log-normal distribution and close to the power-law distribution, F dust (ℓ d ) ~ (ℓ d )
Àα
(where ℓ d is the characteristic size of the grain) with α % 2.5 (see Fig. 5.4).
However, dust mobilization from the plasma-facing components with subsequent
penetration of the dust particles into the plasma volume depends on many circumstances (e.g. the initial location of the dust grains, the plasma parameters, etc.)
[17, 39]. Therefore, the size distribution of dust collected from fusion devices during
ventilation events can be different from that present in the plasma during
the discharges.
Important information about the size and spatial distribution of the dust grains in
the plasma of magnetic fusion devices can be obtained with laser scattering by
utilizing the non-shifted (Rayleigh) channel of the Thomson scattering diagnostics
used for the measurements of electron density and temperature. The first systematic
studies of dust with laser scattering had been performed on the DIII-D tokamak
[13, 22], and later, laser scattering was used for dust detection in FTU [23] and JET
[24]. The distribution of dust particle density found with laser scattering in the lower
divertor of DIII-D is shown in Fig. 5.5. Initial results on the size distribution of the
dust grains, inferred from the laser scattering data using the Rayleigh theory of light
scattering by particles [22], were reconsidered in [25] on the basis of the more correct
Mie theory. In addition, in [25] dust grain ablation under intense laser radiation was
taken into account. It was found that the size distribution function of the dust grains
with radii in the range 0.01–10 μm (the average grain radius was ~ 200 nm) in the
DIII-D plasma can be described by the power-law distribution with α % 2.6 À 2.7.
However, we should notice that larger grains can not be identified by laser
scattering diagnostic due to the saturation of the measured reflected signal. We
also note that similar power-law size distributions of the dust collected from LHD
and measured in DIII-D plasma may be just a coincidence.
Observations of dust with the laser scattering technique show that the dust particle
density at the edge of a magnetic fusion device depends strongly on the operational
mode of the device. For example, in H-mode having rather violent MHD events such
as ELMs (Edge Localized Modes), dust particle density in the SOL is few times
higher than that in the “quiet” L-mode, Fig. 5.6. In Fig. 5.7 one can see the relaxation
of the dust density just after an ELM burst (the solid line is the exponential fit with
Fig. 5.5 Dust particle
density in the lower divertor
of DIII-D. (Reproduced with
permission from [13],
© Elseiver 2007)
92
5 Dust in Fusion Plasmas
Àα
(where ℓ d is the characteristic size of the grain) with α % 2.5 (see Fig. 5.4).
However, dust mobilization from the plasma-facing components with subsequent
penetration of the dust particles into the plasma volume depends on many circumstances (e.g. the initial location of the dust grains, the plasma parameters, etc.)
[17, 39]. Therefore, the size distribution of dust collected from fusion devices during
ventilation events can be different from that present in the plasma during
the discharges.
Important information about the size and spatial distribution of the dust grains in
the plasma of magnetic fusion devices can be obtained with laser scattering by
utilizing the non-shifted (Rayleigh) channel of the Thomson scattering diagnostics
used for the measurements of electron density and temperature. The first systematic
studies of dust with laser scattering had been performed on the DIII-D tokamak
[13, 22], and later, laser scattering was used for dust detection in FTU [23] and JET
[24]. The distribution of dust particle density found with laser scattering in the lower
divertor of DIII-D is shown in Fig. 5.5. Initial results on the size distribution of the
dust grains, inferred from the laser scattering data using the Rayleigh theory of light
scattering by particles [22], were reconsidered in [25] on the basis of the more correct
Mie theory. In addition, in [25] dust grain ablation under intense laser radiation was
taken into account. It was found that the size distribution function of the dust grains
with radii in the range 0.01–10 μm (the average grain radius was ~ 200 nm) in the
DIII-D plasma can be described by the power-law distribution with α % 2.6 À 2.7.
However, we should notice that larger grains can not be identified by laser
scattering diagnostic due to the saturation of the measured reflected signal. We
also note that similar power-law size distributions of the dust collected from LHD
and measured in DIII-D plasma may be just a coincidence.
Observations of dust with the laser scattering technique show that the dust particle
density at the edge of a magnetic fusion device depends strongly on the operational
mode of the device. For example, in H-mode having rather violent MHD events such
as ELMs (Edge Localized Modes), dust particle density in the SOL is few times
higher than that in the “quiet” L-mode, Fig. 5.6. In Fig. 5.7 one can see the relaxation
of the dust density just after an ELM burst (the solid line is the exponential fit with
Fig. 5.5 Dust particle
density in the lower divertor
of DIII-D. (Reproduced with
permission from [13],
© Elseiver 2007)
92
5 Dust in Fusion Plasmas