Effect of caesium seeding in negative
hydrogen ion sources
Marthe Bacal
UPMC, LPP, Ecole Polytechnique, Palaiseau, France
Roy McAdams and Elizabeth Surrey
CCFE/Euratom Fusion Association, Culham Science Center,
Abingdon, Oxfordshire OX14 DBB, UK
Low Temperature Plasma Teleseminar, December 20, 2013
also presented at the 15th Intern. Conf. on Ion Sources,
9-13 Sept. 2013, Japan
1
OUTLINE
I. Motivation
II. Why are neutral beams required for fusion ?
III. Negative ion beams – precursors of high energy
neutral beams
IV. Effect of plasma electrode work function
V. Effects of caesium seeding
VI. Causes of the H- ion current enhancement by
caesium
VII. Effect of gettering the atomic hydrogen by caesium
VIII.Conclusion
2
I. Motivation : magnetic controlled fusion
• On the route to sustainable power from magnetic
confinement fusion, the International Tokamak
Experimental Reactor (ITER) is currently under
construction at Cadarache in France.
• ITER will operate to produce net output of fusion
power that exceeds the heating power by a factor of
Q=10 and produce a self-sustaining plasma for
several hundred seconds.
• However ITER is still an experimental device and will
not produce any electricity.
3
What will follow beyond ITER ?
• Beyond ITER the DEMO machine will produce
electricity and demonstrate the requisite
technologies to allow commercial production
of electrical power.
• The route to fusion power leads from today’s
tokamaks such as JET, moving through ITER
and DEMO to a commercial fusion reactor.
4
Role of DEMO
• DEMO will link a fusion source with electricity
generation and will be the last machine before a
commercial fusion reactor.
• The European Fusion Roadmap calls for construction
of DEMO to commence in 2030 at the point where
ITER has successfully demonstrated the Q=10
performance.
• The European programme is currently considering
two options as of July 2013 for DEMO: a “pulsed”
and a “steady state” option.
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II. Why are neutral beams required for fusion ?
• The neutral beam power is required to heat the plasma
to reach the burn stage and sustain the pulse length by
current drive.
• 33 MW are planned for ITER, while 135 MW will be
necessary for the steady state option of DEMO.
• In ITER a negative ion current of 40 A will be accelerated
to 1 MV before negative ions being neutralized. The pulse
length will be 3600 s.
• The energy of the DEMO beam is also 1MeV at present.
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The ITER beamline
•
Heating beam parameters
–
–
–
–
–
•
DEnergy 1MeV
Accelerated current 40A
Gas neutraliser
Injected power 17MW
Accelerator
– ~10MW of power dumped
in accelerator
– ~700kW of electrons exits
accelerator
– ~900kW of backstreaming
positive ions
•
Extensive development
programme with test stands
operating and being built
– ELISE - IPP Garching
• one half sized
– RFX – Padua
• full sized
7
Neutral beam generation from
positive versus negative ion beams
• A beam of energetic atoms, called also neutral beam, can
be generated by a positive ion beam, crossing a gas
neutralizer. However the efficiency of neutralization of a
positive hydrogen ion declines rapidly as the ion velocity
goes up and past 60 keV/amu the neutralization
efficiency becomes prohibitively low (see next slide).
• The neutralization efficiency of negative hydrogen
ions in a neutralizer cell of optimum thickness remains
acceptable at higher velocities, and is nearly
independent of beam energy above 100 keV/nucleon.
8
Neutralization efficiency of the ions
From Hemsworth and Inoue, IEEETPS, 33, N° 6, p. 1700 (2005)
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III. Negative ion beams – precursors of high energy
neutral beams.
• Since high energy atomic beams are required in fusion
research, the need for their precursors, the negative ion
beams, became urgent.
• This imposed the development of negative hydrogen ion
sources, based on two types of processes :
- In the plasma volume, D- ions are formed by dissociative
attachment of electrons to ro-vibrationally excited molecules
- on surfaces limiting the plasma they are formed by the
interaction of fast particles with these surfaces.
Volume processes alone cannot produce the required current
density for ITER (~30mA cm-2 of D-), therefore efforts concentrated
on the surface production.
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Physics of negative ion surface production
The probability, P, of negative ion production on a
metal surface has been calculated by Rasser et al
(Surface Science, 118, 698 (1982)) as
2 é p (F - EA ) ù
P º expêú
p ë
2av û
F is the surface work function
EA is the electron afinity
v = the particle velocity
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The volume type negative ion source.
The volume negative ion source consists of the following parts as
shown on the following slide :
• a plasma generation region, denoted S (the source region), where a
hot plasma is generated by an arc discharge or by RF.
• a negative ion production region, from where the negative ions are
extracted, denoted E (the extraction region).
• a magnetic filter MF, separating these two regions.
The extraction region is limited by a plate denoted PE (plasma
electrode) containing the extraction opening and separating the source
from the accelerator, which is differentially pumped to allow particle
acceleration.
12
Schematic presentation of a volume type negative ion
source
• This negative ion source is a « volume type » one, operated in
pure hydrogen, driven by the electrons emitted by a filament,
which are accelerated to several tens of eV.
13
IV. Effect of Plasma Electrode Work Function on
extracted H- Ion Current
• After seeding a certain
amount of Cs into a volume
source, it was observed
that the negative ion
current increased with the
plasma grid temperature,
up to 250 - 300°C.
• Thus a work function of 1.5
eV can be obtained not
only in ultrahigh vacuum,
but also under plasma
conditions.
From Hemsworth and Inoue, IEEE TPS, 33, N°
6, 1700 (2005)
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Role of the Plasma Electrode in Surface
Production
• When the source is « caesiated » the plasma
electrode surface work function can be minimized
by the choice of its temperature (usually 150 C).
• Thus the plasma electrode is the main negative ion
producing surface in the source. Its role in the
generation of the extracted current can be optimized
by applying a bias voltage, Vb, with respect to the
plasma potential.
15
Microwave driven source Camembert III
Some of the results to be presented
were obtained in the multicusp
plasma source Camembert III at
Ecole Polytechnique, France.
* The microwave-driven version,
operated with a network of 7 dipolar
sources,1 is shown here.
*In the filamented version2 we performed
the comparison of pure hydrogen and
caesiated operations2.
1Béchu et al, Phys.Plasmas, 20, 101601 (2013)
2Courteille et al, Rev. Sci. Instrum., 66, 2533 (1995)
16
Effect of plasma electrode bias on the extracted currents
in the microwave driven source Camembert III.
Pure hydrogen operation.
Svarnas et al, IEEETPS, 35, N° 4, 1156 (2007)
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The maximum H- ion extraction is obtained for
plasma grid bias, Vb , slightly higher than the local
plasma potential, Vp.
The typical difference Vb – Vp for maximum
extraction < 1 V.
When Vb is further increased the volume produced
ions are accelerated toward the plasma grid and
their density near the plasma grid goes down.
This leads to the decrease of the extracted Hcurrent.
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V. EFFECTS OF CAESIUM SEEDING
The effect of caesium seeding on the
*extracted H- ion curent and
* co-extracted electron current
dependences on the plasma electrode bias,
observed in two ion sources, will be reported.
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V.1. Negative ion current
I
extracted from the
JAERI 10 Amp ion source
Masanobu Tanaka et al,
Report JAERI-M 93-132
(1993)
H2 Pressure in the
source : 0.7 Pa
20
V.2. Negative ion current extracted from the source
Camembert III (Ecole Polytechnique)
from Rev. Sci. Instrum., 69, 932 (1998)
This experiment was
effected at two values of
the extraction voltage,
in. pure hydrogen and
caesium seeded
operations.
The hydrogen pressure in
the source was 0.4 Pa
These data indicate a clear threshold
of the surface component,
i.e. a sudden decrease in I(H-) near
0.5 V
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Effect of Plasma Electrode Bias Voltage on the
Coextracted Electron Current, from Camembert III
• The use of caesium also
reduces the co-extracted
electron current, as can
be seen on the upper
figure.
• This reduction of the
coextracted electron
current is the main
reason for the use of
caesium in negative ion
sources.
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Several characteristics of caesiated operation
• In both JAERI 10 Amp and Camembert III sources
the H- ion current increases due to caesium by
a factor 2.5 at plasma potential.
• In both sources H- ions can be extracted also at
bias voltage above plasma potential (Vb > Vp).
• In Camembert III the extracted current goes down
by a factor 2 only, when the bias voltage increases
above the plasma potential.
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VI. Causes of the H- ion current enhancement
by caesium
• In the whole range of bias voltage: the H- ion
current is enhanced due to the gettering of
atomic hydrogen by caesium.
• At bias voltage below plasma potential: direct
production of H- ions by positive ions and atoms
incident on the caesiated plasma grid surface.
• At bias voltage above plasma potential : the
current of volume produced H- ions is enhanced
by gettering and flow of negative ions from the
bulk plasma.
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IHPure H2
Volume produced ions
Vp -10
Vp -5
Vp
Vp +5
Vp +10
Bias voltage (V)
Cs seeded source
Directly produced H- ions due to
ions and atoms on Cs surface
*enhanced by the gettering
which reduces H- destruction
*reduced by the gettering
via direct production
Vp-10
Vp-5
IHH- ions arriving from the bulk
plasma
Volume produced ions eventually
enhanced by gettering
Vp
Vp+5
Bias voltage (V)
Vp+10
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VII. Effect of gettering the atomic hydrogen by
caesium
• Gettering reduces the destruction of H- ions by associative
and non-associative detachment due to atomic hydrogen.
This enhances the H- ion current extracted in the whole bias
range.
• Another effect of gettering is to reduce the H- ion
direct (surface) production by atomic hydrogen on caesiated
surfaces, i.e. the H- ion current extracted with Vb < Vp.
Thus the consequence of strong gettering is levelling off the
H- current in the whole range of bias voltage variation.
This may explain the weak dependence upon Vb of the Hcurrent in the JAERI 10 Amp source.
26
Recent measurement of the effect of
gettering
• The reduction of the atomic hydrogen density by gettering by
caesium was studied recently by Friedl and Fantz (AIP Conf.
Proc. 1514, 255 (2013)
• They found that the density of atomic hydrogen is reduced by
a factor 2 due to caesium seeding into a plasma produced at
hydrogen pressure of 10 Pa.
• The change in atomic hydrogen density could be much larger
when the hydrogen pressure is as low as in ion source
operation, because the same number of absorbed hydrogen
atoms represents a higher fraction of their initial density at
the lower pressure of 0.3-0.5 Pa than at 10 Pa.
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Are other getters than caesium enhancing
negative ion production ?
• If gettering of atomic hydrogen is the principal
mechanism enhancing H-, any hydrogen getters, like Ti or Ta
should work.
• The graph below shows that the H- currents from a source
with walls covered with Cs and Ta films are comparable, while
a W film leads to reduced H- ion current.
28
e'of'probe'and'CRD'signals
ndence'of'probe'and'CRD'signals
Evidence for H- ion flow from the background
plasma towards the plasma grid
Vbias:1.8V
Vbias:1.8V
Before Before
extraction
extraction
Experiments in NIFS (Toki, Japan) using CRDS and probes
showed that extracted H- ions at
plasma
grid bias
After extraction
After extraction
higher than the plasma potential originate from
the plasma volume beyond the extraction region.
of potentials
tion
of saturation currents
DistributionDistribution
of potentials
n
currents
in
z
direction
in z direction
in z direction
Ez ~ 0V/m
Distribution
of CRD
signals
Distribution
of CRD
signals
in
z
direction
in z direction
Vbias:5.8V
Vbias:5.8V
Ez ~ 0V/m
Before extraction
BeforeAfter
extraction
extraction
After extraction
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VIII. Conclusion ( I )
I. The enhancement due to caesium seeding of the
extracted H- ion current in the plasma grid bias range
above the plasma potential is explained by:
• gettering of atomic hydrogen by caesium
• negative ion flow from bulk plasma.
II. There is a clear threshold of the surface component
of the extracted H- current, i.e. its decrease when
plasma grid bias exceeds the plasma potential.
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Conclusion ( II )
III. Since the extracted current is larger when due to
the surface component, one could expect that in
some ion sources the operation with negative bias
voltage should be optimal.
IV. Strong gettering can level off the extracted
currents in the whole bias variation range. It is
suggested that this is the reason for the weak
dependence of the extracted ion current on Vb
in the JAERI 10 Amp source.
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