CH spectroscopy for carbon chemical erosion analysis in high
density low temperature hydrogen plasma
Citation for published version (APA):
Westerhout, J., Lopes Cardozo, N. J., Rapp, J., & Rooij, van, G. J. (2009). CH spectroscopy for carbon chemical erosion analysis in high density low temperature hydrogen plasma. Applied Physics Letters, 95, 151501-1/3. [151501]. https://doi.org/10.1063/1.3238295
DOI:
10.1063/1.3238295 Document status and date: Published: 01/01/2009
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CH spectroscopy for carbon chemical erosion analysis in high density
low temperature hydrogen plasma
J. Westerhout,1N. J. Lopes Cardozo,1,2J. Rapp,1,3and G. J. van Rooij1,a兲
1FOM Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, Trilateral Euregio Cluster,
P. O. Box 1207, 3430 BE Nieuwegein, The Netherlands
2Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
3Institut für Energieforschung—Plasmaphysik, Forschungszentrum Jülich, Association EURATOM-FZJ,
Trilateral Euregio Cluster, D-52425 Jülich, Germany
共Received 3 August 2009; accepted 8 September 2009; published online 12 October 2009兲 The CH A − X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI 关electron temperature Te= 0.1– 2.5 eV and electron density ne=共0.5–5兲 ⫻1020 m−3兴 with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to ⬎106 due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of ⬃100 for
Te⬍1 eV. The discrepancy is explained as the CH A level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for ne⬎5⫻1020 m−3and 0.1 eV⬍Te⬍1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER. © 2009 American Institute of
Physics. 关doi:10.1063/1.3238295兴
Interaction between the plasma and the material wall is a key issue for the success of the future fusion reactor ITER.1 Carbon can withstand extreme heat loads and is therefore considered for ITER as wall material in the areas of strongest particle and power loads. A serious problem of carbon is that chemical processes induce erosion of the wall even at low incident particle energies. The immediate consequence is the compromise of the life time of the plasma facing component.2A second order effect is that the eroded material will be deposited elsewhere as hydrogen rich amorphous lay-ers and as such form a fuel retention problem.3Finally, dis-integration of these layers, for example by the impact of so-called ELMs 共quasiperiodic burst of power and particles reaching the material wall兲,4
contributes to dust formation and as such to an explosion risk.5
Spectroscopy on the molecular CH A − X Gerö band makes it possible to quantify in situ the chemical erosion of carbon wall elements in contact with hydrogen plasma. CH is the only hydrocarbon that is accessible by emission spec-troscopy in the visible. CH specspec-troscopy relies on the corre-lation between CH radiation and methane particle fluxes,6the main reaction product formed upon chemical erosion of carbon.7The method is widely applied in fusion experiments and provides insight that is presently used to make predic-tions for ITER plasma wall issues. Also for ITER it would be an obvious diagnostic. This requires, however, that it has to be applied in the extreme and unexplored plasma regime of densities⬎1020 m3and temperatures 1–10 eV. Due to steep gradients in the rate coefficients that govern the relation be-tween the CH radiation and the chemical erosion, it is gen-erally regarded as impossible to apply the existing method-ology to plasma temperatures below⬃3 eV.
In this letter, we demonstrate that the interpretation of the spectroscopic data has to be revised for these high den-sity low temperature plasmas on the basis of experiments in
the linear plasma generator Pilot-PSI. We start with a brief analysis of D/XB values 共where D stands for the dissociation rate of the molecule and XB for the excitation rate weighted with the branching ratio, i.e., the inverse photon efficiency that relates photon fluxes to particle fluxes兲 calculated for the above mentioned plasma conditions, which indeed exhibit a steep gradient. Subsequently, experiments on methane seed-ing into the hydrogen plasma of Pilot-PSI are presented, which show still a significant amount of CH A − X light at temperatures of ⬃1 eV. Finally, the difference between the calculated D/XB and measured inverse effective photon effi-ciency is explained on basis of the chemistry underlying the formation of the CH radical.
The onlineHYDKINsolver,8a reaction kinetic solver for the catabolism of hydrocarbons in hydrogen plasma, is used to calculate D/XB values for methane in the plasma condi-tions in Pilot-PSI using the Janev–Reiter database.9The re-sults are plotted in Fig. 1 as a function of Te for ne= 1.0
a兲Electronic mail: rooij@rijnhuizen.nl.
0.1 1 10 100 100 101 102 103 104 105 106 Inver se ph oton effi ci ency
Electron temperature (eV) Pilot-PSI HYDKIN
FIG. 1. D/XB values calculated withHYDKINand the inverse of the effective photon efficiencies measured in Pilot-PSI by relating the CH A − X emission to the methane flux injected into hydrogen plasma. The D/XB shows a steep decrease over orders of magnitude over the range 0.1 eV⬍Te⬍1 eV. The
measured inverse effective photon efficiencies are constant within the error bars in the same temperature range.
APPLIED PHYSICS LETTERS 95, 151501共2009兲
0003-6951/2009/95共15兲/151501/3/$25.00 95, 151501-1 © 2009 American Institute of Physics
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⫻1020 m−3. It shows that indeed the D/XB value varies strongly with temperature, in accordance with the general view that CH A − X spectroscopy becomes difficult to quan-tify at low Te. Evaluation of the underlaying reaction rates gives insight in which processes are important for the ob-served behavior. At low Te, in particular⬍2 eV, the chem-istry simplifies greatly as all electron-neutral processes be-come negligible compared with charge exchange reactions followed by dissociative recombination.9The rate determin-ing step is the charge exchange reaction, which varies much less than an order of magnitude over the temperature range 0.1 eV⬍Te⬍2 eV. The reason that the D/XB does increase steeply toward lower Telies solely in the electron excitation rate of CH, which decreases orders of magnitude.
The behavior of the CH A − X emission as a function of the low temperature high density plasma conditions is ex-perimentally investigated by seeding the hydrogen plasma of the linear plasma generator Pilot-PSI with methane and re-lating the absolute CH A − X emission to the methane flux. A description of the experimental details of Pilot-PSI can be found elsewhere.10 The aspects relevant for the measure-ments presented here are given in Fig.2and caption. The CH photon flux is determined by integrating the CH A − X band from 430.0 to 431.5 nm, multiplied by a factor of 2.8 to obtain the photon flux of the full CH A − X band.13 Re-erosion of carbon deposits on particularly the target also con-tributes to CH A − X emission. This contribution of up to 25% of the total emission is characterized before and after each methane seeding experiment and is corrected for. Figure 3 shows the resulting two-dimensional photon flux profile for the perpendicular view. The peak intensity is located at the injection location and decays exponentially. The axial
e-folding length, illustrated by the upper inset in the plot, is
2 mm. The radial decay length is similar as seen in the left side inset, with a half 1/e width of 1 mm. We note that these numbers indicate a limited spatial resolution of the
spectros-copy measurements. A volume with a diameter of 7.2 mm has to be integrate in order to cover 67% of the total emis-sion. This means that the Te dependencies discussed below concern in fact an average over the Teprofiles, which have a FWHM of typically 12 mm.
Comparison of this perpendicularly measured photon flux profile with one measured in the tangential view 共data not shown兲 learns that up to 5% of the emission is emitted inside the seeding hole. All following analysis has been cor-rected for this effect. Furthermore, similar measurements in a scan of the CH4 seeding flow rate scan from 0.3 to 1.2 SCCM 共SCCM denotes standard cubic centimeter per minute兲 show a perfect linear response of the total emission. This proves that the seeding is small compared to the plasma flux densities and has no effect on the local plasma condi-tions that are relevant for the formation of CH A − X light.
The CH A − X photon flux profiles have been measured in a scan of Tefrom 0.1 to 2.5 eV. Integration over the entire profiles and dividing by the injected methane particle flux gives the effective photon efficiency. The inverse of this quantity is compared with the calculated D/XB values in Fig. 1. The experiments show within the error bars a constant inverse effective photon efficiency of ⬃100 over the range 0.1–1 eV, which is in contrast to the steeply in-creasing calculated D/XB toward lower temperatures. This is explained by taking the chemistry underlying the formation of the CH radical into account. As all electron-neutral pro-cesses become negligible compared to the charge exchange processes,9the main reactions of interest for the production of CH are as follows: H++ CH4→ H + CH4 + , H++ CH4→ H2+ CH3 + .
Both processes are equally important around 0.1 eV, whereas the formation of CH4+is dominant 共⬎85%兲 at 1 eV.8 These molecular ions undergo dissociative recombination, which occurs an order of magnitude faster than the charge exchange step. The products from the dissociative recombination cess are most probably excited due to the nature of the pro-cess, i.e., electron capture to a doubly excited repulsive state of the CHy 共y=0–3兲 molecule.
9
Two of the many possible pathways have sufficient excess energy from the exothermic
Tangential view:
CH spectroscopy
Cu target
Perpendicular view:
Thomson scattering or CH spectroscopy
Window Mirror 30 ° 0.2 m 0.56 m Cascaded arc CH4 seeding
FIG. 2. 共Color online兲 Schematic of the CH4seeding experiments in
Pilot-PSI. Hydrogen plasma that exits the cascaded arc plasma source共Ref.11兲 at sonic speed is confined by 0.4 T magnetic field into a⬃1 cm beam. Thom-son scattering measures radial profiles of neand Teat 18 mm in front of
the target 共Ref. 12兲 Typical conditions for the present experiments: 1 ⫻1020 m−3⬍n
e⬍1⫻1021 m−3, 0.1 eV⬍Te⬍2.5 eV, and 12 mm full 1/e
profile widths. CH4is injected into the center of the plasma column through
the 1 mm diameter channel in the passively cooled copper target at a rate of 1017– 1018 CH
4/s. The temperature of the injection channel is set by the
plasma power between 100 and 500 ° C共estimated from calorimetry on the target cooling water兲. The CH A−X molecular band head at 431 nm is measured along a perpendicular and a tangential view.
8 6 4 2 0 -2 -4 -6 -4 -2 0 2 4 6 CH4
Distance from target (mm)
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Intensity Y = 0 mm Intensity Z = 0.5 mm 1 / e
FIG. 3. 共Color online兲 Emission profile of the CH A−X band measured in the perpendicular view near the Pilot-PSI upon seeding hydrogen plasma 共ne= 1.0⫻1020 m−3, Te= 1 eV兲 with methane at a flow rate of 5.3
⫻1017 CH
4/s. The insets illustrate the penetration depth of the excited CH
into the plasma: ⬃2 mm e-folding length in axial and ⬃1 mm in radial direction.
151501-2 Westerhout et al. Appl. Phys. Lett. 95, 151501共2009兲
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reaction to lead directly to the formation of excited CH A on basis of the excess of energy,9
e + CH4+→ 25% CH + H2+ H + 3.42 eV, e + CH3+→ 14% CH + H2+ 5.1 eV.
The excess energy in these reactions is sufficient to excite the 431 nm radiation of the CH A − X transition. Considering the pathways given above, it requires 5% of the dissociative re-combination events to lead to CH A − X radiation in the Gerö band to explain the measured inverse effective photon effi-ciency of 100. It is noted that also second共and further兲 order charge exchange reactions followed by dissociative recombi-nation have been taken into account for this estimate and contribute up to ⬃30%.
The length scales of the emission plume are also in agreement with the above explanation. The charge exchange rate for H++ CH4 is according to the onlineHYDKIN solver8 equal to 8⫻10−15 m3/s at T
e= 1 eV. For ne= 1⫻1020 m−3 and a velocity of 103 m/s for the hydrocarbon, this gives a mean free path of 1.3 mm, close to the 2 mm e-folding length of the plume in axial direction in Fig.3.
The graph of Fig.1shows also an experiment performed at a plasma temperature of 2.5 eV. Electron excitation of the Gerö band is expected to be dominant at Te= 2.5 eV, so that the measured inverse effective photon efficiency should compare to the calculated D/XB value. However, it is seen in Fig.1that the inverse effective photon efficiency has become even higher, more than 103, instead of dropping to the D/XB value of⬃100. This is due to collisional de-excitation of the CH A level. A quick estimate confirms the effectiveness of collisional de-excitation. The main processes are charge or particle exchange of CH 关total rate is 1.1⫻10−15 m3/s at
Te= 2.5 eV 共Ref.8兲兴 and dissociative excitation of CH into neutrals 关total rate is 1.6⫻10−15 m3/s at T
e= 2.5 eV 共Ref. 8兲兴. The sum of these rates gives at the plasma density of the particular experiment共3⫻1020 m−3兲 a collisional lifetime of 1.2 s, which is close to the radiative lifetime of the CH A level.14 It is noted that these estimates do not take the exci-tation energy of the CH A level into account. Most likely, the dissociative excitation rate has therefore been underestimated and also direct ionization should have been taken into ac-count. Both would have decreased the collisional lifetime even further, which emphasizes the importance of collisional quenching. Thermal decomposition of the methane is not ex-pected to be important at the estimated injection channel temperatures of below 500 ° C. Otherwise, methane could have decomposed into atomic carbon inside the channel, which would also have increased the measured inverse pho-ton efficiency.
Measurements of the effective inverse photon efficiency in scans of ne show that collisional de-excitation becomes important at ne⬎5⫻1020 m−3 in the temperature range Te ⱕ1 eV, i.e., a threshold at higher density compared to the
Te= 2.5 eV case. The effective photon efficiency data in Fig. 1 for Teⱕ1 eV do not contain this effect.
In conclusion, the experiments at Pilot-PSI demonstrate that the interpretation of CH spectroscopy has to be revised for low temperature, high density plasma conditions as will appear in ITER. First, the inverse effective photon efficiency is measured to be ⬃100 for 0.1 eV⬍Te⬍1 eV, indepen-dent of ne for ne⬍5⫻1020 m−3. The constancy of the effective inverse photon efficiency is explained by popula-tion of the CH A level via charge exchange of higher hydro-carbons promptly followed by dissociative recombination. Second, collisional de-excitation increases the value of the inverse effective photon efficiency for densities of ne⬎5 ⫻1020 m−3. One example at T
e= 2.5 eV indicates that this boundary shifts to lower densities for Te⬎1 eV.
The authors acknowledge the assistance of R.S. Al and B. de Groot, H.J. van der Meiden, M.J. van de Pol, A.E. Shumack, W.A.J. Vijvers, and G.M. Wright with the experi-ments and the useful discussions with S. Brezinsek, D. Boro-din, A. Kirschner, V. Philipps, and D.C. Schram. This work was supported by the European Communities under the con-tract of Association between EURATOM/FOM and carried out within the framework of the European Fusion Pro-gramme with financial support from NWO and the NWO Grant No. RFBR 047.018.002. The views and opinions ex-pressed herein do not necessarily reflect those of the Euro-pean Commission.
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