10 11 -10 15 /cm 3 .

4. Atomic Processes

Abstract: Publisher Summary This chapter provides guidelines and examples for determining the dominant atomic processes occurring in and affecting particular plasma. The common models used for analysis are described and the methods of analysis leading to knowledge of either plasma parameters or basic atomic information are exemplified by plasma generated in a theta-pinch device. Such plasma has cylindrical and near-axial symmetry and provides basic data on atomic processes occurring in trace amounts of elements present. Such data are of wide interest to astrophysicists as well as plasma physicists. The importance of knowledge of the correct velocity distribution function (usually assumed to be Maxwellian) for the plasma particles and the experimental methods available for measuring this function are discussed. It is the presence and interaction between ions, neutrals, electrons and photons that lead to the atomic processes which affect the plasma and provide information on the plasma state. The general processes involved are excitation, deexcitation, and ionization of atoms, recombination of free electrons with ions, and transitions of free electrons between continuous energy states.

6. Collisionless Shock Waves in Laboratory Plasmas

Abstract: Publisher Summary Shock waves are of importance for a great number of physical phenomena, on earth as well as in space. They occur in solids, liquids, gases, and plasmas. The chapter discusses the conditions under which they occur, how they propagate, and how they can be generated in the laboratory. Their structure is investigated in detail by theory and experiment and this has led to a well-founded understanding of the dissipation processes occurring in shock waves. In general, the velocity, density and pressure are smooth or continuous functions of position. But under certain conditions a sudden jump or an abrupt change in the flow field variables occurs on surfaces within the flow field. These surfaces are called shock fronts or simply shocks. The state of the gas does not change discontinuously. The diffusive action of dissipative processes broadens the front. Nevertheless, the jump occursover a very narrow region, usually a few mean free paths thick, so that for many applications this layer may be considered as a discontinuity.

10. Instabilities of High-Beta Plasmas

Abstract: Publisher Summary This chapter discusses instabilities of high-beta plasmas. In investigations of plasma confined by a magnetic field, the plasma is potentially able to break out of the confinement system by a large variety of instabilities. In attempts to achieve the confinement of plasma for long times, at the densities and temperatures required in a thermonuclear reactor, the problem of stability has emerged as the most important one at present. Although the problems of heating and confinement are also difficult, there are, in principle, many methods of heating plasmas to high temperatures, and a variety of field configurations which could confine the hot plasma in equilibrium, if they were stable. Microinstabilities involve fluctuating electric or magnetic fields inside the plasma; compared with magnetohydrodynamic (MHD) instabilities they are often slow-growing and not associated with any large scale motion, which makes them more difficult to detect. Future work on MHD stability is likely to be directed toward studying new possible high beta toroidal configurations, capable of producing longer confinement than the linear systems, which have received so much study in the past.

2. Microwave Scattering from Plasmas

Abstract: Publisher Summary This chapter discusses microwave scattering from plasmas. Microwave scattering from gaseous plasmas is a diagnostic which permits the study of collective electron density fluctuations by detection of the induced radiation. The interaction of a wave incident upon or propagating through plasma with other waves also present will result in a transformation of the original wave. The scattering is termed coherent if the frequency of the scattered wave is the same as that of the incident wave, and incoherent if the frequency is different. When the plasma is finite in size, one may refer to a type of scattering peculiar to the reflection process as incoherent rejection, which occurs when the frequencies of incident and reflected waves are unequal, and when the angles of incidence and reflection are unequal. When the wavelength of the incident electromagnetic radiation exceeds the Debye length, it has been demonstrated that the spectrum of the scattered wave will give information about the frequency and wave number spectrum of collective electron density fluctuations in the plasma. The theory shows that the collective electron fluctuations are determined by the motions of both the electrons and the ions. On the other hand, to detect the uncorrelated motion of the electrons, the wavelength of the incoming wave must be small compared with the Debye distance λ D .

Plasma heating by strong shock waves

Abstract: Publisher Summary This chapter discusses the plasma heating by strong shock waves. The chapter discusses the designing and construction of electromagnetic devices. The theory of shock wave phenomena is reviewed. Then the theory of various devices is examined which is developed to create shock waves under laboratory-controlled conditions. The chapter emphasizes on collision-dominated shock waves, although some of the theory and much of the technology is relevant to collisionless shock wave research. The chapter describes and analyzes the technology and component parts of devices which produce strong shock waves. The electromagnetic devices and in particular the coaxial electromagnetic shock tube is considered. The chapter concludes by summarizing the performance of some of these devices and by briefly illustrating results that have been obtained. The speed of shock waves and how laboratory measurements compare with the theoretical prediction for these devices is discussed.

9. Collisional Drift Instabilities

Abstract: Publisher Summary This chapter discusses collisional drift instabilities. The investigation of the collisional drift wave benefits greatly from simple equilibrium, simple boundary conditions, and limitation of the number of predicted unstable modes. In the stable regime observed, critical-fluctuation amplitudes agree with calculated ones. In the unstable oscillatory regime, wave frequency, mode number, n–Φ phase difference, and amplitude patterns are measured to be in agreement with those of the mode of highest growth rate predicted by linear theory. Linear theory, however, considers only disturbances of infinitesimal amplitude. Nonlinear interactions, as in the cases of strong mode-mode coupling or of turbulence, may distort an incipient instability so that the final saturation- stage behavior bears no semblance to the characteristics of the initial perturbation predicted by linear theory. A crucial plasma-confinement experiment is the measurement of anomalous losses. Causal relation between collisional drift waves and enhanced plasma transport is observed. Collisional drift-wave results reveal the evolution of plasma instability.

7. High Frequency Instabilities

Abstract: Publisher Summary High frequency (short wavelength) instabilities tend to have a more “fine-grained” nature than the low frequency instabilities. This difference is sometimes expressed by designating the high frequency instabilities as “microinstabilities” in contrast with the low frequency “macroinstabilities,” but there is no sharp division between the two types. High frequency instabilities manifest themselves through enhanced emission of electromagnetic radiation, increased particle losses and changes in the particle distribution function in the direction of thermodynamic equilibrium. The driving mechanisms for high frequency instabilities in plasmas are discussed. The general types of particle distributions that tend to be unstable are illustrated. High frequency radiation connected with cooperative phenomena has been observed in many experiments. The computer simulation of plasmas is a rapidly growing field. In the study of instabilities, computer simulation lies somewhere between the elegant theoretical formalism and the laboratory experiment.

Preface to Volume 9

Abstract: This volume is devoted to the understandings of law developed from the mid17th century to the end of the 19th century by jurists and legal philosophers working in the civil-law tradition.1 This makes the volume complementary to Michael Lobban’s Volume 8 of this Treatise, where the same subject matter and the same period are covered, but from a common-law perspective instead. This peculiar combination of subject matter and period has made it necessary for us as volume editors to make certain choices in treating the civil-law tradition: These choices concern the volume’s overall design as well as the framing of its single chapters. Which is to say that the thinkers and schools of thought covered are not arranged along a strictly chronological line of development, nor is there an attempt to show how different thinkers and schools of thought have offered different solutions to the same basic questions, concerning the nature, the distinctive traits, and the function of law, since that would not have made it possible to bring out how complex the development was that legal thought went through during the arc of time in question: Such a development cannot be reduced to a linear sequence of theories and ideas, for it is instead broken up by significant discontinuities. One need only recall here, by way of example, how an understanding of law still tied to the late-medieval world eclipsed during this period, making it possible for modern legal science to progressively take hold; or how the institutions of the Ancien Régime fell apart and the modern state became fully established politically as well as administratively; or how legal pluralism survived in Europe until the end of the 18th century, when legal monism came into being with the 19th-century codifications. This makes it necessary—as we take into view the jurists’ and legal philosophers’ understandings of law in the historical period in question—to speak of different epochs of development rather than of different stages within the same epoch. Add to this that the discontinuities just mentioned were marked by different characteristics, came to pass at different times, and had entirely different consequences depending on which levels legal discourse, which areas of cultural influence, and which parts of the legal system we are considering. Let us take a few examples to briefly consider, in the first place, what this means with respect to the different levels of legal discourse. There was the discourse of the practical jurists on the one hand and that of the theoretical ones on the other. And it can be observed in this regard that, while the jurists’

8. Low-Frequency Instabilities

Abstract: Publisher Summary This chapter discusses the low-frequency instabilities. Instabilities with frequency less than the ion cyclotron frequency have been investigated extensively because of the possible link of the instabilities with the diffusion of plasmas across the magnetic field. Experiments by Hoh and Lehnert showed the first unambiguous correlation between diffusion and instabilities (fluctuations). Quite apart from the relation between plasma loss and instabilities, low frequency oscillations have their own interest as manifestation of characteristics of plasmas, such as Landau damping, anisotropy introduced by magnetic field, etc. The chapter discusses the noncollisional plasma instabilities. Experimentally, if instabilities are observed, a procedure as outlined gives a rough guide as to the category to which the instability belongs. Experimental verification of low frequency instabilities is still in its infancy, especially for collisionless instabilities.

Plasma waves and echoes.

Abstract: Publisher Summary This chapter discusses plasma waves and echoes. The complexity of plasma physics is reflected in the waves that exist in plasmas. At any frequency, two transverse modes corresponding to electromagnetic waves and one longitudinal mode can in general be present. The longitudinal waves, which exist only in matter, are most sensitive to the properties of the plasma. Both density and temperature can be measured without affecting the plasma. In some cases, details of the distribution function and fluctuation spectrum can be inferred from wave measurements. The equations for a collisionless plasma comprise Vlasov equations for the electron and ion distribution functions and Maxwell's equations for the fields. For longitudinal modes, the only nontrivial field equation is the Poisson equation. The observation of echoes is inherently no more difficult than observing the primary waves themselves, although some requirements are somewhat more stringent. The plasma must be large and truly collisionless. To observe the primary wave, a plasma only a few damping lengths long suffices. The scale must be increased by an order of magnitude to observe echoes. The great sensitivity of the echo to collisions requires that the mean free path really be longer than the device. If collisional damping is detectable in the primary waves, it will probably destroy the echo.