TOF method in plasma potential measurements by HIBD
The heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) is based on a multiple cell array detector (MCAD), which collects simultaneously a “fan” of secondary ions originated along a primary beam trajectory in collisions with the plasma e...
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| Опубліковано в: : | Вопросы атомной науки и техники |
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| Дата: | 2002 |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2002
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| Цитувати: | TOF method in plasma potential measurements by HIBD / I.S. Nedzelskiy, A. Malaquias, B. Gonçalves, C.A.F. Varandas, J.A.C. Cabral // Вопросы атомной науки и техники. — 2002. — № 5. — С. 148-150. — Бібліогр.: 9 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859778184696823808 |
|---|---|
| author | Nedzelskiy, I.S. Malaquias, A. Gonçalves, B. Varandas, C.A.F. Cabral, J.A.C. |
| author_facet | Nedzelskiy, I.S. Malaquias, A. Gonçalves, B. Varandas, C.A.F. Cabral, J.A.C. |
| citation_txt | TOF method in plasma potential measurements by HIBD / I.S. Nedzelskiy, A. Malaquias, B. Gonçalves, C.A.F. Varandas, J.A.C. Cabral // Вопросы атомной науки и техники. — 2002. — № 5. — С. 148-150. — Бібліогр.: 9 назв. — англ. |
| collection | DSpace DC |
| container_title | Вопросы атомной науки и техники |
| description | The heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) is based on a multiple cell array detector (MCAD), which collects simultaneously a “fan” of secondary ions originated along a primary beam trajectory in collisions with the plasma electrons and separated by the magnetic field of the tokamak. Utilization of the traditional electrostatic energy spectrographs for the plasma potential measurements in experiments with MCAD is very complicated. This paper presents the current results of adaptation and mastering of the alternative time-of-flight (TOF) technique. Three schemes of the measurements are considered: i) “integral” scheme of the average plasma potential measurements by a pulsed primary beam, ii) “quasi-local” scheme of the measurements of plasma potential drop between neighbouring sample volumes, and iii) “local” scheme of plasma potential profile measurements. The electronics used in TOF energy analyzer (TOFEA) consist of charge sensitive and fast shaping amplifiers, constant fraction discriminator and time-toamplitude converter with resolution ∆t/t = 10⁻⁴. The TOFEA resolution ∆t/t = 3×10⁻⁴ has been achieved in mastering experiments with a pulsed (250 ns) primary beam carried out to the primary detector in magnetic field of the tokamak. With plasma the resolution is reduced 2.5 times due to decreasing of signal-to-noise ratio caused by plasma loading of MCAD. The changes of the average plasma potential during discharges with minor disruptions have been obtained by TOF energy analysis. The results of this experiment allow to conclude the reliability of TOF technique in plasma potential measurements by HIBD with MCAD. On the base of the obtained data and experience a four-channel TOFEA for the plasma potential profile measurements has been elaborated.
|
| first_indexed | 2025-12-02T09:20:24Z |
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| fulltext |
TOF METHOD IN PLASMA POTENTIAL MEASUREMENTS BY HIBD
I.S. Nedzelskiy, A. Malaquias, B. Gonçalves, C.A.F. Varandas and J.A.C. Cabral
Associaçao Euratom/IST, Centro de Fusão Nuclear, Instituto Superior Técnico,
1049-001, Lisboa, Portugal
The heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA)
is based on a multiple cell array detector (MCAD), which collects simultaneously a “fan” of secondary ions originated along a
primary beam trajectory in collisions with the plasma electrons and separated by the magnetic field of the tokamak. Utilization
of the traditional electrostatic energy spectrographs for the plasma potential measurements in experiments with MCAD is very
complicated. This paper presents the current results of adaptation and mastering of the alternative time-of-flight (TOF)
technique. Three schemes of the measurements are considered: i) “integral” scheme of the average plasma potential
measurements by a pulsed primary beam, ii) “quasi-local” scheme of the measurements of plasma potential drop between
neighbouring sample volumes, and iii) “local” scheme of plasma potential profile measurements. The electronics used in TOF
energy analyzer (TOFEA) consist of charge sensitive and fast shaping amplifiers, constant fraction discriminator and time-to-
amplitude converter with resolution ∆t/t = 10-4. The TOFEA resolution ∆t/t = 3×10-4 has been achieved in mastering
experiments with a pulsed (250 ns) primary beam carried out to the primary detector in magnetic field of the tokamak. With
plasma the resolution is reduced 2.5 times due to decreasing of signal-to-noise ratio caused by plasma loading of MCAD. The
changes of the average plasma potential during discharges with minor disruptions have been obtained by TOF energy
analysis. The results of this experiment allow to conclude the reliability of TOF technique in plasma potential measurements
by HIBD with MCAD. On the base of the obtained data and experience a four-channel TOFEA for the plasma potential
profile measurements has been elaborated.
PACS: 52.70.-m
I. INTRODUCTION
The heavy ion beam diagnostic (HIBD) is known as the
only tool for direct measurement of the electric potential
in hot plasmas. These measurements are realized by
energy analysis of the secondary ions, which arose in
collisions with the plasma electrons. In HIBD with a
multiple cell array detector (MCAD) [1], which collects a
“fan” of secondary ions originated along a primary beam
trajectory inside the plasma, utilization of traditional
electrostatic energy spectrographs for plasma potential
measurements is very complicated. The alternative can be
the time-of-flight (TOF) energy analysis [2].
This paper presents the current results of adaptation and
mastering of TOF technique for the plasma potential
measurements by HIBD on the tokamak ISTTOK [3].
The structure of the paper is as follows. The principle of
TOF method is reminded in Section II. Three schemes of
plasma potential measurements by TOF energy analysis
are considered in Section III. TOF electronics and
influence of plasma loading effect are characterized in
Section IV. The current results of the TOF measurements
with HIBD on ISTTOK are presented in Section V. The
four-channel TOF energy analyzer elaborated recently is
shortly described in Section VI. Conclusion is formulated
in Section VII.
II. PRINCIPLE OF TOF METHOD
In TOF method the energy dispersion is created in time,
hence, a modulated incident beam must be used with a
detection system capable of determining the time-of-flight
of the injected particles over a known distance (TOF
path). The detector at the entrance of the TOF path creates
“start” (reference) signal, while “stop” signal is obtained
on the detector at the exit of the TOF path. TOF energy
analysis can be generally realized by direct measurements
of “start”-“stop” time delay (tTOF) in a case of discrete
modulated beam (“conventional” TOF), or by
measurements of “start”-“stop” phase-shift (ϕTOF)
accumulated along TOF path by continuously (sinusoid)
modulated beam (“phase-shift” TOF). Resolution of the
measurements is given by:
∆E/E0=2∆t/tTOF =2∆ϕ/ϕTOF, (1)
The simplest beam modulation technique consists of the
fast scanning of the beam across the slit. However, in the
case of continuous modulation the higher harmonics
appear in the signal due to convolution of the beam
profile and slit shape [4], thus resulting in a strong
degradation of resolution. In discrete modulation the
effect of beam energy perturbation during modulation
process can take place when the width of the modulation
pulse is comparable with the time-of-flight of the ion
between the plates. The easy control of the last effect
determines a choosing of the “conventional” TOF for the
implementation in HIBD.
III. TOF PLASMA POTENTIAL MEASUREMENTS
The TOF measurements of plasma potential by HIBD
with MCAD can be generally realized by three schemes
schematically shown in Fig.1.
Fig.1. Schematic of TOF measurements
The simplest “integral” scheme is the measurement of
average plasma potential by primary ion beam. In this
148 Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 148-150
MCAD
Cs+
Stop Start
Modulator
Cs2+
Primary detector
Sample volumes
Plasma
scheme the time-of-flight of the ion across the volume
with some distributed along beam trajectory (ltr)
electrostatic potential (Φ(ltr/Lpl)) is given by the path
integral:
tTOF=tTOFmd(Φ=0)(Lpl/LTOFmd){(1/2)×
×∫[qΦ(ltr/Lpl)/E0]d(ltr/Lpl)+1}, (E0>>qΦmax), (2)
where tTOFmd is the time-of-flight from modulator to
detector, Lpl is the plasma dimension, E0 is the initial
energy and q is the charge of the ion.
In the second “quasi-local” scheme the potential drop (∆
Φ(∆ltr)) between neighbouring sample volumes inside the
plasma can be obtained by measuring the relative time-of-
flight of the secondary ions between the respective
(“start”-“stop”) cells of MCAD. The relation is:
∆tTOF=tTOFsd(Φ=0){(1/2)[q∆Φ(∆ltr)/E0]+1}+∆ttint,
(E0>>q∆Φ ), (3)
where tTOFsd is the time-of-flight of a secondary ion from
the sample volume to MCAD constituted the effective
TOF path. The term ∆ttint is some path integral, which for
ISTTOK HIBD geometry can be as low as 10%,
depending on the achievable resolution of the
measurements.
The third “local” scheme presents the conventional HIBD
measurements of plasma potential (Φ(ltr)) given by:
tTOF=tTOFss(Φ=0){(1/2)[Φ(ltr)/E0]+1},
(E0>>qΦ), (4)
where tTOFss is the time-of-flight between “start” and
“stop” detectors arranged along secondary ion trajectories
outside the plasma.
Notice that the calibration of time-of-flight along the TOF
paths without plasma for all schemes is necessary.
IV. TOF ELECTRONICS AND PLASMA
LOADING EFFECT
A routine application of TOF method in mass and nuclear
spectroscopy naturally determines the use of the already
elaborated approaches and acquisition electronics. The
one-channel TOF energy analyzer (TOFEA) currently
tested in HIBD TOF measurements on ISTTOK is shown
in Fig.2 [4]. It consists of primary beam modulator,
detector, conditioning electronics and a time-to-amplitude
converter (TAC).
Fig.2. One-channel TOFEA
Primary beam modulator constitutes of a pair of
electrostatic plates (8 cm length and 0.8 cm separation),
powered by DEI HV1000 pulser (55 ns - 10µs range), and
the following 2 mm slit. Conditioning electronics are
conventional ORTEC products and include a charge
sensitive preamplifier, a fast shaping amplifier, and a
constant fraction discriminator. A relatively large (of the
order of the beam pulse width) effective collection time of
detector (copper plate) determines the use of a charge
sensitive preamplifier, which is well suited in this
condition of operation [5]. The sensitivity of the
ORTEC142A charge sensitive preamplifier is 45
mV/MeV. The following fast shaping amplifier
(ORTEC474) and constant fraction discriminator
(ORTEC455) allow to optimize the resolution of the
measurements by choosing of the appropriate time-
shaping constants and discrimination levels. The TAC
creates a pulse with amplitude proportional to the time-of-
flight of the primary beam pulse from the modulator to
the primary detector. The time resolution of TAC
(ORTEC457) is (∆t/t)TAC=10-4. The TAC output is
acquired by a multichannel analyzer (LeCroy3001), or by
a 1 µs analog-to-digital converter (ADC) with data
storage in 64 kilobite buffer memory. The time-delay
module allows fast and simple calibration of TOFEA
electronics.
The loading of detector by plasma radiation presents the
main problem of TOF method implementation in
experiments with HIBD. The created current is caused by
photo-electron emission from detector surface. Generally,
it can be considered as an effective leakage current
similar to semiconductor detectors of nuclear
spectroscopy with the same consequences. Usually, this
current presents a pulse of plasma shot duration (20-40
ms in the case of ISTTOK) with amplitude depending on
discharge conditions and geometry of detector
arrangement as to the plasma vision. In standard ISTTOK
discharges and with the present arrangement of MCAD
the value of loading current is high enough to put
preamplifier into saturation.
V. CURRENT RESULTS OF TOF
MEASUREMENTS
In the experiments described below the HIBD operated
with 1.5 (0.8) µA of steady Cs+ (Xe+) beam extracted
from the plasma ion source and accelerated up to 22 keV.
Parameters of the modulator and TOFEA were optimized
in the experiments with a pulsed primary beam carried out
to the primary detector in toroidal magnetic field of
tokamak. Particularly, a width of the beam pulse of 250 ns
is the compromise between limitations imposed by the
beam intensity, detector capacitance and resolution of the
measurements.
Fig.3 presents two TOF spectra obtained in these
experiments and externally delayed by 2 ns. The full
width on half maximum (FWHM) of the spectra is
FWHM~20 ns, and mainly contributed by the electrically
coupled and electromagnetic interference noise (the
signal-to-noise ratio (SNR) is SNR~8). Distinguishing of
2 ns of the delay in spectra demonstrates the TOFEA
capability of resolution ∆t/tTOF~3×10-4 (tTOF = 7.2 µs). The
saturation of preamplifier during plasma shot has been
avoided with inductive high-pass filtration introduced
between the detector and preamplifier. However, the level
149
PULSER
SLIT
PLASMA
CHAMBER
BEAM POSITION
CONTROL
DETECTOR
Cs+ BEAM
20 µ s
~250 n s
HI PASS
FILTER
EARTH
BREAK START ~6 s
DELAY
STOP TAC
FAST
POWER SUPPLY
SHAPING
AMPLIFIER
FAST CHARGE
PRE-AMP.
CONSTANT
FRACTION
DISCRIM.
of the noise increases 2.5 times reducing the TOFEA
resolution to ∆t/tTOF~7.5×10-4, or ∆E/E0~1.5×10-3.
Fig.3. Two TOF spectrums delayed on 2 ns
Changes on the average plasma potential in discharges
with minor disruptions have been observed by TOF
energy analysis in experiments with a pulsed primary
beam crossing the plasma [6]. The TOF (TAC) signal and
plasma parameters time evolutions are presented in Fig.4.
The relative change of the average plasma potential is
described by Eq.(2) derived for two moments (t) and (t+δ
t) of discharge. The resolution of the measurements is
given by ∆Φ/E0≅(∆E/E0)( Lpl/LTOFmd)-1 = 1.1×10-2, or ∆Φ ≅
240 V of the absolute value. Temporal resolution of the
measurements is 20 µs restricted by TAC acquisition
(busy) time.
Fig.4. The TOF signal and plasma parameters time
evolutions in disruptive discharges of ISTTOK
Immediate analysis of TOF signal indicates the drop of
plasma potential to a relatively more negative value
during disruption. The HIBD observations of minor
disruptions on the tokamaks RENTOR [7] and ISX-B [8]
show the drop of plasma potential to zero due to the
shortening of radial electric field when plasma moves and
dumps on the wall of tokamak chamber. Exploitation of
this fact allows to estimate the plasma potential value of
the order of ~(+450) V. Positive electric potential
opposite to the prediction of neoclassic theory was
observed on a number of small tokamaks (in particular on
RENTOR [7]) with reduced plasma parameters, and is
attributed to the fluctuations in the magnetic field which
cause the field lines to stochastically wander out to the
chamber wall, permitting electrons to escape along the
field lines. As a result a positive ambipolar potential is
built up until the electron and ion losses are equal.
However, the estimated value of plasma potential is
higher, than predicted by theory. The presence of
runaway electrons in low-density discharge of ISTTOK
may be an explanation.
VI. FOUR CHANNEL TOFEA
The initial experiments with time-of-flight measurements
of the plasma potential drop between neighbouring
sample volumes have shown that, though the primary
beam intensity has been increased 5 times with a new
beam injection system [9], the SNR for the secondary
beam is still remained too low due to plasma loading
effect. Fig.5 presents the schematic of the four-channel
TOFEA with cylindrical electrostatic steering plates. Such
a design is minimally influenced by plasma loading and
suited to strong mechanical constrains of the ISTTOK
diagnostic port. The conventional electrostatic plates are
foreseen for the careful beam alignment. The “start”
(mesh) and “stop” (plate) detectors of TOFEA are
arranged inside double-shielded box to minimize the
electrically coupled and electromagnetic interference
noise.
Fig.5. Four-channel TOFEA
VII. CONCLUSION
The presented results demonstrate the reliability of TOF
technique in plasma potential measurements by HIBD
with MCAD. Resolving of the plasma loading problem
with a new TOFEA should allow to improve sufficiently
the signal-to-noise ratio and to perform the measurements
of the plasma potential profile.
ACKNOWLEDGMENT
This work has been carried out in the frame of the
Contract of Association between the European
Community and Instituto Superior Técnico and has
received financial support from Fundação para Ciência e a
Tecnologia (FCT). The content of publication is the sole
responsibility of the authors and it does not necessarily
represent the views of the Commission of the European
Union or FCT or their services.
REFERENCES
[1] J.A.C. Cabral et al, IEEE Transactions on Plasma Science,
22, 350 (1994).
[2] I.S. Nedzelskiy, A. Malaquias, J.A.C. Cabral, C.A.F.
Varandas, Rev. Sci. Instrum., 72, 572 (2001).
[3] C.A.F. Varandas et al, Fusion Technology, 29, 105 (1996).
[4] L.I. Krupnik, I.S. Nedzelskiy, A.N. Procenko, Sov. Journal
of Tech. Physics, 32, 6 (1988).
[5] H. Spieler, IEEE Transactions on Nuclear Science, NS-29
(3), 1142 (1982).
[6] I.S. Nedzelskiy, A. Malaquias, J.A.C. Cabral, C.A.F.
Varandas, In: 2000 ICPP, Quebec City, Canada, CP1.120 (2000)
[7] P.M. Schoch, RPDL Report No.83-23, 1983.
[8] J. Matthew et al, Report ORNL/TM-9386, 1986.
150
10
6
2
10 14 18 22 26
Time (ms)
A
m
pl
itu
de
s (
a.
u.
)
Ipl
ne
TOF
Vloop
MHD
Cs2+ Stop Start
Stop Start
700 mm
70
50
30
10
20 40 60 80 100 120
Time (ns)
A
m
pl
itu
de
(a
.u
.)
[9] J.A.C. Cabral, I.S. Nedzelskiy, A. Malaquias, B. Gonçalves,
C.A.F. Varandas, In: HTPD 2002, Madison, USA, DP04 (2002).
151
I. Introduction
II. Principle of TOF method
III. TOF plasma potential measurements
Fig.1. Schematic of TOF measurements
IV. TOF electronics and plasma
loading effect
Fig.2. One-channel TOFEA
Fig.3. Two TOF spectrums delayed on 2 ns
VI. Four channel TOFEA
|
| id | nasplib_isofts_kiev_ua-123456789-79288 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-02T09:20:24Z |
| publishDate | 2002 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Nedzelskiy, I.S. Malaquias, A. Gonçalves, B. Varandas, C.A.F. Cabral, J.A.C. 2015-03-30T09:43:33Z 2015-03-30T09:43:33Z 2002 TOF method in plasma potential measurements by HIBD / I.S. Nedzelskiy, A. Malaquias, B. Gonçalves, C.A.F. Varandas, J.A.C. Cabral // Вопросы атомной науки и техники. — 2002. — № 5. — С. 148-150. — Бібліогр.: 9 назв. — англ. 1562-6016 PACS: 52.70.-m https://nasplib.isofts.kiev.ua/handle/123456789/79288 The heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) is based on a multiple cell array detector (MCAD), which collects simultaneously a “fan” of secondary ions originated along a primary beam trajectory in collisions with the plasma electrons and separated by the magnetic field of the tokamak. Utilization of the traditional electrostatic energy spectrographs for the plasma potential measurements in experiments with MCAD is very complicated. This paper presents the current results of adaptation and mastering of the alternative time-of-flight (TOF) technique. Three schemes of the measurements are considered: i) “integral” scheme of the average plasma potential measurements by a pulsed primary beam, ii) “quasi-local” scheme of the measurements of plasma potential drop between neighbouring sample volumes, and iii) “local” scheme of plasma potential profile measurements. The electronics used in TOF energy analyzer (TOFEA) consist of charge sensitive and fast shaping amplifiers, constant fraction discriminator and time-toamplitude converter with resolution ∆t/t = 10⁻⁴. The TOFEA resolution ∆t/t = 3×10⁻⁴ has been achieved in mastering experiments with a pulsed (250 ns) primary beam carried out to the primary detector in magnetic field of the tokamak. With plasma the resolution is reduced 2.5 times due to decreasing of signal-to-noise ratio caused by plasma loading of MCAD. The changes of the average plasma potential during discharges with minor disruptions have been obtained by TOF energy analysis. The results of this experiment allow to conclude the reliability of TOF technique in plasma potential measurements by HIBD with MCAD. On the base of the obtained data and experience a four-channel TOFEA for the plasma potential profile measurements has been elaborated. This work has been carried out in the frame of the Contract of Association between the European Community and Instituto Superior Técnico and has received financial support from Fundação para Ciência e a Tecnologia (FCT). The content of publication is the sole responsibility of the authors and it does not necessarily represent the views of the Commission of the European Union or FCT or their services en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Plasma diagnostics TOF method in plasma potential measurements by HIBD Article published earlier |
| spellingShingle | TOF method in plasma potential measurements by HIBD Nedzelskiy, I.S. Malaquias, A. Gonçalves, B. Varandas, C.A.F. Cabral, J.A.C. Plasma diagnostics |
| title | TOF method in plasma potential measurements by HIBD |
| title_full | TOF method in plasma potential measurements by HIBD |
| title_fullStr | TOF method in plasma potential measurements by HIBD |
| title_full_unstemmed | TOF method in plasma potential measurements by HIBD |
| title_short | TOF method in plasma potential measurements by HIBD |
| title_sort | tof method in plasma potential measurements by hibd |
| topic | Plasma diagnostics |
| topic_facet | Plasma diagnostics |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/79288 |
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