CHAPTER 1
INTRODUCTION
The prospect of generating abundant amounts of energy using ubiquitous natural resources is an extremely attractive proposition given the state of the energy environment today. In this regard the science of magnetic confinement for achieving controlled nuclear fusion has presented researchers with means to investigate ways to harness the power of the atom. Nuclear fusion propounded in the late 19xx pertains to combining elements of light nuclei by overcoming their mutual electrostatic repulsion to release vast amounts of energy – a mechanism which is fundamental to energy production in the sun.
Magnetic confinement research, which has been motivated from the perspective of achieving this goal in a controlled manner, has been pursued in the United States for almost 40 [KC1]years in over 7 different confinement schemes. The promise of the most investigated of all concepts, the tokamak, is by far the most developed and closest to realization of a practical working reactor. However, other closed and open ended confinement concepts have also been pursued at various levels at large governmental research labs and as parts of various University university programs.
The Madison Symmetric Torus is one such example of a close-ended confinement concept which belongs to the class of devices known as an reversed field pinch (RFP). The name stems from the unique feature of the reversal of the toroidal component of the magnetic field near the outer edge of the plasma. The RFP concept has been pursued in the United States for 20 [KC2]years, however in recent years MST has been the sole RFP program in existence in the United States. The attractiveness of the RFP confinement scheme stems from two distinct engineering advantages namely, (a) a higher plasma beta and (b) lower required confining magnetic fields.
The former of the two quantities is an important aspect of generating fusion energy because it directly [KC3]pertains to the power density achieved in a fusion reactor. A high beta is desirable because it implies that of the finite amount of energy in a magnetic system, more of it will be used to heat the plasma than to use in the production of magnetic fields required to confine it. The implication of (b) is that the engineering design of a working RFP-fusion reactor will be significantly cheaper to manufacture (compared to a Tomakak) because of the low costs associated with the required magnet technology [ref].
With the promise of the RFP comes a host of issues to be solved before its manifestation as a practical working fusion device. The most dominant of these issues pertains to the degradation of plasma confinement by magnetic turbulence. Despite this fact, a 9-fold confinement improvement has been recently obtained in the MST RFP paving the way for further active control of magnetic turbulence surpressionsuppression.
In addition to these achievements, the[KC4] MST has become an extremely well diagnosed machine, thus enabling scientists to correlate the various plasma parameters that are important for understanding the nature of confinement in the RFP.
One such plasma parameter that provides an immediate understanding of the confined species is the plasma potential. The plasma in a magnetically confined device is quasineutral. However[KC5], the mobility of one species over the other allows for rapid loss of this species due to some phenomenon in the plasma. In the RFP, the rapid loss of electrons along a stochastic field line,thus causingwhich causes the plasma to charge up positively, has been theoretically predicted and experimentally measured inside the reversal surface [KC6][ref].
The material presented in this thesis presents first -ever detailed experimental results of the measurement of the plasma potential inside the core of a hot RFP. While plasma potential measurements and even electric field measurements have been made in MST in the outer edge region, the direct measurement of the core potential has only been made possible through the application of the heavy ion beam probe diagnostic (HIBP). The measurement of the plasma potential presents novel information for RFP researchers. While the core potential has been anticipated to be positive in standard discharges in MST-RFP, its level compared to the electron temperature was not known. It has been hoped that the measurement of potential can provide some information about the level of stochasticity in the core and the changes in the stochasticity over the time periods associated with changes in magnetic topology, including some rather abrupt ones. This is an interesting and an important area of RFP research because RFP’s in general are well known for their high levels of magnetic fluctutation driven transport. Hence the extent of the rapid loss of electrons along stochastic or chaotic field lines is closely related to the study of magnetic stochasticity in the core and perhaps the evolution of the plasma potential. If the plasma potential is positive throughout the discharge, the result would then indicate that the RFP magnetic equilibrium is always stochastic[KC7].
The measurement of the radial electric field (gradient in potential) is more valuable because of itsit can have a stabilizing influence on turbulent particle loss at the edge of MST in certain enhanced confinement discharges[KC8]. In this regard the experimental investigation of the nature of the radial electric field in the core of MST has been also been aggressively pursued. Certain theories on stochastic transport processes suggest that the ambipolar electric field established in the core of a stochastic plasma should be a function of the density and temperature gradients [ref]. While fFundamental relations like the ion-momentum balance suggest that the electric field should be closely related with plasma rotation as well as the pressure gradients [ref]. Additionally the measurement of the radial electric field provides a way to quantify the magnitude of the generated flows in MST.
The application of the HIBP to make measurements of the core equilibrium potential profile in an RFP is the first ever such endeavor for the HIBP. Given the plethora of challenges from the onset of the design process, the successful application of the HIBP has been just as important as obtaining meaningful results and successfully interpreting them[KC9].
The organization of the material presented in this thesis begins with an introduction to the measurement of plasma potential using the HIBP diagnostic on a variety of different devices in the United States. Particular emphasis is placed on the importance of the measurement from the standpoint of its application on a reversed field pinch plasma confinement device. In this regard, the third chapter deals with the phenomenology of the RFP configuration, the operation and the operational space of MST-RFP. The various different types of plasma discharges are described in detail with emphasis on theose discharges types which are investigated in this study. In chapter 3, the principles of the heavy ion beam probe diagnostic and the principles of HIBP measurement of plasma potential is discussed. Some important aspects of the MST-HIBP system design are also explained in this chapter.
The measurement of the plasma potential and the radial electric field time evolution is discussed in chapter 4 in high current standard discharges. Similar measurements (although only equilibrium measurements averaged over 2 ms) are also discussed in a variety of low current discharges. Given that one of the challenges for the HIBP measurement of the plasma potential profile is the knowledge of the magnetic field chapter 5 deals with utilizing recently established techniques for generating magnetic fields and using it to determine HIBP measurement locations. The relationship between the radial electric field and plasma flow will be discussed in chapter 6. Finally the sources of uncertainty in the measurement of the plasma potential and potential profile are discussed at length in the appendices[KC10].
[KC1]1It has been longer than this. You also need a reference for the history of fusion. There are many to choose from.
[KC2]1Check this number too. You might want to mention at least the machines at Los Alamos and where the concept came from originally.
[KC3]1Poor wording. Just say what beta is.
[KC4]1I looked in MST papers and they do not say ‘the MST’ as far as I can tell.
[KC5]1This is a very poor sentence. Try again or drop the sentence.
[KC6]1Using what diagnostic?
[KC7]1‘Always’ seems kind of strong. You need to provide some kind of reference that stochasticity is more likely to affect electrons, probably because their small larmor orbits do not average out the effects very well.
[KC8]1Reference? Why do we know this?
[KC9]1You should just state the two major tasks – implementation and acquiring meaningful data – which are both difficult.
[KC10]1You still seem to have trouble writing simple sentences describing things. Each chapter has a specific purpose. Just, state it simply.