2. PMI basics

The basic interaction of a plasma bounded by a material surface is the balance of charged particles that arrive at a given time. In a magnetized plasma, the incident ions gyro-orbit around magnetic field lines that intersect material surfaces at ultra-shallow angles between 1 and 3 with respect to the surface, resulting in the so-called "Chodura sheath" where incident ions arrive with a distribution of incident angles and energies ranging from 20 to 60 with respect to surface normal and energies between 10 and 100 eV, respectively. The incident ions (mostly hydrogen fuel particles) implant at depths between a few nm to hundreds of nm. Sputtering of the wall material will depend on these conditions for both light and heavy mass target materials. The incident hydrogen particles will also reflect or backscatter from the surface and carry a finite amount of energy also resulting in a balance of implanted versus recycled fuel particles in a fusion device. In this section, we briefly discuss some of the most salient sputtering and reflection mechanisms with realistic (e.g., rough) surfaces found in a fusion device.

each other? The information we wish to know has many facets as well. What species is liberated? What is its energy? What angle does it leave the surface with respect to the surface normal and with respect to the incoming trajectory? Does it come off as a neutral, an ion, a dimer, or a molecule? With so many variables, exhaustive experimental determination of these quantities is impossible. What is possible is a computer model based on the physics of the interactions and then tested against experimental data. If a model can be shown to agree with experiments over a wide variety of ion-target pairs, there is some confidence that it will accurately predict PMI variables even for situations that may be impossible to directly measure. Such a computer scheme exists—Monte-Carlo simulations based on a binary collision approximation. Monte-Carlo simulations are ideal for ion-surface interactions. The physics of any one interaction is straightforward. Stringing many together while randomizing the impact parameter according to the physical parameters of the situation can be done with relative ease. Both the incident particle and all particles, which receive more than some pre-set amount of kinetic energy, are then followed after the collision. After every particle in this cascade is tracked until they come to rest or leave the surface, the final location and velocity of each atom is recorded. The transport of ions in matter (TRIM) simulation code has been one of the most successful PMI codes to simulate the interaction of energetic particles with surfaces and in the

Fundamentals of Plasma-Material Interactions in Magnetic Fusion Devices

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

51

context of PMI-simulating effects such as ion implantation and sputtering [3].

Expanding from the successful TRIM simulation platform, many variances have emerged over many decades and one of them is the incorporation of fractal geometry to mimic realistic surfaces [4]. Figure 1a shows the reflection of 50 eV H from an Ni surface as a function of fractal dimension [5]. Rn is the fraction of particles that reflect and Re is the fraction of energy that is reflected. Note the precipitous drop in both Rn and Re when some roughness is added, especially when the incident particle strikes the surface at a grazing incidence (75 from the normal). Initially, roughness reduces reflection as expected. The gradual rise in reflection for very rough surfaces is attributed to there being less of a chance for an upward-moving atom to be recaptured due to the lower average density of the material near the surface. Planar TRIM is akin to the D = 2.00 case for normal incidence. Some interesting comparisons [4] are shown in Figure 1b. Here, D is fixed at 2.30 and planar TRIM is compared to fractal TRIM (FTRIM) for three different incident energies as a function of incident angle. Specular reflection tendencies are clearly seen with TRIM but not in FTRIM. At 50 eV, the calculation was repeated using both generator A and generator B (see Figure 1) to show that the results did not depend on the generator, just on the dimension. Finally, a comparison is made with a molecular dynamic simulation at 10 eV. The similarity of those results shows that FTRIM can be used with some confidence even at low energies. Comparisons of predicted reflection to experiment are not possible because reflection measurements have not been done in this energy range. The reflected particles come off neutral and are very difficult to detect. A better comparison to

Figure 2a shows the FTRIM prediction [6] for physical sputtering of 300 eV H on C and normal incidence and at 60 incidence. Two experimental points [7] are also shown where the fractal

2.3. Effects of roughness on PMI

experiment can be made when sputtering is considered.
