⇦Chinese Physics Letters, 2016, Vol. 33, No. 6, Article code 065205⇨ Observation of Molybdenum Emission from Impurity-Induced Long-Lived $m=1$ Mode on the Experimental Advanced Superconducting Tokamak * Yong-Cai Shen(沈永才)1,2, Bo Lyu(吕波)1,3**, Fu-Di Wang(王福地)1, Yue-Jiang Shi(石跃江)4,5, Bin Wu(吴斌)1, Ying-Ying Li(李颖颖)1, Jia Fu(符佳)1, Bao-Nian Wan(万宝年)1, the EAST team Affiliations 1Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031 2School of Physics and Electronic Engineering, Anqing Normal University, Anqing 246011 3Hefei Science Center, Chinese Academy of Sciences, Hefei 230031 4School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026 5Department of Nuclear Engineering, Seoul National University, Seoul 151-742, Korea Received 8 March 2016 ^*Supported by the National Magnetic Confinement Fusion Science Program of China under Grant Nos 2013GB112004 and 2015GB103002, the Natural Science Research Key Project of Education Department of Anhui Province under Grant No KJ2016A434, the Doctoral Scientific Research Foundation of Anqing Normal University under Grant No 044-140001000024, the National Natural Science Foundation of China under Grant Nos 11275231, 11305212, 11405212 and 11261140328, the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology under Grant No 2014FXCX003, and the Hefei Science Center CAS Users with Potential Project under Grant No 2015HSC-UP007.
^**Corresponding author. Email: blu@ipp.ac.cn Citation Text: Shen Y C, Lyu B, Wang F D, Shi Y J and Wu B et al 2016 Chin. Phys. Lett. 33 065205 Abstract We observe the spectra of molybdenum for the first time since the first wall of our experimental advanced superconducting tokamak (EAST) was changed mainly to molybdenum tiles. A large amount of molybdenum accumulated in the central plasma where the long-lived $m=1$ mode instability bursts is shown. Molybdenum is proved to be the main impurity species observed during the formation and lifetime of impurity-induced long-lived $m=1$ mode. This may indicate that a close relationship exists between the high-$Z$ impurity accumulation and the occurrence of long-lived $m=1$ mode in EAST plasmas. DOI:10.1088/0256-307X/33/6/065205 PACS:52.25.Vy, 52.55.Fa, 29.30.Kv © 2016 Chinese Physics Society Article Text Impurity behavior plays a key role in magnetic confined fusion research as impurity radiation can break the power balance, cool down the plasma and degrade the wall materials. Spectroscopy in the soft x-ray and extreme ultraviolet range supply a useful tool to survey core impurity radiation from high-temperature magnetic confined fusion plasmas.[1-3] To measure line emissions from these ranges, a soft x-ray spectrometer and an extreme ultraviolet spectrometer which form an XEUV spectrometer system with certain time-resolution and spatial-resolution have been established on the EAST. Line emissions from both low-$Z$ impurities such as C and O and medium-impurities such as Fe and Mo can also be observed by the system.[4] Spectra from highly-ionized molybdenum in the wavelength range of 5–13 nm were first observed in EAST plasmas during recent experiments. Magnetohydrodynamic (MHD) instabilities have been proved to be important in core plasmas as they can degrade the confinement performance. The $m=1$ mode was the most complicated mode. It has been broadly observed on many magnetic confined fusion devices[5-10] since the long-lived $m=1$ mode was first found.[11] An $m/n=1/1$ ideal kink mode locking was also observed on EAST plasmas for the first time.[12] In the Alcator C-Mod tokamak, observation of $m=1$ snakes was related to accumulation of heavy ions,[13] which can produce high radiated power loss. The basic principle is as follows: impurity is contained in the island, and the increasing radiation of impurities cools down the island's interior. As a result, the local resistivity increases, the helical current perturbation increases and then the island grows.[14] The EAST is the first non-circular fully superconducting tokamak in the world with major $R \sim1.85$ m, minor radius $a=0.45$ m, and an elongation ratio $\kappa < 1.9$.[15] Lower hybrid current drive (LHCD) and ion cyclotron range of frequency (ICRF) systems have been equipped on the EAST since 2008. During the experiment in 2012, lower-hybrid waves (LHWs) were injected by a 20-waveguide launcher capable of delivering up to 2 MW of power at 2.45 GHz, and four 1.5 MW ICRF transmitters with a range of 25–70 MHz were put into operation. The main materials of plasma facing surfaces (PFM) are carbon on divertor and molybdenum on the first wall, which makes carbon and molybdenum the main impurities on the EAST. Carbon line emission can be easily observed while molybdenum spectra are hardly seen without using LHCD and ICRF heating. Molybdenum spectra and line emission profiles were observed by XEUV spectrometers and the 1/1 mode is analyzed by soft x-ray signals measured by three soft x-ray cameras.[13] An extreme ultraviolet spectrometer mainly consists of three parts: an entrance slit, a varied-lines-space (VLS) spherically concaved holographic groove grating and a back-illuminated full frame CCD.[16,17] The XEUV system is installed at port C, attached to the end of the pumping duct, as shown in Fig. 1. The upper part is the soft x-ray spectrometer and the lower one is the extreme ultraviolet spectrometer. As can be seen from Fig. 1(b), the distance between the plasma center and the entrance slit is 8335 mm and the distance between entrance slit and the CCD is 472 mm. The current setup is able to cover 0–450 mm above the equatorial plane. Spatial resolution is illustrated for the pinhole imaging scheme and is determined by the height of entrance slit and the CCD's pixels binned for one channel.[18]

Fig. 1. Schematic drawings of space-resolved XEUV system's installation in the EAST (a) in side view and (b) principle of spatial resolution in vertical view.

Fig. 2. Typical Mo spectrum in the wavelength range of 5–13 nm.

Mo spectral lines have been observed since the first wall is made up of molybdenum. Figure 2 shows the typical Mo spectrum observed in the H-mode discharge from shot 41931 by XEUV covering 5–13 nm. The spectroscopy mainly consists of the charge state of Mo XXIV–XXXII. Mo XXX to Mo XXXII in 10–13 nm is easy to identify while the blow-up spectrum of 6.5–8.5 nm is not easy to distinguish and the research in detail can be found in another device.[19] The strongest line that can be identified is Mo XXXI at 11.59 nm and the ionization energy is 1726 eV. It has been found that the main molybdenum ionization charge states for a core temperature of interest ($T_{\rm e,0}\sim$1–3 keV) are Mo$^{31+}$, Mo$^{32+}$, and Mo$^{33+}$, respectively.

Fig. 3. The temporal evolution of shot 41931: (a) plasma current, (b) soft x-ray radiation intensity, (c) Mo XXXI intensity, and (d) C VI intensity.

Fig. 4. SXR brightness time history profiles of the molybdenum long-lived $m=1$ mode.

Figure 3 shows the temporal waveforms of shot41931 in which LHW and ICRF are injected. Plasma current stays at 500 kA, and the central electron density is about 5.8–6 (10$^{19}$ m$^{-3}$). MHD instability begins at $t=5.75$ s and ends at 5.95 s. The long-lived $m=1$ mode happens during $t=5.8$ s and 6.0 s. Figure 4 shows the time history of SXR profiles of the molybdenum long-lived $m=1$ mode and Fig. 5 gives the reconstructed 1/1 mode at $t=5.78$ s using the tomography of SXR signals. Intensity of Mo XXXII (11.59 nm) increases to a high level before the snake from $t=5.4$ s to 5.7 s and decreases during the snake from $t=5.7$ s to 5.9 s. From Fig. 3(d) the intensity of C VI at 3.373 nm observed by the XEUV system decreases from 5.5 s to 5.7 s and then keeps constant, which means that carbon plays a negligible role in inducing the long-lived $m=1$ mode. The intensity of line integrated Mo XXXI brightness profile obtained at different times is shown in Fig. 6. The core intensity of Mo XXXII increases from $t=5.5$ s to $t=5.7$ s, which means the peaking of molybdenum density before the snake formation. Then the core profile becomes very flat at $t=5.9$ s during the snake. A slightly peaking core profile is obtained at $t=6.1$ s after the snake.

Fig. 5. Reconstructed 1/1 mode at $t=5.78$ s using tomography of SXR signals.

Fig. 6. The intensity of line integrated Mo XXXI brightness profile obtained at different times.

The main contribution of the SXR signal is line emission and the fraction is about 90%. As for the relatively high electron temperature in core plasma ($\sim$1 keV), the main line radiation must be from high-$Z$ impurities, particularly molybdenum. An assessment of radiated power density of molybdenum before the snake formation (at $t=5.7$ s) was carried out. If a molybdenum cooling factor of $L_{\rm Mo}\sim4\times 10^{-32}$ W/m$^{3}$ for the core temperature and the core electron density of $4\times 10^{19}$ m$^{-3}$ is considered, the molybdenum density and its concentration can be as high as $4\times 10^{16}$ m$^{-3}$ and 0.1%, respectively. The core molybdenum radiated power density will be 64 kW/m$^{3}$. As mentioned above, such a long-lived $m=1$ mode is produced by high impurity radiation. It can be assumed that the molybdenum charge states in the time range of 5.5–6.0 s play the most important role in core emission, and the small region of localized and enhanced molybdenum density is the main reason for the long-lived $m=1$ mode's formation. During the snake time, molybdenum ions are being flushed from the core by transport. As the time resolution of the XEUV system is limited, the more accurate coincidence between Mo XXXII and soft x-ray signal is hard to obtain and the more precise work during the transit of the snake is still to be studied. In summary, the XEUV system which includes a soft x-ray spectrometer and an extreme ultraviolet spectrometer has been developed on the EAST tokamak for impurity behavior research. Mo spectrum has been first observed since the changing of the first wall from carbon to molybdenum. Time history of Mo XXXII in the central channel and its profiles at different times prove that high-$Z$ ions in core plasma play an important role in the formation of the long-lived $m=1$ mode. The observation and its interpretation may illuminate ways for the research of long-lived $m=1$ mode's formation in tokamak plasmas. References EUV spectroscopy on NSTXSpectroscopic comparison between 1200 grooves∕mm ruled and holographic gratings of a flat-field spectrometer and its absolute sensitivity calibration using bremsstrahlung continuumDensity and impurity profile behaviours in HL-2A tokamak with different gas fuelling methodsImpurity Emission Behavior in the Soft X-Ray and Extreme Ultraviolet Range on EASTSnake-like density perturbations in JETSnake-like phenomena in Tore Supra following pellet injectionMagnetic island structures and their rotation after pellet injection in FTURecent experiments on Alfvén eigenmodes in MASTSaturated ideal modes in advanced tokamak regimes in MASTSnake perturbations during pellet injection and LHCD in the HL-1M tokamakPoloidal distribution of impurities in a rotating tokamak plasmaObservation of 1/1 impurity-related ideal internal kink mode locking in the EAST tokamakMolybdenum emission from impurity-induced m = 1 snake-modes on the Alcator C-Mod tokamakOrigin of Tokamak Density Limit ScalingsOverview of steady state operation of HT-7 and present status of the HT-7U projectSpace-resolved extreme ultraviolet spectrometer system for impurity behavior research on experimental advanced superconducting TokamakSpatially-resolved flat-field soft X-ray spectrometer on experimental advanced superconducting tokamakSpace-resolved extreme ultraviolet spectrometer for impurity emission profile measurement in Large Helical DeviceLine analysis of EUV Spectra from Molybdenum and Tungsten Injected with Impurity Pellets in LHD

[1]	Lepson J K, Beiersdorfer P, Clementson J, Gu M F, Bitter M, Roquemore L, Kaita R, Cox P G and Safronova A S 2010 J. Phys. B: At. Mol. Opt. Phys. 43 144018
[2]	Chowdhuri M B, Morita S, Goto M, Nishimura H, Nagai K and Fujioka S 2007 Rev. Sci. Instrum. 78 023501
[3]	Cui Z Y, Zhou Y, Li W, Feng B B, Sun P, Dong C F, Liu Y, Hong W Y, Yang Q W, Ding X T and Duan X Y 2009 Chin. Phys. B 18 3473
[4]	Shen Y C, Lyu B, Du X W, Li Y Y, Fu J, Wang F D, Pan X Y, Chen J, Wang Q P and Shi Y J 2015 Plasma. Sci. Technol. 17 183
[5]	Gill R D, Edwards A W, Pasini D and Weller A 1992 Nucl. Fusion 32 723
[6]	Pecquet A L, Cristofani P, Mattioli M, Garbet X, Laurent L, Geraud A, Gil C, Joffrin E and Sabot R 1997 Nucl. Fusion 37 451
[7]	Giovannozzi E, Annibaldi S V, Buratti P, Frigione D, Lazzaro E, Panaccione L and Tudisco O 2004 Nucl. Fusion 44 226
[8]	Gryaznevich M P, Sharapov S E, Lilley M, Pinches S D, Field A R, Howell D, Keeling D, Martin R, Meyer H and Smith H 2008 Nucl. Fusion 48 084003
[9]	Chapman I T, Hua M D, Pinches S D, Akers R J, Field A R, Graves J P, Hastie R J, Michael C A and the MAST Team 2010 Nucl. Fusion 50 045007
[10]	Liu Y, Qiu X M, Dong Y B, Guo G C, Xiao Z G, Zhong Y Z, Zheng Y J, Fu B Z, Dong J F and Liu Y 2004 Plasma Phys. Control. Fusion 46 455
[11]	Wesson J A 1997 Nucl. Fusion 37 577
[12]	Xu L, Hu L Q, Chen K Y, Li E Z, Wang F D, Zhong G Q, Chen Y J, Xi Y and the EAST Team 2013 Plasma Phys. Control. Fusion 55 032001
[13]	Delgado-Aparicio L, Bitter M, Granetz R, Reinke M and Beiersdorfer P 2012 Rev. Sci. Instrum. 83 10E517
[14]	Gates D A and Delgado-Aparicio L 2012 Phys. Rev. Lett. 108 165004
[15]	Wan Y X, HT-7 Team and HT-7U Team 2000 Nucl. Fusion 40 1057
[16]	Shen Y, Du X, Zhang W, Wang Q, Li Y, Fu J, Wang F, Xu J, Lu B, Shi Y and Wan B 2013 Nucl. Instrum. Methods Phys. Res. Sect. A 700 86
[17]	Shen Y C, Lu B, Du X W, Li Y Y, Fu J, Wang F D, Zhang H M, Xiong Y W, Wang Q P, Shi Y J and Wan B N 2013 Fusion Eng. Des. Design 88 3072
[18]	Dong C F, Morita S, Goto M and Zhou H Y 2010 Rev. Sci. Instrum. 81 033107
[19]	Chowdhuri M B, Morita S, Goto M, Nichimura H, Nagai K and Fujioka S 2007 Plasma Fusion Res. 2 S1060