Interaction of CLAM Steel with Plasma in HT-7 Tokamak During High Parameter Operation

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Plasma Science and Technology, Vol.9, No.4, Aug. 2007

Interaction of CLAM Steel with Plasma in HT-7 Tokamak DuringHigh Parameter Operation∗

LI Chunjing(���)1, HUANG Qunying(���)1, FENG Yan(��)1,LI Jiangang(���)1, KONG Mingguang(���)2

1 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China2 Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

Abstract A Plasma Surface Interaction (PSI) experiment on China Low Activation Marten-

sitic (CLAM) steel was done to check if CLAM steel could be used as a Plasma Facing Material

(PFM). A specimen with a diameter of 45 mm was exposed to 897 shots of deuterium plasmas

with a total duration of 712 sec at a minor radius of 30 cm in HT-7 tokamak. During the expo-

sure experiment, no observable influence was found on plasma performance. After exposure, the

surface of the specimen seemed as smooth as before but with some colour change at the margin

of the specimen. Even though some micro-damage, such as dense blisters, melting, splashing,

depositions, and dust, was found on local surfaces with Scanning Electron Microscopic (SEM)

observation. The reflectivity of the specimen decreased only slightly. All of these shows CLAM

steel has good stability and irradiation resistance. With further optimization, it could possibly be

used as the first mirror material for plasma diagnostics in tokamaks.

Keywords: CLAM, PSI, PFM

PACS: 52.40.Hf�68.37.-d

1 Introduction

The study of fusion materials, especially the plasmafacing materials (PFMs), is one of the key issues re-lated to the realization of fusion energy and has beencarried out extensively over the world [1]. Low Z ma-terials, e.g., graphite, carbon-fibre reinforced carbon(CFC), etc. are widely used in current tokamaks asPFMs. Their lifetime is limited by a high sputteringyield of carbon. In addition, carbon based materialsshould be used as less as possible in fusion reactors dueto its high tritium retention rate. High Z materials areattractive as PFMs for their high sputtering thresh-old, but the their tolerable concentration in plasma israther low. However, the use of tungsten coating inthe ASDEX-Upgrade tokamak didn’t spoil the plasmaand the concentration of tungsten in the core plasmawas very low and even hardly detected. This indicatesthat the concentration of PFMs in plasma is not onlyrelated to their erosion yield but also to their trans-port behaviour in the plasma [2]. However, tungsten isvery difficult to manufacture due to its brittleness underroom temperature. If the structural material in a fusionreactor can be used as PFM directly, the cost and thedifficulty in fabrication for the reactor will be reducedgreatly. The Reduced Activation Ferritic/Martensiticsteels (RAFMs) are being widely studied and devel-oped in the world. They are considered as the primarycandidate of structural materials for the DEMO reactorand the first fusion power plant [3∼5]. Much work hasbeen done on the direct use of RAFMs as PFMs in

Fig.1 Position of the specimen in HT-7 device

tokamaks [6,7] in order to study their behaviour underplasma irradiation and the possibility to be used asPFMs.

The Chinese Low Activation Martensitic (CLAM)steel[8] is a kind of RAFMs and developed by Insti-tute of Plasma Physics (ASIPP), Chinese Academy ofSciences under wide cooperation with other institutesand Universities in China. It has been chosen as thestructural material of the International ThermonuclearExperimental Reactor (ITER) liquid LiPb Test Blan-ket Module of China [9∼11]. A preliminary experimenton the interaction of CLAM steel with plasma was car-ried out in HT-7 tokamak during its operation in 2005.The major radius R and minor radius a of the toka-mak are 122 cm and 27 cm, respectively, as shown inFig. 1. The specimen of CLAM steel (HEAT 0408B)was exposed to deuterium plasma for 897 shots and a

∗supported by the National Natural Science Foundation of China (No. 10375067) and the Knowledge Innovation Program of ChineseAcademy of Sciences

LI Chunjing et al.: Interaction of CLAM Steel with Plasma in HT-7 Tokamak during High Parameter Operation

Table 1. Compositions of CLAM steel (HEAT 0408B)

Element Cr W V Ta Mn C Si S P Fe

wt% 8.91 1.44 0.20 0.15 0.49 0.12 0.13 0.0034 0.0042 Bal.

Table 2. Main parameters during plasma exposure

Item Average value Minimum value Maximum value

Vertical field coils amperage (A) 4006.27 3703.33 4269.09IP (kA) 130.41 52.15 227.01Discharge length (sec) 0.85 0.13 1.95Electron Density (1013/cm3) 4.43 0.09 81.79LHCD injection power (kW) 36.00 0 211.12LHCD pulse length (ms) 168.25 0 1774.94

total irradiation time of 712 sec during the high param-eter operation of HT-7. The preliminary results showedthat CLAM steel did not disturb the discharges of HT-7and its corrosion rate was moderate.

2 Experimental setup

The specimen was taken from one 25 kg ingot(HEAT 0408B) of CLAM steel smelted in a vacuuminduction furnace and its composition is listed in Ta-ble 1. CLAM steel was austenitized for 30 minutes at980 oC and cooled in air to room temperature, and thentempered at 760 oC for 90 minutes and again cooled inair to room temperature. The specimen with a size ofφ 45 mm×5 mm was machined. All its surfaces wereground and its plasma facing surface was mirror pol-ished.

The specimen was washed in ultrasonic alcohol bathfor 5 minutes and then baked at 50 oC for 3 hours in air.A magnet transfer device was used to place the speci-men into the HT-7 device through a horizontal port tothe site at a distance of 30 cm from the plasma centre asshown in Fig. 1. The guard limiter was located 28.2 cmfrom the plasma centre and the specimen was set at1.5 cm behind the guard limiter in the radial directionand 265 mm in the toroidal direction along the direc-tion of ion movement. The statistical parameters of theplasma during the interaction of CLAM steel with theplasma are listed in Table 2. The surface inspections bySEM, optical microscopy and Energy Dispersive X-raySpectroscopy (EDS) were performed before and afterthe plasma exposure. The weight and reflectivity ofthe specimen were measured and analysed as well.

3 Results and discussion

3.1 Mass loss due to interaction and ir-radiation

The weight of the specimen was 56.5587 g beforeplasma irradiation. The mass loss was 0.0014 g for the

Fig.2 Macroscopic photo of the specimen

irradiation. According to the X-ray diffraction results,the crystal structure of CLAM steel was body centralcubic and its lattice parameter was 2.866A. Assum-ing that the erosion only occurred on the plasma-facingsurface and there was no deposition, the reduced thick-ness of the sample would be 0.1 μm, which means about400 layers of the lattice of the CLAM steel sample weretaken away due to the interaction of the sample withplasma.

3.2 Surface macro-inspection

Before the experiment the specimen surface was verysmooth except for some tiny scratches as a result of thepreparation of the specimen. After the exposure, thespecimen surface was still very smooth and shining ex-cept some colour changes in small areas at both ionside and electron side as shown in Fig. 2. The colourchanges might originate from constructive and destruc-tive light interference in the deposition of the carbonbased film as mentioned in Ref. [12].

3.3 Observation of sample surface byoptical microscopy and SEM

The surface of the specimen was inspected by SEMbefore and after plasma irradiation and results areshown in Fig. 3(a) and Fig. 3(b)∼(f), respectively.

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Plasma Science and Technology, Vol.9, No.4, Aug. 2007

(a) As-machined surface (b) Dust due to exposure (c) Elec-

tron side after exposure (d) Middle surface after exposure

(e) Electron side after exposure (f) Ion side after exposure

Fig.3 SEM micrographs before and after plasma exposure

Fig. 3(a) shows the surface of the specimen beforeplasma irradiation and it was smooth and without anydust on it. Some dust was found on the specimen un-der SEM after irradiation, as shown in Fig. 3(b). A fewmelting and splashing spots were found on local surface(Fig. 3(c), (d)) and their sizes generally ranged from∼1 μm to ∼10 μm. These defects probably formed dur-ing unusual plasma transport. A very tense meltingand splashing spot with a size of ∼20 μm occurred nearthe hole for assembly at the electron side as shown inFig. 3(e). It might result from a relatively large plasmadisruption. A liquid layer formed and the surface tem-perature of the liquid layer probably exceeded the equi-librium vaporization temperature. This overheating re-sulted in the growth and explosion or vaporization ofvolumetric bubbles in turn, leading to the ejection andloss of parts of the melted layer [13].

Blisters on the specimen surface occurred on the ionside. The bubble density was very high and the regionsranged from ∼0.1 μm to several μm in size, as shown inFig. 3(f). At the middle of the specimen surface, therewere also some blisters but with a much lower density,as shown in Fig. 3(d); while on the electron side, al-most no blister could be found. A possible reason forthe difference in the blister density was that the CLAMsteel specimen changed the magnetic field distributionnearby, which leads to a more intense deposition of theH+ and D+ ions at the sample surface at the ion side.

Fig.4 EDS results

3.4 Composition of dust and the samplesurface

The composition of two pieces of dust and threepoints at the specimen surface without any defects afterthe plasma exposure were analysed by EDS as shown inFig. 4. The composition of the dust is close to that ofCLAM steel, especially the ratio of concentration of Feto Cr. So the dust is thought to be from the sputteringand redeposition of the specimen surface material dueto the interaction.

Concentrations of carbon and silicon on the samplesurface are much higher than those of the sample bulk.Because the limiters are made of doped graphite withSiC coating, the carbon and silicon existing on the sam-ple surface are considered to be from the sputtering andredeposition of SiC coating on the limiters. In addition,a high concentration of oxygen was found on the surfaceat the electron side, which might result from the oxida-tion of CLAM steel under high temperature during theplasma exposure.

3.5 Possible application

About 50 percent of all the methods for plasma diag-nostics in ITER will be based on the analysis of charac-teristics of electromagnetic radiation in different partsof the spectrum. Due to intensive irradiation from theneutron and gamma rays, all components of the opticalschemes nearest the burning plasma must be metallicmirrors because, in contrast to characteristics of the re-fractive optical components (such as lens and prisms),the optical properties of metals are not significantly af-fected by any of these deeply-penetrating radiations. Ineach optical scheme one mirror has to be the plasma fac-ing component, and this mirror (first mirror, FM) willbe subjected to much more intensive plasma irradiation.The selection of the FM materials and research on thebehaviour of FM under plasma irradiation are currentlyunderway in the world. As mentioned in Ref. [14], thestainless steel 04Cr16Ni11Mo3Ti has a good capacitycompared to refractory metals for maintaining a smoothsurface for long term sputtering probably due to its finegrain structure, while CLAM steel also has a fine grainstructure with a grain size of ASTM 11 and is expected

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LI Chunjing et al.: Interaction of CLAM Steel with Plasma in HT-7 Tokamak during High Parameter Operation

Fig.5 Specular reflectivity

to have a good capacity of maintaining a smooth sur-face and keeping its specular reflectivity. In addition,the CLAM steel is a kind of low activation materialunder neutron irradiation and can be dealt with moreeasily.

Specular reflectivity of the CLAM steel sample in ul-traviolet and visible spectral regions was measured be-fore and after exposure in HT-7 and results are shownin Fig. 5. After interaction with the high parameterplasma in HT-7 for 712 sec, the degradation of the re-flectivity was pronounced at the nearly ultraviolet re-gion and the specular reflectivity at the wavelength of200 nm was reduced by about 56% after irradiation.While the reduction in the visible spectral region wasrelatively low and it was about 6.3% degradation at thewavelength of 700 nm. The mechanism of the reflectiv-ity change needs further investigation. The relativelylow specular reflectivity of the CLAM steel sample atthe nearly ultraviolet region is expected to increase af-ter special cleaning of the sample surface. It mightbe hopefully suitable to be used as the first mirror forplasma diagnostics in the near future.

4 Conclusions

From the interaction experiment of CLAM steel withhigh parameter plasma in HT-7 for 897 shots and a to-tal shot time of 712 sec, some phenomena were observedand preliminary analysis was derived as follows:

a. No noticeable effect of the sample on plasma per-formance or configuration was observed during the ex-periment. The magnetization of the sample did notdisturb the discharge of HT-7.

b. Traces of melting and blisters were observed onthe surface under SEM, which accelerated the corrosionof the specimen. But the mass loss of the specimen wasmoderate.

c. There were deposition of C, Si, etc. and dust onthe specimen surface after the plasma exposure, whichprobably originated from the graphite tiles with SiC

coating over the limiters, and redeposition of the ero-sion yield of the specimen itself.

d. The specular reflectivity was slightly decreasedafter the plasma exposure, which was probably due tothe deposition of carbon, silicon, etc. on the samplesurface. It might be suitable to use CLAM as the firstmirror material for plasma diagnostics in tokamak withmore careful preparation of the surface.

Further studies will be carried out to check the effectof the magnetization of CLAM steel on plasma confine-ment and the possibility of using CLAM steel as PFMand FM materials.

Acknowledgements

The authors thank Prof. YU Jinnan from China In-stitute of Atomic Energy for helpful discussion.

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(Manuscript received 15 September 2006)E-mail address of LI Chunjing: lcj@ipp.ac.cn

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