Fusion Engineering and Design 74 (2005) 299–303

Diagnostics of the cesium amount in an RF negative ion sourceand the correlation with the extracted current density

U. Fantz∗, H.D. Falter, P. Franzen, M. Bandyopadhyay, B. Heinemann,W. Kraus, P. McNeely, R. Riedl, E. Speth, A. Tanga, R. Wilhelm

Max-Planck-Institut fur Plasmaphysik, EURATOM Association, Boltzmannstr. 2, D-85748 Garching, Germany

Available online 1 August 2005

Abstract

The formation of negative ions in plasma sources by the surface process requires covering the extraction grid with a materialof low work function. This can be achieved by cesium evaporation but for operational reasons the consumption of cesium shouldbe minimised. In order to quantify the amount of cesium in the RF discharge optical emission spectroscopy is used as diagnostictool. Suitable diagnostic lines and their analysis to obtain particle densities and particle fluxes are described. Influences of argonadmixtures to hydrogen and deuterium discharges on cesium emission are shown. A correlation of the cesium emission with theextracted negative ion current density is discussed.© 2005 Published by Elsevier B.V.

Keywords: Cesium; Current density; RF discharge

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. Introduction

Neutral beam injection based on negative ionources will be a major heating system of ITER[1].n contrast to positive ion sources, which are rou-inely used in today’s fusion experiments, negative ionources are still under development. Promising candi-ates to meet the ITER target parameters are arc sourcesnd inductively coupled RF sources. The status andlans for the development of an RF negative ion source

or ITER are described in[2].

∗ Corresponding author. Tel.:+49 89 3299 1958;ax:+49 89 3299 2558.

E-mail address: fantz@ipp.mpg.de (U. Fantz).

Negative ions, H− and D−, are formed in hydrogen and deuterium plasmas by volume and suprocesses. The reaction chain for volume processbased on the dissociative attachment of vibrationexcited hydrogen molecules in the ground s(H2(v) + e→ H2

− → H + H−). The surface proceneeds atoms or ions, which interact with a surwith a low work function, where cesium is commoused: H + surface e (Cs)→ H−. The destruction onegative ions with their binding energy of 0.75 eVdominated by volume processes: the electron strip(e + H− → 2e + H) being very effective forTe in therange of a few eV and the mutual neutralisa(H+ + H− → H + H) which depends slightly on iotemperature. The losses can be reduced by reducinTe,

920-3796/$ – see front matter © 2005 Published by Elsevier B.V.doi:10.1016/j.fusengdes.2005.06.184

300 U. Fantz et al. / Fusion Engineering and Design 74 (2005) 299–303

but mutual neutralisation will take over. This resultsin a short survival length of negative ions (cm range).As a consequence, the H− formation by the surfaceprocess directly at the extraction grid is favoured. Oneof the main tasks is to achieve a thin and homogeneousCs coverage of the extraction grid by Cs evaporationin the discharge volume with the boundary conditionof minimising the Cs consumption. Therefore, in situdiagnostics of the layers and the Cs amount in thedischarge are desirable. Optical emission spectroscopyprovides a tool to measure Cs in the plasma volume.First observations at an arc source and some trendswith discharge parameters are summarised in[3].

This paper is focussed on a quantification of theamount of cesium in the RF driven ion source byoptical emission spectroscopy. Results of systematicinvestigations in hydrogen and deuterium dischargesare presented.

2. Diagnostic technique and line analysis

The inductively coupled RF discharge (f = 1 MHz,Pmax= 140 kW) operates in the pressure range of0.2–1 Pa. The plasma in the discharge chamber canbe separated into the driver region, where the plasmais generated, and the expansion region. Details ofthe RF discharge are shown in[2]. Typical parame-ters (obtained from optical emission spectroscopy) forhydrogen discharges with 120 kW input power at 0.5 Paa ,de rid.D ions onew thed d (d m-e Them≈ rdedw res-o ,λ lowr ,λ

ces-s onic

states 6p2P3/2, 7p 2P3/2 and 7p2P1/2 at 852.1, 455.5and 459.3 nm, with excitation energies of 1.45, 2.70and 2.72 eV, respectively. Since the ionisation energy(Eion = 3.894 eV) is comparable withTe close tothe grid, emission from cesium ions (CsII, Xe-like,Eion = 25.076 eV) is expected also. Strong lines are,for example, at 522.7 nm (6p[1/2]1–6s[1/2]2 transi-tion) and 460.3 nm (6p[21/2]3–6s[11/2]2 transition)with excitation energies of 15.68 and 16.01 eV,respectively.

From the absolutely calibrated line emission:εpk = n(p)Apk (Apk: transition probability from levelp to level k) the population density of the excitedstaten(p) can be determined. The relation with theground state population, and thus with the particledensity, is obtained from population models. Here,the corona model can be applied where the electronimpact excitation (with rate coefficientX0p(Te))from the ground staten0 is balanced by sponta-neous emission: n0neX0p(Te) = n(p)

∑q<pApq.

Introducing the emission rate coefficientXem

pk (Te) = X0p(Te) × Apk/∑

q<pApq the line radia-tion is given byεpk = n0neX

empk (Te) and depends on

ne, Te and particle density. The basis for a quantitativeanalysis of line radiation is the availability of thecorresponding rate coefficient or cross section. For the6p and 7p levels of CsI, calculated electron impactexcitation cross sections are given in[4]. Assuming aMaxwell energy distribution, the corresponding emis-s -t arem

F s fors

re Te≈ 10 eV andne≈ 5× 1018 m−3 in the driverecreasing toTe≈ 3 eV andne≈ 5× 1017 m−3 in thexpansion region a few cm above the extraction giagnostic flanges with optical windows for emisspectroscopy are available in different locations:ith a line of sight through both the centre ofriver and expansion regions onto the extraction griz-irection). The other lines of sight (LOS, 1 cm in diater) are parallel to the grid (4 cm above the grid).asked extraction grid (74 cm2 area)[2] contributes to40% to the length of the LOS. Spectra are recoith absolutely calibrated spectrometers: a highlution survey spectrometer (�λFWHM = 20–35 pm= 200–750 nm, one spectrum per pulse) and a

esolution survey spectrometer (�λFWHM ≈ 1–1.8 nm= 200–870 nm, 100 ms time resolution).The most intense lines of cesium (CsI) in the ac

ible λ-range are resonance lines of the electr

ion rate coefficients are calculated (Fig. 1). Cross secions for the CsII lines in the visible spectral rangeissing.

ig. 1. Ionisation rate coefficient and emission rate coefficientelected lines of CsI.

U. Fantz et al. / Fusion Engineering and Design 74 (2005) 299–303 301

The evaporation of cesium leads to a Cs coverageof the surfaces where Cs can be chemically erodedand physically sputtered during the discharge. Cs neu-trals penetrate into the plasma and are ionised. Sincethe situation is comparable to the plasma wall inter-action in fusion experiments (hydrogen particles oncarbon surfaces) a similar analysis method is appli-cable for the determination of particle fluxes fromspectroscopic measurements. The basis of this methodis the, so-called, photon efficiency which describesthe destruction events per emitted photon and whichrelates a measured photon fluxΓ ph (LOS perpendicu-lar to the surface) to the particle fluxΓ P [5]. In case ofCs the destruction is described by ionisation, resultingin ΓCs = S(Te)/Xem

pk (Te)Γph, where the ionisation ratecoefficientS(Te) (Fig. 1) is calculated from the ionisa-tion cross section taken from[6].

3. Results

In the standard configuration CsI 455 and CsI459 nm as well asH� and H� are observed with thehigh resolution survey spectrometer using the LOS par-allel to the grid. CsII lines are rarely obtained; here the460 nm line is favoured due to its neighbourhood to

the CsI lines. Time traces of CsI 852,H� andH� arerecorded with the low resolution spectrometer.

Fig. 2(left part) shows a sequence of hydrogen dis-charges at a filling pressure of 0.6 Pa. Variations in inputpower and temperature of the Cs oven are plotted in theupper part. The lower part shows normalised values (to#19776) of the extracted H− and electron current den-sity,jH− andje, respectively, together with the spectro-scopic signals CsI 455 andH�. The first 10 dischargesare without additional Cs evaporation but Cs was evap-orated the day before. All four signals increase withincreasing power. With the beginning of the additionalCs evaporationjH− increases, whereasje decreases dueto an enhanced H− formation by the surface process.CsI 455 increases and seems to correlate withjH− ,whereasH� decreases, and seems to followje. Balmerline radiation depends onne, Te andH density and rep-resents therefore the plasma behaviour. Since line ofsight averaged plasma parameters,neandTe, are knownfrom other spectroscopic techniques the Cs densitycan be deduced. Thus, the Cs density increases from9× 1012 m−3 (#19776) to 1.2× 1013 m−3 (#19788)further to 2.6× 1013 m−3 (#19800). These low densi-ties are unexpected, however it must be kept in mindthey refer to the line averaged Cs amount in the plasmavolume 4 cm above the extraction grid. This also means

F je in a ration( alised t

ig. 2. CsI line (455 nm) and Balmer line (H�) intensities,jH− andleft) and with admixtures of argon (right). All signals are norm

sequence of hydrogen discharges without and with Cs evapoo their value at discharge #19776.

302 U. Fantz et al. / Fusion Engineering and Design 74 (2005) 299–303

that a direct correlation withjH− cannot be expectedeither. This is confirmed by a continuation of 120 kWdischarges where CsI 455 increases further butjH− andje remain almost constant. The line ratio (CsI 455/CsI459) is 4± 0.5 and is therefore in good agreement withthe ratio of the corresponding emission rate coeffi-cients (Fig. 1). Vertically a line ratio (CsI 852/CsI 455)of 300± 50 is measured, whereas the predicted valueis around 40. Systematic errors in the spectroscopicsystems can be excluded since the agreement inH�

is better than a factor of 1.5. Thus, analysis of CsI 852would result in Cs densities around 1014 m−3. All threeCsI lines show a very similar dependence on dischargeparameters.

The right part ofFig. 2 shows a drastic increaseof CsI 455 when argon is added to the hydrogendischarge (without Cs evaporation). This can beattributed to an enhanced sputtering of Cs by argon(due to its higher mass) in comparison to hydrogenparticles; similar, higher Cs signals in pure deuteriumdischarges are seen than in pure hydrogen. With theenhanced CsI signals the CsII lines appear clearly anda line ratio (CsI 455/CsII 460) of 6–10 is observed,depending on discharge parameters. The density ratiohas been estimated without precise knowledge of theCsII emission rate coefficient by taking advantageof the similarity of the electron configuration andelectronic transition of CsII with those of argon (2p1level). At Te = 3 eV the estimated ratio of the emissionrate coefficients (CsI 455/CsII 460) is 300, resultingi ds,t hert uchw

d# tralp nΓ

p nsitya d tob th av ssp reasef

t ts eak

Fig. 3. Time traces of a CsI line (852 nm) and a Balmer line (H�)parallel to the grid for various discharge parameters.

decreaseH� remains constant, which represents sta-ble plasma conditions. CsI 852 increases rapidly whenthe extraction voltage (Uextr = 6.5 kV) is turned on andincreases further. A saturation of the Cs signal with dis-charge time of up to 5 s is not observed as shown in thecentral part ofFig. 3, which reflects also the good repro-ducibility of the discharge (identicalH� signals). Thedependence on input power is plotted in the upper partfor deuterium discharges. The increase ofD� is due toan increase ofne with power. CsI 852 increases as welldue to the higher ion bombardment. A comparison ofa deuterium with a hydrogen discharge at similar con-ditions yields enhanced signals for deuterium, whichcorrespond to a higherne for deuterium.

4. Conclusions

The amount of cesium in RF discharges in hydro-gen and deuterium was determined using emissionspectroscopy as diagnostic tool. CsI lines at 455.5and 852.1 nm are recommended for an analysis. Thecorresponding rate coefficients are presented and

n a density ratio of 0.02–0.033. In other worhe Cs ion density is roughly a factor of 30 highan the Cs neutral density but the intensity is meaker.The analysis of particle fluxes from CsI 852 (inz-

irection) results inΓ Cs= 3× 1018 m−2 s−1 (discharge19780). Using the thermal velocity of the neuarticlesvth = 450 m s−1 (T = 1000 K) and the relatio= n/4× vth a density of 2.5× 1016 m−3 for the Cs

articles starting at the surface is deduced. The det a distance of 4 cm to the surface is determinee two orders of magnitudes lower. Estimations wiery simplified model where ionisation is the only lorocess of Cs particles result the same density dec

or a distance of 9 cm.Time traces of CsI 852 andH� (y-direction) during

he discharge are compiled inFig. 3. The lower parhows the typical temporal behaviour: after a w

U. Fantz et al. / Fusion Engineering and Design 74 (2005) 299–303 303

quantitative analysis methods are introduced. Theneutral density of Cs (typically around some 1014 m−3)4 cm above the extraction grid is rather low, aboutfive orders of magnitudes below the neutral hydrogendensity. It was estimated from CsII emission (460.3 nmline) that roughly 30 times more ions are present thanneutrals 4 cm from the grid surface. The analysis of Csparticle fluxes results in some 1018 m−2 s−1. Measureddensities and fluxes go together when cesium isassumed to be released from the surface by sputteringof plasma particles and penetrates into the plasmawhere it ionises in a few cm. Enhanced sputtering isobserved in deuterium and increases drastically whenargon is added to the discharge. Time traces ofH�

demonstrate stable plasma conditions, whereas CsIlines increase typically during the discharge, showingalso a dependence on the extraction voltage. A directcorrelation of Cs emission with the extracted negativeion current density is not observed since line intensitiesreflect the sputtering of cesium, whereasjH− correlateswith the surface process directly, i.e. the Cs coverage ofthe extraction grid. Here, lines of sight directly abovethe grid should be favoured. In conclusion, the dynam-ics of Cs, which is released from surfaces during the

discharge is better described by particle fluxes than bydensities.

References

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[3] T. Mosishita, M. Kashiwagi, M. Hanada, Y. Okumura, K. Watan-abe, A. Hatayama, M. Ogasawara, Mechanism of negative ionproduction in a cesium seeded ion source, Jpn. J. Appl. Phys. 40(2001) 4709–4714.

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[5] K. Behringer, H.P. Summers, B. Denne, M. Forrest, M.Stamp, Spectroscopic determination of impurity influx fromlocalized surfaces, Plasma Phys. Control. Fusion 31 (1989)2059.

[6] E.J. McGuire, Scaled electron ionisation cross sections in theBorn approximation for atoms with 55≤ Z ≤ 102, Phys. Rev. A20 (1979) 445–455.