IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

Plasmagenerator PSI-2
Performing plasma diagnostic
measurements with Langmuir
probes one has to take into consideration
possible disturbances
of the plasma by the probe itself.
Apart from the probe tip, there
is especially the probe shaft that
could influence the results of a
measurement. To resolve this question, measurements were
made of the electron temperature and density using two
Langmuir probes. The probes were arranged in two positions:
In the first case, they were aligned in azimuthal direction
of the PSI-2 plasma column. In the second case, they
were placed in the same axial plane but tilted to each other
in azimuthal direction. Data was acquired keeping one
probe at fixed position while the second one was scanning
the plasma radially. We have found that, irrespective of the
geometry, the electron temperature is not affected by the
probe, however the electron density is. This result can be
explained within a theoretical transport model in which the
probe shaft acts as an additional particle sink (figure 1). It is
interesting to note that the disturbance of the plasma by the
probe takes place on a global scale.
n e and T e [normalized]
1.1
1
0.9
0.8
0.7
T
e
n
e
Model
The flow behaviour of plasmas in contact with a target surface
was investigated. The ion velocity distribution function (ivdf)
of Ar + ions was measured at different displacements from
the target using LIF techniques (see Annual Report 2006).
In addition to the ivdf, the electron temperature was measured
by means of a Langmuir probe. Assuming T e =T i and combining
the results from the ivdf and Langmuir probe measurements
the Mach number M=u/c s was evaluated (u streaming
velocity, c s =[(T e +T i )/m i ] 1/2 speed of sound). Figure 2 shows M
and the electron density at different axial positions z (spatial resolution
Δz~0.5 mm). Close to the target (z=0) the Mach number
approaches unity in agreement with the prediction by Bohm.
Humboldt-University of Berlin
Arbeitsgruppe Plasmaphysik
Head: Prof. Dr. Gerd Fußmann
-40 -20 0 20 40
Radial Position x [mm] of Disturbing Probe
Figure 1: Normalized electron density (red) and temperature (blue) measured
at fixed position while a second probe was radially driven. The calculated
density distribution is plotted in black.
The plasma physics group at the Humboldt
University operates the plasma generator PSI-2
and the electron beam ion trap (EBIT). Research
activities comprise basic plasma physics, plasmamaterial
interactions and highly charged ion
processes relevant to fusion experiments. Further
effort is dedicated to the study of plasmoids produced
on water surfaces at atmospheric pressure.
111
Our measurement is thus the
first confirmation of Bohm’s
criterion for conditions relevant
to fusion experiments. An unexpected
result is the short distance
(Δz~5 mm) over which the
final acceleration of the ions to
M=1 takes place. Using a theoretical
presheath model the measured
profiles can be reproduced
by relying on D=20 m 2 s -1 for the perpendicular diffusion coefficient.
However, this D value is unrealistically large to explain
the profiles solely by diffusion. A more refined analysis of our
results suggests that radial electric fields build up in front of the
target causing strong particle transport onto the target surface.
M LIF
M
n LIF
n
-50 -40 -30 -20 -10 0 0
z [mm]
Figure 2: Mach number and electron density as a function of axial position
Electron Beam Ion Trap (EBIT)
To complement earlier work on the line emission from highly
charged tungsten ions (see Annual Reports 1999 and 2000),
x-ray spectra from Si-like W 60+ to Ne-like W 64+ were measured
in the 1-2 Å wavelength region. Our study includes the directly
excited L-shell spectra as well as the associated satellite
emission originating from dielectronically excited W ions.
Precise knowledge of such data is essential owing to the increasing
use of tungsten as wall material in nuclear fusion
devices. The tungsten ions in the EBIT were produced by
directing a continuous flow of W(CO) 6 gas towards the trap
and ionizing the injected carbonyl compound by the monoenergetic
electron beam. Spectra were obtained for a number
of beam energies between 10 and 20 keV and analyzed
using high resolution x-ray spectroscopy. As an example, in figure
3(a) we present a 15 keV electron beam energy spectrum
showing lines from W 60+ to W 64+ recorded between 1.18 and
1.57 Å. To support our identification of the lines, wavelengths
and intensities were calculated for the ions under investigation
using an atomic structure computer package combined
with a collisional-radiative model (in cooperation with
the division Tokamak Edge and Divertor Physics in Garching).
n 0
M 0
1
0.8
0.6
0.4
0.2
M or n/n 0