Open access peer-reviewed chapter

Noise-Free Rapid Approach to Solve Kinetic Equations for Hot Atoms in Fusion Plasmas

By Mikhail Tokar

Submitted: November 16th 2017Reviewed: March 21st 2018Published: November 5th 2018

DOI: 10.5772/intechopen.76681

Downloaded: 227

Abstract

At the first wall of a fusion reactor, charged plasma particles are recombined into neutral molecules and atoms recycling back into the plasma volume where charge exchange (cx) with ions. As a result hot atoms with chaotically directed velocities are generated which can strike and erode the wall. An approach to solve the kinetic equation in integral form for cx atoms, being alternative to statistical Monte Carlo methods, has been speeded up by a factor of 50, by applying an approximate pass method to evaluate integrals, involving the ion velocity distribution function. It is applied to two-dimensional transfer of cx atoms near the entrance of a duct, guiding to the first mirror for optical observations. The energy spectrum of hot cx atoms, escaping into the duct, is calculated and the mirror erosion rate is assessed. Computations are done for a molybdenum first mirror under plasma conditions expected in the fusion reactor DEMO. Kinetic modeling results are compared with those found with a diffusion approximation valid in very cold and dense plasmas. For ducts at the torus outboard a more rigorous kinetic consideration predicts an erosion rate by a factor up to 2 larger than the diffusion approximation.

Keywords

  • fusion
  • plasma
  • neutral
  • atoms
  • kinetic equation
  • numerical solution

1. Introduction

In devices for thermonuclear fusion research, for example, of the Tokamak type, particles of hydrogen isotopes, deuterium and tritium, are in the form of a hot fully ionized plasma [1, 2]. To avoid a destruction of the machine wall, a special region, the so-called scrape-off layer (SOL), is arranged at the plasma edge, where particles stream along the magnetic field to the special target plates [3]. Normally, it is done by using additional magnetic coils to form a divertor configuration (see Figure 1a).

Figure 1.

The cross section of toroidally symmetric fusion device of the Tokamak type with the SOL region formed by the presence of a divertor (a) and the processes near the opening in the wall for a duct guiding to a first mirror (b).

By reaching the divertor target plates, plasma electrons and ions are recombined into neutral atoms and molecules which are finally exhausted from the device by pumps. However, in a future fusion reactor like DEMO [1, 2], only a minor fraction of 1% of neutrals generated at the targets will be pumped out. The rest of them is ionized again in the plasma near the targets. This “recycling” process significantly restrains the parallel plasma flow in the SOL [4]. Therefore, a considerable fraction of plasma particles lost from the plasma core will reach the vessel wall before they are exhausted into the divertor. Plasma fluxes to the wall saturate it with fuel particles in a time much shorter than the discharge duration, and a comparable amount of neutral species will recycle back from the wall into the plasma. Recycling neutrals are not confined by the magnetic field and penetrate at several centimeters into the SOL. Here, charge exchange (cx) collisions of them with ions generate atoms of energies much higher than that of primary recycling neutral particles. A noticeable fraction of such secondary cx atoms hit the vessel wall and erode it.

Statistical Monte Carlo methods [5] are normally used to model cx atoms at the edge of fusion devices. A crucial obstacle to apply these approaches for extensive parameter studies, for example, with the aim to optimize the duct geometry, has too long calculations needed to achieve reasonably small accident errors. This is, however, necessary, for example, to couple neutral parameters with the plasma calculations. In a one-dimensional geometry, an alternative approach, based on iteration procedure to solve the kinetic equation represented in an integral form, has been elaborated decades ago [6]. Being free from statistical noise and permitting the convergence of iterations to the error level defined by the machine accuracy, this method, nonetheless, is also time-consuming. The reason is the necessity to assess integrals in the velocity space from functions involving the ion velocity distribution function, additionally to integrations in the normal space. Recently [4], an approximate pass method has been applied to evaluate these integrals, and the acceleration of kinetic calculations by a factor of 50 has been achieved.

The amendments, outlined above, have allowed to perform calculations of the plasma parameters in the DEMO SOL with cx atoms described kinetically, by varying the input parameters, for example, the plasma transport characteristics, in a broad range [4]. In addition the results of these computations have been thoroughly compared with those obtained with cx species described in the so-called diffusion approximation. This approximation, often used in diverse edge modeling approaches to save CPU time (see, e.g., [7, 8]), is strictly valid under plasma conditions of low temperature and high density where the time between cx collisions of atoms with ions is much smaller than that till their ionization by electrons. Across the DEMO SOL, the plasma density and the temperatures of electrons and ions change, however, by orders of magnitude [4].

The usage of a diffusion approximation for cx atoms, generated from species recycling from the wall, becomes especially questionable by considering the situation near an opening in the vessel wall. Such openings will be made in a reactor for diverse purposes, for example, for ducts leading to the first mirrors, collecting light emitted by impurity species in the plasma (see Figure 1b). These installations are inaccessible for charged plasma particles, moving mostly along magnetic field lines and penetrating into the duct at a distance of 1 mm. However, hot cx atoms, unconfined by the magnetic field, can freely hit and erode these installations. The erosion rate is very sensitive to the energy spectrum of cx atoms which can be significantly dependent on the modeling approach. At the position of the duct opening, the inflow of recycling neutrals is actually absent, and the transfer of cx atoms has two- or even three-dimensional pattern. By calculating the density of cx atoms in the vicinity of a circular opening from a 2D diffusion equation, the erosion rate of a first mirror of Mo has been assessed in [9]. In the present paper, we extend the approaches, elaborated in [4] to model cx atoms kinetically in the 1D case, on a 2D geometry.

The results are compared with those of [9]. Although there cx atoms have been considered in the diffusion approximation to reduce CPU time, the new approach allows to perform more exact kinetic calculations even by orders of magnitude faster.

2. Basic equations

Although the concept of magnetic fusion is based on the idea that charged particles can be infinitely long confined within closed magnetic surfaces, there are diverse physical mechanisms leading to the losses across these surfaces (see, e.g., [10]). Therefore, electrons and ions escape through the separatrix into the SOL and may reach the first wall of the machine vessel. Here, these species fast recombine into neutral atoms, with an energy comparable with that of ions. Partly, these atoms escape immediately back into the plasma, and these species are referred as backscattered (bs) atoms. The rest transmits the energy to the wall particles and may be bounded into molecules, desorbing back into the plasma as the wall, and is saturated with the working gas. Here, the densities of recycling backscattered atoms and desorbed molecules decay with the distance from the wall because diverse elementary processes such as ionization, charge exchange and dissociation in the latter case, caused by collisions with the plasma electrons and ions, lead to disintegration of neutrals [11]. By the dissociation and ionization of molecules, the so-called Franck-Condon fc atoms are generated of a characteristic energy Efcof 3.5eV[3, 11]. By charge exchange collisions of the primary neutral species, listed above, with the plasma ions, cx atoms of higher energies and with chaotically oriented velocities are generated. Although, namely, the latter are of our particular interest, to describe them firmly, all the species introduced have to be considered. In the vicinity of a circular opening in the wall (see Figure 1b), any neutral particle is characterized by the components Vxand Vρof its velocity, where xand ρare distances from the wall and opening axis (see Figure 1b).

2.1. Recycling molecules and atoms

Recycling species, lunched from the wall, comprise backscattered atoms and desorbed molecules. Henceforth, we assume that the xcomponents of their velocities have single values Vbs,mand distinguish two groups of particles moving outward and toward the opening axis, with Vρ=Vbs,mand Vρ=Vbs,m, correspondingly. The magnitudes of Vbs,mare predefined by the generation mechanism for the species in question. Within the plasma the point with certain coordinates xand ρcan be reached only by particles, coming from the wall points 0ρ+xfor Vρ<0and 0ρxfor Vρ>0, respectively. By taking into account the attenuation processes with neutrals in the plasma, one gets

nbs,mxρ=nbs,m0ρ+x+nbs,m0ρx2exp0xνa,mVbs,mdyE1

where νa=nkiona+kcxaand νm=nkdism+kionm+kcxmare the decay frequencies for atoms and molecules, correspondingly, with nbeing the plasma density assumed the same for electrons and ions, kiona,mare the rate coefficients of the ionization by electrons, kcxa,mthose for the charge exchange with ions, and kdismthat for the dissociation of molecules.

2.2. Franck-Condon (fc) atoms

The fc atoms are born by the destruction of molecules and have positive and negative values both of Vρand of Vx. Henceforth, we distinguish four groups of these species with the velocity components VxVρ=±Vfc±Vfc, where Vfc=Efc/mand mis the atom mass. The source density of fc atoms of each group is as follows:

Sfc=n2kdism+kionm+kcxmnm/4

and depends on ρthrough nm. The particle densities nfc±±, where the first subscript ±corresponds to the sign of Vxand the second one – of Vρ, change along the characteristics ±xρ=constaccording to the continuity equation:

2Vxlnfc±±=Sfclνanfc±±

where lis the length of the corresponding characteristics. The boundary conditions take into account that fc atoms are reflected with the probability Rfcfrom the wall, x=0, and are absent far from the wall, x. For the total density of fc atoms, nfc=nfc+++nfc++nfc++nfc, one can obtain

nfcxρ=0Sfcyρx+y+Sfcyρ+xyVfcμ0dy++Rfc2Θρ+xρ00Sfcyρ+x+y+Sfcyρ+xyVfcμ1dy++Rfc2Θρxρ00Sfcyρx+y+SfcyρxyVfcμ1dyE2

with

μ0=expUxUyVfc,μ1=expUx+UyVfc,Ux,y=0x,yνadz,

the Heaviside function Θξ0=1,Θξ<0=0and ρ0being the opening radius (see Figure 1b).

2.3. Charge exchange cx atoms

The velocity distribution function of cx atoms, fcxxρVxVρ, is governed by the kinetic equation:

Vxxfcx+Vρρρρfcx=ScxπVth2expVx2+Vρ2Vth2νafcx,E3

Here, Scx=Scx0+Scx1is the total density of the source of cx atoms, with Scx0=nkcxmnm+kcxanbs+nfcand Scx1=nkcxancxbeing the contributions due to charge exchange collisions with ions of primary neutrals and cx atoms themselves, correspondingly, where

ncx=fcxdVxdVρ;

is the density of the latter; the velocity distribution of the source is assumed as the Maxwellian one with the ion temperature Ti, and Vth=2Ti/mis the ion thermal velocity.

2.3.1. Integral-differential equations for the density of cx atoms

To solve Eq. (3), Vρis discretized as ±Vρl, with Vρl=ΔVl1/2, ΔV=Vmax/lmax, and l=1,,lmax. The Vxdistribution functions φl±of particles with Vρin the range ±VρlΔV/2Vρ±Vρl+ΔV/2are governed by the equations, following from the integration of Eq. (3) over these ranges:

Vxxφl±±Vρlρρρφl±=Scxδl2πVthexpVx2Vth2νaφl±,E4

with

δl=erfVρl+ΔV/2VtherfVρlΔV/2Vth/erfVmaxVth.

From the equation above, one gets the following ones for the variables φl=φl++φland γl=Vρlφl+φl:

Vxxφl+1ρρργl=ScxδlπVthexpVx2Vth2νaφl,E5
xγl+Vρl2Vxρρρφl=νaVxγl,E6

The latter equation is straightforwardly integrated with respect to x, and for the ρ-component of the flux density, one gets

γl=Vρl2Vxρx0xρρφlexpUyUxVxdy.

Here, x0is the position, where the boundary condition is imposed; since there is no influx of cx atoms from the wall, γlx=0=0for Vx>0, neutral species are absent deep enough in the plasma and γlx0for Vx<0. The found expression for γlcan be made more convenient if (i) the x-variation of terms involved is approximated by linear Taylor series:

ρρφlyρρφlx+xρρφlxyx,UyUx+νaxyx,

and (ii) by taking into account that the width of the region, occupied by cx atoms, is of several mean free path lengths (MFPL), that is, xx0>Vx/νatypically. As a result we get

γlDl/ρ×ρρφl,

with Dl=Vρl2/νa.

By substituting the last expression into Eq. (5), this is reduced to the following one:

VxxφlDlρρ2ρφl=ScxδlπVthexpVx2Vth2νaφl.E7

Equation (7) can be integrated as the first-order equation with respect to x, with the boundary conditions similar to those for γl, that is, φlx=0=0for Vx>0and φlx0for Vx<0. Finally, for the density of cx atoms within the range VρlΔV/2VρVρl+ΔV/2, ηl=φldVx, one obtains the following integral-differential equation:

Dlνaρρ2ρηl=0ScxδlπVthIαdyηl,E8

where Iα=0Fαdu, with Fαu=u1expu2α/uand α=UyUx/Vthy. For the total density of cx atoms, one has ncx=l=1lmaxηl.

2.3.2. Diffusion approximation

By neglecting the ρderivative and by considering a single Vρvalue, Eq. (8) is reduced to an integral one obtained in the 1D case (Eq. (16) in Ref. [6] or Eq. (11) in Ref. [4]). In this case l1,δl=1and ncx=η1. As it was demonstrated in Ref. [4], this integral equation can be reduced to an ordinary differential diffusion equation, if (i) the electron temperature is low enough and the ionization rate is much smaller than that of the charge exchange, kionakcxa, and (ii) the characteristic dimension for the plasma parameter change is much larger than the typical MFPL for cx atoms Vth/νa. The same procedure can be performed in the case under the consideration, providing the following 2D diffusion equation:

DthρρρρncxxxncxTiνam=Scx0kionanncx,E9

with Dth=Vth2/2νa. The boundary conditions to Eq. (9) imply as follows: (i) cx atoms leave the plasma with the velocity close to the ion thermal one, and (ii) neutral species are absent far from the wall, that is, ncxx=0. Additionally, ρηl=0and ρncx=0for ρ=0and ρfor Eqs. (8) and (9), respectively.

2.3.3. Assessment of the velocity space integral Iα

By inspecting Eq. (8), one can see the cause for large calculation time for kinetic computations for cx atoms. Even in the 1D case, for each grid point, one has to calculate enclosed double integrals over the ion velocity space and over the whole plasma volume, 0yrw, and repeat this, in an iterative procedure, with respect to the whole profiles of ncxx. In a 2D situation, the runs through the grid points in the ρVρphase subspace are involved additionally into calculations. These can be, however, efficiently parallelized. The integral Iαis appeared due to the generation of cx atoms with the ion Maxwellian velocity distribution and consequent attenuation by ionization and cx collisions. With a high accuracy, it can be assessed by an approximate pass method [12]. The function Fαuis shown in Figure 2a for α=0.3, 1, and 3. One can distinguish four uintervals, where Fαubehaves principally differently: uu1, u1<uum, um<uu2, and u2<u. The characteristic points u1,2and umare defined by the conditions Fαum=0and Fαu1,2=0for the derivatives Fα=Fαg1/u2and Fα=Fαg2/u4with

g1u=2u3+uα,g2u=g1+u22+6u2u2.

Figure 2.

The functions Fαu=u−1exp−u2−α/u for α=1 (solid curve), α=0.3 (dashed curve), and α=3 (dashed-dotted curve) (a); the integral Iα=∫0∞Fαudu estimated by the pass method (solid curve) and computed numerically (dashed curve) (b).

For umone can use the Cardano formula for the real root of a cubic equation um=b+a3ba3with a=α/4and b=a2+1/216. By searching for u1,2, we notice that for the roots of interest the equation g2u1,2=0reduces to the following ones:

g1u1+u1+u12+6u12=0,g1u2+u2u22+6u22=0.

With properly selected initial approximations accurate enough, solutions of these equations are found after three to five iterations with the Newton approach.

To assess the integrals Iα, we approximate function Fαuas a linear one at uu1and by polynomials of the fifth order at the intervals u1<uumand um<uu2. The coefficients are selected to reproduce Fαm,1,2=Fαum,1,2, Fα1,2=Fαu1,2, Fαm=Fαum, and Fαum=Fαu1,2=0. In addition, for u>u2the factor expu2leads to a very fast decay of Fαwith increasing u. Therefore, for the aim to assess the corresponding contribution to Iα, we approximate Fαby expu2α/u2/u2. This results in the following expression:

IαFα122Fα1+Fα1δ1+Fαmu2u1+Fα2δ22++Fα1δ12Fα2δ2210+Fαmδ13+δ23120+π21erfu2u2expαu2,E10

where δ1,2=umu1,2. Figure 2b shows Iαversus αfound with the pass method, outlined above and evaluated numerically. By approximating the numerical results very accurately, the approximate pass method procedure allows to reduce the CPU time by a factor of 50 compared to direct estimates.

Alternatively to the usage of formula (10), one can calculate the integral Iαin advance and save the results in a table. Then, for any particular α, the integral Iαis interpolated from the values in this table. However, some time is needed to find the proper αinterval, and this time grows up with α0since Iαas lnα. Thus, the density of the data in the table has to be increased as α0, and it is not obvious that such a way is more time-saving than formula (10).

2.3.4. Numerical procedure

The procedure to solve Eq. (9) numerically is organized as follows: For the problem in question, with the plasma profiles assumed unaffected by the processes in the vicinity of the duct opening, the integral Iαxycan be assessed once in advance. Then,

  1. the source density Scxxρis calculated, in the first approximation with ncx=0;

  2. for all x the first term on the right-hand side of the equation is computed as a function of ρ, and the second-order ordinary Eq. (8) is solved by the method outlined in [10] for all Vl;

  3. the next approximation for ncxxρand Scxxρis computed, and the procedure is repeated till the relative error in, for example, ncx00, becomes smaller than a desirable one.

A typical behavior of the error with the iteration number for calculations presented in the next section is shown in Figure 3. One can see that with consequent iterations the calculation error reduces steadily without any noise and the machine accuracy can be actually achieved.

Figure 3.

The reduction of the relative error in calculation with the iteration number.

Normally, a diffusion approximation is used to describe cx atoms to reduce CPU. The present approach to solve kinetic equation makes this unnecessary. To get the density of cx atoms, ncxxρ, with a relative error below 106, kinetic calculations require much less CPU time than it is needed to solve the diffusion equation. Thus, for 500 and 30 grid points in the xand ρdirections, respectively, lmax=10 and Vmax=3Vth, the CPU times for a 1 GHz processor are 30 s and 112 min, correspondingly. Of course, the time for kinetic calculations increases significantly if neutrals affect the plasma background, and Iαxyhas to be evaluated by each iteration. Even in this case kinetic computations for cx atoms are done faster by factor of 4.

3. Results of calculations

Figure 4 shows the profiles of the plasma parameters, assumed homogeneous on the flux surfaces, across the surfaces, computed in [13] with the input parameters from the European DEMO project [1, 2]. The distance xfrom the first wall to a certain flux surface depends on the poloidal angle θ(see Figure 1a) and in Figure 4 xcorresponds to the torus outboard, θ=0, where this distance is minimal. The profiles of the cx atom density found with these parameters and for the duct radius ρ0=0.05m, at the opening axis, ρ=0, and far from it, ρ=0.25mare shown in Figure 5. There is a noticeable difference between the profiles of ncxcalculated in the diffusion approximation (5a) and kinetically (5b). In particular, the difference between the density profiles at these two positions is significantly larger in the diffusion approximation than for those computed kinetically.

Figure 4.

The profiles of the plasma density (solid curve), the ion (dashed curve), and electron (dashed-dotted curve) temperatures versus the distance from the wall at the DEMO torus outboard.

Figure 5.

The profiles of the cx atom densities far from the opening (solid curve) and at its axis (dashed curve) computed in the diffusion approximation (a) and kinetically (b).

In the latter case, ncxis still noticeable in much deeper and hotter plasma regions. This circumstance is of importance for the erosion of installations in ducts due to physical sputtering because this is very sensitive to the energy of impinging particles. To demonstrate this the erosion rate of a Mo mirror, imposed into the duct of three different radii ρ0, at the distance hfrom its opening (see Figure 1b) is assessed. To do such assessment, consider an infinitesimally thin toroid within the plasma, with a square cross section of the width , thickness dx, and the radius ρ, situated at the distance xfrom the wall. Cx atoms with the energies within the range E,E+dEare generated in the toroid with the rate:

dRcxxρE=ScxxρφixE2πρdρdxdE.E11

where

φixE=2EπTi1.5xexpETix.E12

is the Maxwellian energy distribution function of ions.

Since cx atoms move in all directions, some of them hit the mirror inside the duct (see Figure 1b). Henceforth, we analyze the surface position at the duct axis. One can see that cx atoms generated only in toroids with ρρmax=ρ01+x/hcan hit this mirror point. The contribution of the plasma volume in question to the density of the cx atom flux perpendicular to the mirror surface is

djcxxρE=dRcxsexpλ/s4πx+h2+ρ2,E13

where s=1+ρ2/x+h21/2is the cosine of the atom incidence angle with respect to the duct axis. The exponential factor in Eq. (13), with λxE=Ux/2E/m, takes into account the destruction of cx atoms in ionization and charge exchange collisions on their way through the plasma. By ionization the atom disappears. By charge exchange a new cx atom is generated. The latter process is taken into account by the contribution Scx1in the source density Scx.

The energy spectrum of cx atoms, hitting the mirror, is characterized by their flux density γ=djcx/dEin the energy range dE, where the integration is performed over ρand x. By proceeding from ρto s, according to the relation ρ=x+h1/s21, one gets

γhρ0E=0rwφixEdxh/h2+ρ021Scx2expλsds.E14

The density of the outflow of mirror particles eroded by physical sputtering with cx atoms is as follows:

Γsphρ0=0dE0rwφixEdxh/h2+ρ021Scx2expλsYspEsdsE15

Here, Yspis the sputtering yield, whose dependence on Eand sis calculated by applying semiempirical formulas from Ref. [14]. The erosion rate, measured henceforth in nmper full-power year nm/pfy, is hsp=Γsp/nsp, with the particle density of molybdenum nsp. It is shown in Figure 6 versus the distance hbetween the wall position and the mirror point in question. One can see that in the former case the erosion rate is by a factor of 2 larger than in the latter one. It is of importance to notice that hsp, found with ncxcomputed kinetically, does not unavoidably excides that obtained with ncxcalculated in the diffusion approximation. The results above have been gotten for a mirror positioned in a duct at the plasma outboard, that is, at the largest major radius R(see Figure 1a). Here, the local gradients of the plasma parameters are the largest because the distance between two particular flux surfaces has the minimum. Therefore, cx atoms penetrate, before they are ionized, into plasma regions with higher ion temperatures. The situation is different at the torus top where the corresponding distance has maximum and neutral species are attenuated already at significantly low nand Ti. This leads to noticeably smaller hsp(see Figure 7). In this case, oppositely to the situation at the outboard, hspis larger if ncxis computed in the diffusion approximation.

Figure 6.

The erosion rate of a Mo mirror versus the distance from the wall position to the mirror surface for ducts positioned at the torus outboard computed with the density of cx atoms found in the diffusion approximation (a) and kinetically (b).

Figure 7.

The erosion rate of a Mo mirror versus the distance from the wall position to the mirror surface for ducts positioned at the torus top computed with the density of cx atoms found in the diffusion approximation (a) and kinetically (b). (Note that the hsp scale is different than in Figure 6).

Due to technical requirements, an acceptable mirror erosion rate in a future fusion reactor should not exceed 1nm/pfy. As one can see in the case of a duct at the torus outboard, this target level is exceeded significantly for all hand ρ0under consideration. Seeding of the working gas into the duct is considered as a possible way to diminish the erosion rate below the maximum level allowed. The energy of cx atoms, coming into the duct, is reduced through elastic collisions with gas molecules, before cx atoms hit the mirror. Elastic collisions between neutral species lead to scattering on large angles. Consider a cx atom which, without gas in the duct, can hit the mirror directly. If the gas is seeded, in a narrow duct in question, with ρ0h, any collision of the cx atom with gas molecules leads to such a change of the atom velocity that with an overwhelming probability the atom will many times strike the duct wall before it gets the mirror. Through the collisions with the wall, the cx atom loses its energy so dramatically that it cannot contribute to mirror erosion. The reduction of the flux jEof incoming cx atoms with the energy Eby collisions with the gas in the duct is governed by the equation:

djE/dl=σelEngjE,

where σel3.81019E0.14m2eVis the cross section for elastic collisions between cx atoms and molecules of hydrogen isotopes [15]. Consequently, the density of the outflow of mirror particles eroded by physical sputtering with cx atoms is modified, compared with Eq. (15), as follows:

Γsphρ0=0dE0rwφidxh/h2+ρ021Scx2expλ+σelnghsYspEsdsE16

In Figure 8, the calculated dependences of the erosion rate hspon hand ρ0are shown for several magnitudes of the density ngof the working deuterium gas in the mirror duct. One can see that the enhancement of ngabove a level of 21019m3should lead to the reduction of hspbelow the target level of 1nm/pfy. The question, to what extent the local plasma parameters may be changed by the outflow of the gas from the duct, has to be investigated in the future on the basis of approaches developed in [16]. There, it has been demonstrated that the ionization of the gas outflowing into the SOL can lead to dramatic growth of the local density and cooling of the plasma to a temperature of 1eV. Such cold dense plasma cloud can affect the transfer of cx atoms in the plasma near the opening in the wall.

Figure 8.

The erosion rate of a Mo mirror versus the distance from the wall position to the mirror surface for ducts positioned at the torus outboard computed with cx atoms treated kinetically for different gas densities in the duct: ng=0 (solid line), 3⋅1017m−3 (dashed line), 3⋅1018m−3 (dotted line), 1019m−3 (dashed-dotted line), and 3⋅1019m−3 (dashed-double dotted line).

4. Conclusions

The iteration approach to solve 1D kinetic equation for cx atoms, proposed decades ago [6], has been elaborated further to describe the transport of these species in a 2D geometry, in the vicinity of a circular opening in the wall of a fusion reactor. Unlike the Monte Carlo methods, this approach does not generate statistical noise so that calculation errors can be reduced to the level restricted by the machine accuracy. In order to perform calculations for a broad range of input parameters and do a thorough comparison with the results, obtained in the diffusion approximation for cx atoms [13], the solving procedure has been accelerated by a factor of 50, by applying an approximate pass method to assess integrals in the velocity space from functions, involving the Maxwellian velocity distribution of plasma ions.

The found possibility to speed up kinetic calculations is of importance, in particular, to perform firm assessments of the erosion rate of the first mirrors in future fusion reactors like DEMO. For a mirror located at the torus outboard, more accurate kinetic calculations predict by a factor of 2 higher erosion rate than the approximate diffusion approach. The erosion rate can be reduced very strongly either by putting the mirror duct at the torus top or by seeding the working gas into the duct. In the latter case, the elastic collisions with molecules in the gas reduce significantly the fraction of cx atoms which can hit and erode the mirror.

Nomenclature

Dl,thDiffusivity of cx atoms in kinetic and diffusion descriptions, correspondingly
EEnergies of ions and atoms
Fαu≡1uexp−u2−αu,Iα≡∫0∞Fαudu
hDistance from duct opening to the mirror
hspMirror erosion rate in nm per full-power year
γhρ0EFlux density of cx atoms onto the mirror
Γhρ0Outflow density of sputtered mirror particles
kcxa,mCharge exchange rate coefficients
kdismMolecule dissociation rate coefficient
kiona,mIonization rate coefficients
λxE=Ux/2E/m
mMass of atoms
nPlasma density
nm,bs,fc,cxDensities of different neutral species—desorbed molecules, backscattered atoms, Franck-Condon, and charge exchanged atoms, respectively
νm,aDecay frequencies for molecules and atoms
RcxxρEThe generation rate of cx atoms in the energy range E,E+dE within an infinitesimally thin plasma toroid of the width dρ, thickness dx, the radius ρ, and situated at the distance x from the wall
ρDistance from the opening axis
ρ0The opening radius
sCosine of the cx atom incidence angle with respect to the duct axis
Sfc,cxSource densities of fc and cx atom species
Scx0,1cx atom source density contributions due to charge exchange of primary neutral species and cx atoms
σelCross section for elastic collisions between cx atoms and molecules of hydrogen isotopes
Te,iElectron and ion temperatures
Ux,y≡∫0x,yνadz
Vm,bs,fcAbsolute velocities of neutral species
Vx,ρVelocity components perpendicular to the wall and to the opening axis, respectively
Vth=2Ti/mThermal velocity of ions
φixEEnergy distribution function of ions
xDistance from the wall

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Mikhail Tokar (November 5th 2018). Noise-Free Rapid Approach to Solve Kinetic Equations for Hot Atoms in Fusion Plasmas, Plasma Science and Technology - Basic Fundamentals and Modern Applications, Haikel Jelassi and Djamel Benredjem, IntechOpen, DOI: 10.5772/intechopen.76681. Available from:

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