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

Since evaporation of cesium and the formation of Cs layers
on metal surfaces play a key role in negative hydrogen ion
sources, basic investigations on Cs behaviour in hydrogen
plasmas have been started at the University in 2007. To
determine the thickness of a Cs layer, a specially designed
quartz-crystal-microbalance is used that measures the frequency
difference between two oscillating crystals. Figure 3
(inlay) shows the measured correlation between Cs evaporation
using a Cs dispenser at different currents and the
change in frequency and thus the film thickness. In contrast
to studies in high-purity conditions, the growth of Cs onto
the examined surfaces (Cu, Mo, W, steel) is inhomogeneous;
the cesium forms local, drop-like structures with a size of
several μm. SEM pictures show that only 20-30 % of the
metal is covered by those structures; consequently the measured
thickness represents an effective thickness. In addition,
a method for in-situ measurements of the work function
using the photoelectrical effect (Fowler method) has been
established. As shown in figure 3 the work function decreases
with the Cs coverage of the molybdenum surface; however
the work function does not fall below 2.8 eV towards the
expected 2.2 eV for pure Cs.
Work function [eV]
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
0.0
Frequency change [Hz]
600
500
400
300
200
100
Plasma Surface Interaction Studies
0
6.5A
dispenser
current
7A 7.5A
0 300 600 900 1200
Dispenser on-time [s]
0 10 20 30 40 50 60
Effective cesium thickness [nm]
Figure 3: Work function of a cesium layer on molybdenum as a function of
the effective thickness. The inlay shows the thickness of the Cs layer during
Cs evaporation using a dispenser.
In strong collaboration with the materials research group at
IPP, the ongoing work on chemical erosion of different
doped carbon materials in low pressure ICP hydrogen and
deuterium plasmas has now been focused on the one hand on
the effect of the dopant distribution, i.e. carbide grain size
and on the other hand on manufacturing and surface temperature
effects on the erosion yield (released carbon particles
per incident ion). Doping of carbon leads to a reduction of
the effective carbon surface and thus to a reduced erosion
University of Augsburg
60
50
40
30
20
10
0
Cesium thickness [nm]
108
yield in hydrogen plasmas. In-situ measurements of the fluence
resolved erosion yield were carried out by optical
emission spectroscopy (CH and C 2 band emission) in combination
with weight loss measurements and RBS measurements.
The incident ion flux towards the surface was measured
using an energy resolved mass spectrometer in combination
with a Langmuir probe. The effect of the dopant distribution
was studied with amorphous carbon layers with atomically
disperse distribution and different concentrations of the
dopant materials (V, Ti, W). Another set of these samples
was previously annealed at up to 1100 K leading to the formation
of carbide grains with a size of several nm depending
on dopant material and concentration. Figure 4 shows the
results for vanadium doped samples with a surface temperature
of 300 K and incident ion energy of 30 eV in deuterium
plasmas. The solid line represents the erosion yield of undoped
amorphous carbon. The yields show a strong decrease
with the fluence and depend strongly on the dopant concentration.
This strong reduction of the erosion yield can be
explained by a surface enrichment of the dopant material
which was proven by RBS measurements at the IPP. The
pre-annealed samples always show significantly higher
yields compared to samples without annealing; an effect
which is less pronounced for titanium doped layers due to a
less effective grain formation by annealing.
Erosion yield G C /G ion [%]
10
1
8.5%V
Scientific Staff
8.5%V
pre-annealed
1.5%V
3.5%V
Dopant: V
3.5%V
pre-annealed
a-C
1.5%V
pre-annealed
0.1
0.0 0.5 1.0 1.5 2.0
Fluence [1024 m-2 ]
Figure 4: Erosion yields of vanadium doped amorphous carbon with atomically
disperse dopant distribution and nm-sized carbide grains (preannealed)
considering different concentrations of the dopant in ICP discharges
with deuterium
U. Fantz, P. Starke, S. Dietrich, S. Briefi, J. Ebad-Allah,
D. Filimonov, S. König, A. Manhard, P. Schmidt.
Energy scenarios group: J. Herrmann, F. Botzenhart, B. Grotz,
A. Hämmerle, T. Hartmann.