NlSi AOOJU - International Atomic Energy Agency

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NlSi
AOOJU
ASSOCIAATTMIE EURATOM-FOM
FOM-INSTITUUT VOOR PLASMAFYSICA
RIJNHUIZEN - NIEUWEGEIN - NEDERLAND
TIME-OF-FLIGHT ANALYSIS OF
CHARGE-EXCHANGE NEUTRAL
PARTICLES FROM
THE TORTURII PLASMA
by
H.J.B.M. Broeken
Rijnhuizen Report 81-136
ASSOCIATIE EURATOM-FOM October 1981
FOM-INSTITUUT VOOR PLASMAFYSICA
RIJNHUIZEN - NIEUWEGEIN - NEDERLAND
TIME-OF-FLIGHT ANALYSIS OF CHARGE-EXCHANGE NEUTRAL
PARTICLES FROM THE TORTURII PLASMA
by
HJ.B.M. Broeken
Rijnhuizen Report 81-136
This work Mas performed as part of the research programme of the association agreement ot I uratom api' the "Slithlinp voor
f undarnenleel Onderzoek der Materie" (l-'OM) with financial support from the "Nederlandse Organisatie voor/uiver-Weien-
schappeli|k Onder/nek" l/WO)and I uralom
ft is also published as a thesis of the I niversily of I 'trccht.
TIME-OF-FLIGHT ANALYSIS OF CHARGE-EXCHANGE NEUTRAL
PARTICLES FROM THE TORTUR II PLASMA
by
H.J.B.M. Broeken
Association Euratom-FOM
FOM-Instituut voor Plasmafysica
Rijnhuizen, Nieuwegein, The Netherlands
ABSTRACT
A disc chopper for time-of-flight analysis of fast neutral par
ticles was constructed for the determination of the ion energy spectrum
at lower energies than can be obtained by conventional electro-magnetic
analyzers. The method has been applied to the TORTUR II tokamak. The
chopper and detection system are described and the measurements are
presented. For the interpretation of the results of the measurements a
data analysis procedure was developed. The influence of reflections of
neutrals at the liner wall showed to be important in the calculations
of the neutral density profile at the plasma edge. The neutral energy
spectrum in the lower energy range is strongly pronounced by this
effect.
-1-
I. INTRODUCTION
Analysis of the energy spectrum of charge-exchange neutrals es
caping from a plasma is a commonly used method for the determination
of ion energy spectra ' ' ' ' . Usually, complicated devices as multi
channel analyzers with an electric or a magnetic field or with combina
tions of those fields are used to measure the spectra of the charge-
exchange neutrals. Besides the high costs and technical problems in
herent in these analyzers, they all are equipped with a stripping cell,
or in recent applications, a stripping foil to ionize the charge-ex-
chnnge neutrals emerging from the plasma. The rather low stripping ef
ficiency turns out to be a serious obstacle in the determination of the
lower energy range of the neutral emission spectrum. With a stripping
gas cell the lower limit of the measured neutral emission spectrum is
about 200 eV or higher. The lower energy range, however, is important
in the determination of the neutral density in the plasma edge region;
it is highly depending on the plasma temperature and density profiles
but also on the conditions of the liner wall. When the plasma profiles
are known, the analysis of the lower energy range of the neutra lemis
sion spectrum can contribute to plasma-wall interaction studies. The
time-of-flight method presented is a solution to the problem to de
termine the lower energy range of the neutral spectrum and, therefore,
a welcome completion of the usual analyzing techniques.
A technical description of a time-of-flight system will be given
in Chapter II. The data analysis is discussed in Chapter III and the
numerical methods for tne interpretation of the measurements are pre
sented with the results in Chapter IV, followed by a discussion in
Chapter V. A difference up to a factor 50 is found between the calcu
lated and the measured spectra in the energy range below 400 eV. An
explanation of this difference is sought in the recycling of neutrals
at the plasma edge.
-2-
II. THE TIME-OF-FLIGHT SPECTROMETER
Conventional neutral-particle analyzers make use of a stripping
gas cell or foil to ionize the charge-exchange neutral particle flux
from the plasma. The stripping gas cell, the small entrance slits and
the small solid angle of acceptance limit the use of these analyzers to
energies above approximately 200 eV {Ref. 6?. When stripping foils are
used, e.g. carbon or alumina foils of about 10 nm, the strippin geffi-
7)
ciency is greatly improved . However, the energy straggling of foils
affects the resolution, particularly at low energies. This is enhanced
by non-uniformity of the foils, which is difficult to avoid in the thin
foils needed for particles below 200 eV to pass. From these considera
tions it appears preferable not to ionize the neutrals but to detect
them directly and to determine their energies from the flight time for
a fixed distance. Another advantage of measuring neutrals is the in-
sensitivity to the effects of disturbing stray or fringing electric and
magnetic fields.
One of the methods to measure the time of flight of particles
over a given distance is the use of a chopping disc as is known in
8)
other branches of physics . This technique is adopted in our device.
It consists of a stainless steel chopper disc of 160 mm diameter,
directly coupled to and driven by a two-phase hysteresis synchronous
motor (75A1003-2, Globe Co.). A power amplifier and an oscillator
supply the power to the motor. The motor has a rotor frequency of 400 Hz.
The disc, made by a photo-etching method, has 80 slits of 0.4 mm at a
radius of 67.5 mm with an interdistance of 4.90 nun. The slits are 15 mm
in height and have an accuracy better than 0.25%. There is a slit of
20*0.1 mm2 in front of the disc. At the entrance port of the torus one
finds a fixed pumping resistance of 10x1 mm2 and 72 mm in length, keep
ing out the filling gas of the torus. The function of the pumping re
sistance, not represented in Fig. 1, is rather important. Without it
the filling gas of the torus builds up a pressure in the flight tube
such that it ionizes and scatters all the low energy neutrals flying
along the 2 m of the flight tube. The pumping resistance and the chop
per entrance slit also collimate the neutral flux from the plasma, pre
venting scattering against the flight tube wall.
The dimensions of the chopper disc slits used at maximum rota
tional velocity lead to opened times of 2.36 ps and to closed times of
28.89 MS. The chopper and the flight tube of 2 m make a spectrometer
energy range between 24 eV and several keV for hydrogen particles. The
energy resolution of the time-of-flight spectrometer is given by:
-3-
-20 kV
disc-chopper system
plasma dump
-eference system
transducer
Fig. 1. Schematic representation of the time-o f-flight spectrometer,
üE , At
= (22 < E < 1500) (1)
E t
where At is the open time and t is the flight time of the neutral par
ticles .
A small light source at one side and an optical electric trans
ducer at the opposite side of the chopper blade produce the reference
signals for open and close positions of the chopper slits.
The detector is of the Daly type (see Fig. 2). An aluminum foil
of 9 um placed under 45 degrees, with respect to the direction of the
chopped beam, emits secondary electrons due to the bombardment of neu
tral particles. A foil target is chosen to minimize reflections of hard
X-rays from the plasma. The aluminum target, put at a potential of
-25 kV, is placed opposite a scintillator photomultiplier combination.
The secondary electrons accelerated by the electric field, penetrate
the Al-foil of 3 ym on top of the scintillator, thereby losing about
10% of their energies. A dump for the hard X-rays is placed behind the
foil target. VUV light and soft X-rays can be reflected by the foil
target, producing a large signal in the P.M.-scintillator combination.
Provided that this signal does not cause an overload of photomulti-
pliers and following amplifiers, it can be used as an accurate indica
tion of the open position of the chopper slits. Corrections for pertur
bations due to this signal can be performed in the analysis of the data.
An example is shown in Fig. 3.
-4-
- 20hV
;20Mft
H* x-i ays
x-rays dump
vacuum
At. layer ' scintillator
pyrex
^ .,:.-^gfSL
y
Fig. 2. Schematic representation of the Daly-particle detector
used in the time-of-flight spectrometer.
160
1 1 1 l l I
1
i _
140 h «
/I •i
Cpart./jus) 120
\
\
- 1 \ i
100 t 1 i
_ / \
i
80 h
\ ƒ I
60
-
40 - w
-
20 open time I /
^ Ï 1— i 1—tJ—
i
V 5 10 15 20 25 30 35
*• t t>t3)
I toil.) • 1 L _l_ _L
1000 200 100 50 25
+• E (eV)
Kig. 3. Signal from the detector during a cycle time (= open time
+ close time) of the detector after a zero-line correction
(dotted line) and the signal after corrections for VUV
light and X-rays (solid line). Corrections for the secon
dary emission coefficient and energy resolution ftsme not
been carried out.
-5-
The corrections were performed as follows. The zero-line correc
tion was made by subtraction of a measurement with a closed valve be
tween the time-of-flight spectrometer and the torus. The correction for
light needs more explanation. The light from the plasma is converted
into a trapezium-shaped signal at the photomultiplier. The signal from
the photomultiplier is integrated by an RC-circuit over a time equal
to twice the sample time of the analogue digital converter. The inte
grated signal has a maximum at the end of the open time. The time be
tween two of these maxima is equal to a chopper cycle time. From this
and the chopper disc geometry the open-time duration can be determined.
The shape of the siqnal due to the contribution of light can be calcu
lated from the open time and the RC-time given. The actual height and
its location on the time axis are calculated by fitting the analytical
shape with the ADC points of the measurement. In this manner, the open
time and the height of the integrated signal caused by light are de
termined with the best possible accuracy. The final signal is obtained
by subtracting the light contribution from the signal measured.
The chopper timing parameters are chosen in such a way that
virtually no slow particles interfere with the high-energy part of the
next spectrum tc record. As neutrals are involved, the secondary emis
sion coefficient is restricted to the kinetic emission. As shown in
Pig. 4, the kinetic emission coefficient vanishes below approximately
15 eV (Ref. 9). The analysis of the spectrum was restricted to the
energy range from 24 eV up to 1 keV.
The calibration of the detector system is performed with 500 -
1500 eV ions from a Penning ion source. It has been performed with a
pulse-heicJit analyzing technique along the same lines as described in
5)
Rijnhuizen Report 80-122 . For the evaluation of the data, the
secondary emission coefficient of the Daly disc had to be known. As low-
energy neutrals were not available, secondary emission coefficients of
hydrogen neutrals on Al for the lower energy range were adopted from
literature data ' (see Fig. 4). A good agreement between the cali
bration and the secondary emission coefficient in the energy range be
tween 500 and 1500 eV was found.
The output signals of the photomultiplier are led through a
line driver, integrated over a time equal to twice the sampling time
of the ADC, amplified and registered. The measurements can be pre-
processed immediately after a recording and sent to a PDP 11/70 com
puter. It performs the integrations over the open time, the corrections,
it arranges the measured spectra and performs the data analysis.
6-
•*- E («V)
Fig. 4. The kinetic secondary emission coefficient
of hydrogen atoms on Al as a function of
the energy.
-7-
III. ANALYSIS OF THE NEUTRAL FLUX
The energy spectrum of the charge-exchange neutral flux emerging
from a plasma yields information about the internal ion temperature and
the ion density. Several authors have developed methods to calculate
the relation between the ion energy distribution and the charge-ex
change neutral flux emitted by the plasma ' ' * . The difficulty
in these calculations lies in the determination of the neutral density
profj-le which has to be consistent with the radial profiles of the
plasma density and temperature. Hence, for a determination of the ion
profiles, results of other diagnostics are required. Apart from the de
termination of the central ion temperature derived from the asymptotic
behaviour of the energy spectrum at higher energies, the lower range
of the energy spectrum - which is highly profile-dependent - can be
profitably used to complete the profiles parameters obtained from other
diagnostics for the plasma-edge regions. We shall return to this aspect
in Chapter IV.
When the lower energy range of the neutral spectrum is examined,
the main problem is the determination of the neutral density profile.
Most authors give an estimate of this density profile. They ignore the
time dependence as the plasmas may be considered stationary and they
describe the neutral distribution function f(r,v) by means of a slab
14)
model . Accepting this slab approximation and ignoring the time de
pendence, the continuity equation for the neutral distribution function
is given by:
v 3f(5'v) + s(r,v)f(r,v) = h(r,v)<Mr,v) (2)
3r
i ... . ^_e
s(r,v) = (<Ö_ V>. + <o, v>. + <o . v> )n(r) is containing all loss
V
6X X XOii X X Oil 6
processes like ionization and charge exchange. All formative processes
like resonant charge-exchange and recombination processes are repre
sented by: h(r,v) = <c v> n (r) +• <a v> n{r). f{r,v) and <ft(r,v) rep-
•* ex o o ree e r
resent, respectively, the neutral distribution function and the ion
distribution function for which:
n (r) = f(r,v)dv3; n(r) = <Mr,v)dv3 , (3)
0
n(r), n (r) are the plasma and neutral density profiles. <o v> , ,
o , ex l
<a. v> , <ai v^ / <arecv>e are' reDPectivelv r tne collision rate coef
on i
ficients for resonant charge exchange, electron ionization, ion ioniza
tion, and radiative recombination. A collision rate at velocity w is
defined by e.g.:
-8-