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Effect of caesium seeding in negative hydrogen ion sources

Marthe BacalUPMC, LPP, Ecole Polytechnique, Palaiseau, France

Roy McAdams and Elizabeth SurreyCCFE/Euratom Fusion Association, Culham Science Center,

Abingdon, Oxfordshire OX14 DBB, UK

Low Temperature Plasma Teleseminar, December 20, 2013also presented at the 15th Intern. Conf. on Ion Sources,

9-13 Sept. 2013, Japan

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OUTLINEI. MotivationII. Why are neutral beams required for fusion ?III. Negative ion beams – precursors of high energy

neutral beamsIV. Effect of plasma electrode work functionV. Effects of caesium seedingVI. Causes of the H- ion current enhancement by

caesiumVII. Effect of gettering the atomic hydrogen by caesiumVIII.Conclusion

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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.

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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.

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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

– D-

– Energy 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

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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 hydrogenions in a neutralizer cell of optimum thickness

remains acceptable at higher velocities, and is nearly independent of beam energy above 100

keV/nucleon.

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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

F is the surface work functionEA is the electron afinityv = 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.

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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.

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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.

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Microwave driven source Camembert III

Some of the results to be presented were obtained in the multicuspplasma source Camembert III at Ecole Polytechnique, France.* The microwave-driven version,operated with a network of 7 dipolarsources,1 is shown here.*In the filamented version2 we performedthe comparison of pure hydrogen andcaesiated operations2.1Béchu et al, Phys.Plasmas, 20, 101601 (2013) 2Courteille et al, Rev. Sci. Instrum., 66, 2533 (1995)

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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 H- current.

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V. EFFECTS OF CAESIUM SEEDING

The effect of caesium seeding on the *extracted H- ion curent and * co-extracted electron currentdependences on the plasma electrode bias,observed in two ion sources, will be reported.

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IV.1. Negative ion current 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

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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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VpVp -5Vp -10 Vp +5 Vp +10

Pure H2

Volume produced ions

VpVp-5Vp-10 Vp+5 Vp+10

Cs seeded source

Volume produced ions eventually enhanced by gettering

IH-

IH-

H- ions arriving from the bulk plasma

Bias voltage (V)

Bias voltage (V)

Directly produced H- ions due to ions and atoms on Cs surface

*enhanced by the getteringwhich reduces H- destruction

*reduced by the getteringvia direct production

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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 theH- current in the whole range of bias voltage variation.This may explain the weak dependence upon Vb of the H-

current in the JAERI 10 Amp source.

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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.

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Evidence for H- ion flow from the background plasma towards the plasma grid

Experiments in NIFS (Toki, Japan) using CRDS and probes showed that extracted H- ions at plasma grid biashigher than the plasma potential originate from the plasma volume beyond the extraction region.

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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 extractedcurrents 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.