talk

abstract

Plasma, the 'fourth state of matter', is a large collection of ionized particles and neutral species. Quasi-neutrality and collective behaviour are two important properties of plasma, make it different from a normal ionized gas. It is often said that almost 99% matter of this universe exists in the plasma state. Examples of naturally occurring plasmas include solar wind, gaseous nebulae and most of the interstellar hydrogen, while electric arcs, neon and fluorescent lamps, rocket exhaust are some common examples of man-made plasmas. A typical electron-ion plasma is composed of electrons, positively charged ions and neutral atoms. However, plasmas formed in electron-attaching gases, some negatively charged ions are also present as an additional charged species. When the density of negative ions becomes significant enough to influence the plasma dynamics, the plasma is defined as electronegative plasma. Negative ions in electronegative plasmas are mainly classified into two categories based on their production mechanism: volume produced negative ions and surface produced negative ions. Plasmas containing volume negative ions are widely used in surface modification techniques; however, surface produced negative ions are regarded as the primary source of negatively charged ions in negative ion sources for negative ion-based neutral beam injector (n-NBI) systems, owing to their high extraction efficiency. Volume produced negative ions are formed within the plasma volume through a process, called dissociative attachment. When the plasma formed by an electronegative gas interacts with a solid surface, surface produced negative ions are generated through the resonant tunnelling of electrons from the surface.

Plasma generated in practical devices, is contained in a vacuum chamber of finite size. At the interface between plasma and any material surface, a thin positively charged layer, known as sheath, forms. The existence of negative ions has a large effect on the structure of the sheath as well as on the transport and spatial distribution of charged particles. A study has been carried out to investigate the structure of a plasma sheath in front of a caesium coated metallic plate using a simple theoretical model. Along with the electrons, the considered plasma is composed of multi-species of positive ions, and surface and volume produced negative ions. When plasma sheath of a negative ion plasma is observed from the wall side, multiple possible values of the sheath edge potential can be obtained for a specific range of electronegativity. This occurs when the ratio of electron temperature to negative ion temperature 𝛾 exceeds (5 + √24). In this study, it has been observed that, the width of multi- valued region increases with the increase in surface production yield. The effect of volume electronegativity on the sheath profile has been found to play a critical role. It determines the shift from electropositive to electronegative sheath. The influence of surface production yield on the volume electronegativity value, in which the transition from electronegative to electropositive sheath occurs, has also been observed here.

In partially ionized plasmas, a sufficient number of neutral particles coexists with positively and negatively charged particles. Hence, inside the sheath zone, thermal motion of neutral particles and an accelerated motion of positively charged particles towards the wall ensures the presence of ion-neutral collision at that region. A study also has been done to observe the effect of ion-neutral on the plasma sheath for an electronegative plasma in presence of surface produced negative ions. It has been found that, the collision reduces the ion velocity towards the metallic plate and consequently reduces negative ion production from the surface. The space charge profile shows peak near the sheath edge in contrast to the common case, where it has a peak near the wall. Fractional loss of ion impact energy is also has been seen to increase with increasing collision

Talk

abstract

The plasma research is currently at its peak in diverse fields of applications mainly in fusion for power generation since the idea of power generation from fusion in laboratory scale came into consideration. The objective is to achieve a positive power from the fusion reactor by controlled thermonuclear fusion which demands self-sustaining plasma conditions in the reactor. Different instabilities such as transient ELM events (Energy density ~ 0.2 - 2.5 MJ/m2) are generated during the magnetic confinement of the plasma in these fusion reactors. The high heat load during ELM events causes significant damage to material surface of the reactor. There might be surface erosion, melting or vaporization of the material surface which degrades the material with time and also act as a source of impurity generation. Different mock-up plasma devices are operating around the world as heat sources to study the interaction of plasma with fusion- relevant material target. The Pulsed Plasma Accelerator, a QSPA type device developed at CPP-IPR is presently capable of producing a pulsed plasma with an energy density of ~ 0.22 MJ/m2 in hydrogen medium. It is one of the promising heat source for simulating the heat load of transient ELM events. However, optimization of the system is the primary requirement for achieving the maximum efficiency and obtaining a continuous uniform plasma stream. Hence, emphasis is given for the optimization that is carried out by studying the different plasma characteristics.

Passive diagnostics technique of OES and its different spectroscopic methods are used to extract information on different plasma characteristic parameters. The formation of different ionization species in plasma and its excitation process, the Doppler shift method for plasma velocity estimation, analysis of stark broadened profiles for plasma density, and electron excitation temperature from Boltzmann plot under p-LTE assumption are reported. Apart from this, a collisional radiative model-based code FLYCHK is presented for a basic understanding of the code, its applicability conditions, and the approach to determining electron temperature. The evolution of plasma density, Texc, and Te are also reported to draw a conclusion on plasma characteristics for different time frames of plasma lifetime. The influence of an externally applied longitudinal magnetic field on the characteristic parameters of plasma is studied. A comparative study on the behavior of neutral and ionized argon species excitations, plasma density, electron excitation temperature, and electron temperature both in the absence and presence of an external longitudinal magnetic field is carried out thereby providing information on the interaction of the magnetic field with the plasma species. The dependence of the plasma characteristics on the gas pressure (plenum pressure) and discharge voltage is also studied. Apart from this, a time evolution study of plasma behavior in the absence and presence of magnetic field is also studied. Another diagnostics technique of high-speed imaging carried out using a High-speed video camera is discussed. A high-speed video camera is used to study the morphology of the plasma stream. Under a special arrangement of the electromagnet, the formation of different rotating structures of plasma is observed. In this study, three distinct cases of interaction are observed and outlined - one is the rotating plasma, second is the formation of time dependent low intense core which appears as a hollow region surrounded by intensely emitting plasma particles and, the third is the development of pressure dependent plasma lobe structures.

The interaction of hydrogen plasma with fusion-relevant tungsten targets is studied. Initially, the process of optimization of the plasma is discussed for its application in plasma-material interaction. The process involves the estimation of the different plasma parameters including plasma density and energy density under varied experimental conditions. The surface modifications of plasma-irradiated tungsten targets such as blister formation, development and propagation of cracks, impurity re-deposition, strain propagation to material depth, and accumulation of tensile residual stress in the material are discussed in detail. The results are summarized at the conclusion and an outlook is provided on prospects and scopes of upgradation of PPS to 300 KJ to achieve higher energy density to simulate higher heat load of ELM events and also explore other research fields for its efficient application.