1. Introduction

The interaction of plasmas and materials is one of the most interesting and critical subjects in the field of plasma technology. In fact, plasma processing has been the hallmark of plasma technology and its impact on the semiconductor industry. Plasmas are ubiquitous in nature and are responsible for some of the most fundamental interactions known to man. For example, in astrobiology, the early formation of the stars from interstellar media (ISM) dust and the presence of hydrogen molecules in interstellar space are conjectured to be governed by carbonaceous dust grain surface interactions with plasma energetic particles (hydrogen, helium) forming more complex organic molecules [1, 2]. Ion-induced etching in modern high-density plasma-processing tools is driven by the complex energetic multi-particle interaction with

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

material surfaces, and fluorescent lamps operate by a balance of the plasma-material interaction (PMI) under high gas pressure containers.

under "burn" conditions (e.g., more power out than power in) but when used to generate

The plasma-material interface is one important factor to the realization of nuclear fusion power. At this interface, high particle and heat flux from the fusion plasma can limit the material's lifetime and reliability and therefore hinder operation of the fusion device. This region is critical to the operation of a nuclear fusion reactor since material can be emitted both atomistically (e.g., evaporation, sputtering, etc.) and/or macroscopically (i.e., during transient events, such as disruptions or edge-localized modes). The environmental conditions of a future nuclear fusion reactor interacting with the plasma-material interface are extreme. The incident plasma will carry heat fluxes of the order of hundreds of MWm<sup>2</sup> and particle fluxes that can

800C, and the incident energy of particles will vary from a few eV ions to MeV neutrons. To exacerbate this, another challenge is the management of damage over the course of time. Operating at reactor-relevant conditions means the wall material would need to perform over the course of not just seconds or minutes (i.e., as in most advanced fusion devices today and the near future) but months to years. Therefore, plasma-material interface is a dynamic, evolving, reconstituted region of material that is constantly eroded and re-deposited a million

Another important factor is the limited attention given not only to the structural properties of refractory metals such as tungsten but especially surface-dominating properties (e.g., erosion, ion mixing, hydrogen- and helium-induced bubbles and swelling at the surface, surface diffusion, surface chemistry, morphology, and nanoscale patterning) that ultimately dictate particle recycling emitted back to the edge plasma consequently cooling the fusion plasma. In addition, understanding the relevant plasma-facing component issues, which vary with respect to fusion device design, is also lacking in fusion material's R&D efforts. For example, the differences of plasma edge conditions for first wall versus divertor fusion materials are quite different. Incident He fluxes to the first wall of future plasma-burning devices may vary between 1018

<sup>1</sup> and energies 100 and 1000 eV and at the divertor, fluxes between 1022 and

. How-

<sup>1</sup> and energies 5 and 100 eV. These flux and energy regimes induce distinct damage

mechanisms that must be understood in the development of advanced fusion materials. Of the various material options at the plasma-material interface (i.e., graphite, liquid metals, etc.), refractory metals (molybdenum, tungsten, etc.) are attractive for use during steady-state, hightemperature (700–1000C) operation with heat flux ranging between 10 and 20 MW/m2

ever, one major challenge for the application of solid refractory metals in future burning plasmas is the large production of helium ash in a fusion reactor. He implantation and generation of cavities, bubbles, surface morphology (e.g., fuzz), and blisters is of major concern for the application of commercial-grade tungsten materials. The surface nano- and mesoscale morphology could be detrimental to the performance of nuclear fusion reactor operation given

Given the broad area of plasma-material interactions and its importance to many different disciplines, we limit our treatment in this chapter to the interactions and plasmas in magnetic fusion devices. We also limit our coverage of PMI to one specific set of mechanisms mainly

the possible micro- and macroscale emission of W particulate (dust) into the plasma.

focused on physical sputtering and incident-particle reflection.

times over rendering our current understanding of material damage quite limited.

. The fusion reactor wall would need to operate at high temperatures near

Fundamentals of Plasma-Material Interactions in Magnetic Fusion Devices

http://dx.doi.org/10.5772/intechopen.77157

49

electricity operate at high-duty cycles (e.g., weeks or months of continuous power).

average 10<sup>24</sup> m<sup>2</sup> s

and 1019 m<sup>2</sup> s

1023 m<sup>2</sup> s

1

Fusion reactors also witness significant modification of wall materials when exposed to highdensity high-temperature magnetized plasmas. The nuclear fusion reaction in the core plasma generates a plethora of energetic particles including He and neutrons for hydrogenbased fuels. Both highly energetic He and neutrons can penetrate deep inside the material structure. However, the charged particles that remain inside the reactor are controlled by the boundary plasma with the reactor wall, and through what is known as a magnetic sheath, the charged particles are driven to the surface by several mechanisms that dictate both their incidence angle and energy to the material surface. Fusion devices are only experimental and operate under a pulsating configuration that enables only pulsed fusion plasmas that range from a few seconds up to about 50–60 s for the most modern, super-conducting fusion devices. During the pulse, the plasma can drive many complex interactions at the plasma edge and at the wall surface can implant energetic particles that range from a few eV up to several kilo-electron volts (keV). These energetic particles are mostly fuel particles such as hydrogen or deuterium but also in some cases helium. Other particles are mostly impurities from the ambient (e.g., water) such as oxygen and material atoms sputtered and redeposited in the fusion device. In this chapter, we examine in detail the interactions of these energetic particles and material surfaces to understand the basic mechanisms that drive plasma-material interactions.

Beyond energetic particle interaction on the wall material surface, there are additional complex interactions that take place and evolve during plasma-material interaction. The surface kinetics of impurity-driven modification can change the surface chemistry and morphology significantly. Over time, fusion devices also can drive transient events that consist of instabilities from the plasma, releasing large amounts of energy in the form of heat to the wall material surface. In a fusion reactor, deuterium-tritium plasma is confined by strong magnetic fields at a temperature of hundreds of millions of degrees Celsius. Therefore, in the exhaust of such a reactor, the so-called divertor, the plasma-facing surfaces are subjected to extremely high and intermittent heat loads (10 MW/m2 time-averaged, with periodic excursions in the GW/m2 level on sub-millisecond timescales), while simultaneously being bombarded by extreme fluxes of energetic particles (hydrogen isotopes, helium, neutrons). The radiation interaction with matter will be dynamic, imposing time-dependent changes on the structure, composition, and chemistry of both bulk and surface region of material components. Performance and lifetime limits of nuclear fusion materials will ultimately need to survive >100-dpa and >1000 appm He production over the high-duty cycle operation of the reactor. Currently, no material can meet such requirements namely limited by critical material properties including creep resistance, fracture toughness, surface erosion/re-deposition, corrosion, chemistry, thermal conductivity, and many others.

Although progress has been made in the last decade in establishing an understanding of plasma-material interactions, there remain critical knowledge gaps as it relates to predicting and designing for the behavior at the plasma-material interface under the so-called "reactorrelevant" plasma conditions anticipated in a future plasma-burning neutron-dominated environment. Ultimately, a magnetically confined fusion plasma must be able to not only operate under "burn" conditions (e.g., more power out than power in) but when used to generate electricity operate at high-duty cycles (e.g., weeks or months of continuous power).

material surfaces, and fluorescent lamps operate by a balance of the plasma-material interac-

Fusion reactors also witness significant modification of wall materials when exposed to highdensity high-temperature magnetized plasmas. The nuclear fusion reaction in the core plasma generates a plethora of energetic particles including He and neutrons for hydrogenbased fuels. Both highly energetic He and neutrons can penetrate deep inside the material structure. However, the charged particles that remain inside the reactor are controlled by the boundary plasma with the reactor wall, and through what is known as a magnetic sheath, the charged particles are driven to the surface by several mechanisms that dictate both their incidence angle and energy to the material surface. Fusion devices are only experimental and operate under a pulsating configuration that enables only pulsed fusion plasmas that range from a few seconds up to about 50–60 s for the most modern, super-conducting fusion devices. During the pulse, the plasma can drive many complex interactions at the plasma edge and at the wall surface can implant energetic particles that range from a few eV up to several kilo-electron volts (keV). These energetic particles are mostly fuel particles such as hydrogen or deuterium but also in some cases helium. Other particles are mostly impurities from the ambient (e.g., water) such as oxygen and material atoms sputtered and redeposited in the fusion device. In this chapter, we examine in detail the interactions of these energetic particles and material surfaces to understand the basic mechanisms that drive

Beyond energetic particle interaction on the wall material surface, there are additional complex interactions that take place and evolve during plasma-material interaction. The surface kinetics of impurity-driven modification can change the surface chemistry and morphology significantly. Over time, fusion devices also can drive transient events that consist of instabilities from the plasma, releasing large amounts of energy in the form of heat to the wall material surface. In a fusion reactor, deuterium-tritium plasma is confined by strong magnetic fields at a temperature of hundreds of millions of degrees Celsius. Therefore, in the exhaust of such a reactor, the so-called divertor, the plasma-facing surfaces are subjected to extremely high and intermittent heat loads (10 MW/m2 time-averaged, with periodic excursions in the GW/m2 level on sub-millisecond timescales), while simultaneously being bombarded by extreme fluxes of energetic particles (hydrogen isotopes, helium, neutrons). The radiation interaction with matter will be dynamic, imposing time-dependent changes on the structure, composition, and chemistry of both bulk and surface region of material components. Performance and lifetime limits of nuclear fusion materials will ultimately need to survive >100-dpa and >1000 appm He production over the high-duty cycle operation of the reactor. Currently, no material can meet such requirements namely limited by critical material properties including creep resistance, fracture toughness, surface erosion/re-deposition, corrosion, chemistry, thermal

Although progress has been made in the last decade in establishing an understanding of plasma-material interactions, there remain critical knowledge gaps as it relates to predicting and designing for the behavior at the plasma-material interface under the so-called "reactorrelevant" plasma conditions anticipated in a future plasma-burning neutron-dominated environment. Ultimately, a magnetically confined fusion plasma must be able to not only operate

tion (PMI) under high gas pressure containers.

48 Plasma Science and Technology - Basic Fundamentals and Modern Applications

plasma-material interactions.

conductivity, and many others.

The plasma-material interface is one important factor to the realization of nuclear fusion power. At this interface, high particle and heat flux from the fusion plasma can limit the material's lifetime and reliability and therefore hinder operation of the fusion device. This region is critical to the operation of a nuclear fusion reactor since material can be emitted both atomistically (e.g., evaporation, sputtering, etc.) and/or macroscopically (i.e., during transient events, such as disruptions or edge-localized modes). The environmental conditions of a future nuclear fusion reactor interacting with the plasma-material interface are extreme. The incident plasma will carry heat fluxes of the order of hundreds of MWm<sup>2</sup> and particle fluxes that can average 10<sup>24</sup> m<sup>2</sup> s 1 . The fusion reactor wall would need to operate at high temperatures near 800C, and the incident energy of particles will vary from a few eV ions to MeV neutrons. To exacerbate this, another challenge is the management of damage over the course of time. Operating at reactor-relevant conditions means the wall material would need to perform over the course of not just seconds or minutes (i.e., as in most advanced fusion devices today and the near future) but months to years. Therefore, plasma-material interface is a dynamic, evolving, reconstituted region of material that is constantly eroded and re-deposited a million times over rendering our current understanding of material damage quite limited.

Another important factor is the limited attention given not only to the structural properties of refractory metals such as tungsten but especially surface-dominating properties (e.g., erosion, ion mixing, hydrogen- and helium-induced bubbles and swelling at the surface, surface diffusion, surface chemistry, morphology, and nanoscale patterning) that ultimately dictate particle recycling emitted back to the edge plasma consequently cooling the fusion plasma. In addition, understanding the relevant plasma-facing component issues, which vary with respect to fusion device design, is also lacking in fusion material's R&D efforts. For example, the differences of plasma edge conditions for first wall versus divertor fusion materials are quite different. Incident He fluxes to the first wall of future plasma-burning devices may vary between 1018 and 1019 m<sup>2</sup> s <sup>1</sup> and energies 100 and 1000 eV and at the divertor, fluxes between 1022 and 1023 m<sup>2</sup> s <sup>1</sup> and energies 5 and 100 eV. These flux and energy regimes induce distinct damage mechanisms that must be understood in the development of advanced fusion materials. Of the various material options at the plasma-material interface (i.e., graphite, liquid metals, etc.), refractory metals (molybdenum, tungsten, etc.) are attractive for use during steady-state, hightemperature (700–1000C) operation with heat flux ranging between 10 and 20 MW/m2 . However, one major challenge for the application of solid refractory metals in future burning plasmas is the large production of helium ash in a fusion reactor. He implantation and generation of cavities, bubbles, surface morphology (e.g., fuzz), and blisters is of major concern for the application of commercial-grade tungsten materials. The surface nano- and mesoscale morphology could be detrimental to the performance of nuclear fusion reactor operation given the possible micro- and macroscale emission of W particulate (dust) into the plasma.

Given the broad area of plasma-material interactions and its importance to many different disciplines, we limit our treatment in this chapter to the interactions and plasmas in magnetic fusion devices. We also limit our coverage of PMI to one specific set of mechanisms mainly focused on physical sputtering and incident-particle reflection.
