ISTTOK as a liquid metal plasma facing component test device

Transcription

1 ISTTOK as a liquid metal plasma facing component test device H. Fernandes, J. Loureiro, C. Silva, R. Gomes, E. Alves, R. Mateus, T. Pereira, H. Figueiredo, H. Alves Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

2 Motivation The high power load on the wall of the reactors is one of the main unsolved issues in nuclear fusion. The European Fusion Development Agreement has defined in the fusion roadmap Heat and particle exhaust as one of 8 key issues to be solved. Solid Be W-coated CFC Bulk W 2017 March Slide 2

3 Plasma facing components Causes: Fusion is a compact power source (high power flows in a thin surface) Tolerance to off-normal events (which lead to surface temperature excursions) Constraints to usable materials as plasma facing components (PFC): Neutron fluence Intense flux of hydrogen radicals High melting point Low Z Issues: Surface melting Physical sputtering Redeposition Chemical erosion Hydrocarbon formation, dust Tritium retention Element transmutation 2017 March Slide 3

4 Issues with solid PFC Heat load and Particle fluence Termal Cycles Neutron fluence Unusual events 2017 March Slide 4 Issues with the conventional solid metal plasma facing components. Erosion: Cracking: Newsletter posted October 25th 2006 Embrittlement: Shin-ichi Komazaki, et al (2005) J. Eng. Mater. Technol. 127(4), Melting: B. Lipschultz, et al (2012) J. Nuc. Fus. 52(12)

5 Liquid Metals as PFC Material flows in making a self-healing surface Flowing liquid metal is immune to surface damage Simultaneous conduction and convection of heat 2017 March Slide 5

6 Materials for a Liquid Divertor Li, Ga and Sn are potential candidates: Li temperature limit is around 450ºC (evaporation rate). Ga and Sn could work up to ºC while Li-Sn could work up to 750ºC. What is the compatibility of these materials with hydrogen plasmas? Particularly we re interested in investigating the deuterium retention of this material and the contamination of the plasma by this alloy. From report by R. Majeski, Liquid Metal Walls, Lithium, And Low Recycling Boundary Conditions In Tokamaks, PPPL, March Slide 6

7 ISTTOK tokamak: ISTTOK Instituto Superior Técnico TOKamak Geometrical parameters: Iron core tokamak Major radius (R): 46 cm Minor radius (a): 8.5 cm (typical) Toroidal field: 0.5 T Typical plasma parameters: Current: 5 ka Averaged density: ~5x10 18 m -3 Electron temperature at r=0:~125 ev q(a): March Slide 7

8 CODAC Plasma Position: Tomography Mirnov coils Electric probes Cosine coil Plasma current: Mirnov coils Rogowsky Plasma density: Interferometer ATCA acquisition board: 32 ADC 2 MSample/s) 8 DAC 50 MSample/s) Xilinx Virtex-4 FPGA 512 MB DDRII SDRAM 11 Aurora fast serial links 8 RS-485 slow serial links 2017 March Slide 8 Radiated power: Tomography H-alpha detector Transformer flux: Primary current Loop voltage

9 ISTTOK MARTe GAM execution Time/data acquisition Wait for next cycle Data collection PS and puffing references Time-windows state machine 100 µs control cycle Control matrix Waveform generation Diagnostics pre-processing Observers calculation #CPU Core 0 Core 1 Core 2 Core 3 Process Linux services Drivers for acquisition boards MARTe services ISTTOK real-time thread The MARTe framework synchronizes the GAM execution, GAMs are executed in sequence; MARTe stores their execution times. The control cycle starts with the data acquisition at each 100 µs. TimewindowsGAM selects the operation mode and the corresponding reference waveforms (scenario or current control). Observers get data from multiple diagnostics. Depending on control type the controller transforms the waveforms in commands to the actuators (scenario or current control). Data is stored and MARTe controls the start of the next control cycle March Slide 9

10 Repeatability of the AC discharges 2017 March Slide 10

11 Magnetic diagnostics Rogowski coil: There are two coils of this type, with 500 turns each and 3.25 mm in diameter, measuring both the internal (plasma) and the external (total) toroidal currents; Yields the plasma current (by integration of the signal) Sine and cosine probes: The sine and cosine probes are similar to Rogwosky coils apart from the variable wiring density in the coil. Linearized signal with respect to the plasma vertical positioning Mirnov coils: Poloidal array of 12 Mirnov coils used to study the plasma MHD activity and to evaluate the plasma position; Yields the Magnetic field and the plasma current (by integration of the signal) Toroidal loop: This diagnostic is constituted by a single loop around the torus measures the loop voltage March Slide 11

12 Electric probes Poloidal array Rake array Radial array 2017 March Slide 12

13 Interferometer Interferometer: A single channel interferometer, at 100 GHz, with heterodynic detection used to determine the line-averaged density; Although more complex, this method has the advantage of reducing the frequency to be acquired by the data acquisition system and improves the sensibility of the system March Slide 13

14 Heavy-ion beam diagnostic Heavy ion beam: Xe ions with energy: kev Faraday-cup like cell detector Low noise trans-impedance amplifiers BW: 550 khz 50 amplifiers Fast beam modulation Local measurements in 12 sample volumes 2017 March Slide 14

15 Visible spectroscopy High resolution spectrometer: CVI Laser DK480I imaging spectrograph. This spectrometer is equipped with a triple-grating turret system, blazed at 300, 500 and 750 nm and 1200 gr/mm each. The 68x68 mm 2 gratings provide a maximum resolution of 0.06 nm and a reciprocal dispersion of approximately 1.60 nm/mm Andor s ixon 888 EMCCD camera: 1024 x 1024 pixels, with each pixel having 13x13 um. The full figure can be acquired at 26 fps or with crop mode higher speeds can be achieved. Broadband spectrometer: Sarspec Spec Res+ Detection from 180 to 1100 nm 3648 pixels linear array CCD detector optical resolution of 0.2 nm 2017 March Slide 15

16 Visible spectroscopy II Exhaustive search in the visible range of Sn compatible lines. No Sn I lines were identified but some Sn II candidates where found. Line ratio method to determine electron temperature Ratios of He I lines 4 3 S 2 3 P at nm and 4 1 S 2 1 P at nm 2017 March Slide 16

17 LM experiments on ISTTOK 2017 March Slide 17

18 LM experiments on ISTTOK Z = 3 Z = 31 Z = 50 Atomic weight = 6.9 Atomic weight = 69.7 Atomic weight = Melting point = ºC Melting point = 29.8 ºC Melting point = 232 ºC Boiling point = 1342 ºC Boiling point = 2204 ºC Boiling point = 2602 ºC Vapor pressure=10-7 mbar at 370ºC Vapor pressure=10-7 mbar at 850ºC Vapor pressure=10-7 mbar at 1000ºC 2017 March Slide 18

19 Experimental campaign in ISTTOK Build a sample holder setup at ISTTOK with heating capabilities. Pyrometer to monitor surface temperature of the sample. Make spectroscopic measurements to infer the impurity transport in the plasma. Characterize the plasma conditions of the discharge using the standard available diagnostics March Slide 19

20 Pyrometer Used to monitor surface temperature of the sample. Spatial resolution of 2 mm 2. Temporal resolution of 1 ms March Slide 20

21 Hydrogen retention in gallium samples Implemented systems: Preparation chamber Sample conditioning / manipulating system Gallium samples exposed at ~33 ºC, for 1, 3, & 10 s. ERDA+RBS (2 MeV 4He beam) analysis to measured hydrogen profiles. Two distinct regions: superficial/diffusive layer. Measured hydrogen content displays evidences of saturation 2017 March Slide 21

22 ISTTOK Liquid Metal Loop LML design constraints imposed by gallium properties & vacuum conditions. High stability liquid gallium jets produced by hydrostatic pressure (1.3 m gallium column). Jet characteristics: 2.3 mm, 13 cm BUL and 2.5 m/s velocity. Although a large number of ISTTOK discharge (>3000) have been performed with gallium no chamber condition degradation is noticed (doesn t seem to significantly affect plasma performance). Full assessment of gallium as PFC requires testing in larger/higher power device. ISTTOK results clearly indicate that free flows are not suitable as PFC March Slide 22

23 Experimental setup With the purpose of realizing deuterium retention studies, two types of specimens were used: Pure Sn ( % Sn) and Li-Sn eutectic alloy (30 at.% Li). Irradiation under D plasmas either in liquid or solid state. Liquid state: Pure 250ºC and Li-Sn 385ºC. Surface area of ~125 mm March Slide 23

24 Experimental conditions 2017 March Slide 24

25 Ion beam diagnostics Nuclear reaction analysis (NRA) of the samples to determine the deuterium retention. Uses a 1.2 MeV 3 He beam to measure retention profiles. Possible usage of other techniques such as RBS, XRD, PIXE and PIGE March Slide 25

26 NRA spectrum of all exposed samples 2017 March Slide 26

27 Deuterium retention Sample D retained D retained Error D retention/ (at/cm 2 ) (at. %) * (%) D incident Sn solid 3 sec (1.03 ± 0.21) 10-3 Sn liquid 3 sec (2.76 ± 0.56) 10-4 Li-Sn solid 4 sec (6.10 ± 1.23) 10-4 Li-Sn liquid 4 sec (1.73 ± 0.36) 10-4 *at. % evaluated for a pure Sn matrix The obtained retention ratios are all extremely low, all below 0.1 at.% By itself this suggests that all studied cases make good candidates for a plasma facing component. For comparison, tungsten has a retention of 0.1 at.% while CFC retains at.%. However the results for both materials indicate that the retention is lower by almost a factor of 4 in the case where samples are kept in liquid phase. A comparison between pure Sn and Li-Sn shows that for each state the later has almost half the retention for each individual state March Slide 27

28 RBS yield (a.u.) NRA yield (a.u) Lithium segregation in Li-Sn alloy A sample of virgin material was tested and compared with another material with Li (to calibrate the NRA spectrum). Then this sample was scanned at different positions to access homogeneity of the material. Finally we compare the spectrums of exposed and unexposed samples. The peaks in the NRA spectrum related to Li double in intensity 7 Li(p i ) 7 Li( 3 He,p i ) 9 Be 6 Li(p i ) 6 Li( 3 He,p i ) 8 Be 2017 March Slide MeV 3 He beam 7 Li(p 3 ) 7 Li(p 2 ) LiNbO data 48 3 mm SnLi data virgin 50 mm 7 Li(p 1 ) SnLi unexposed SnLi 7 Li(p 0 ) MeV H + beam Energy (kev) Li SnLi - 3 sec exposure SnLi exposed SnLi unexposed O 2 D(p 0 ) 2 6 D(p 0 ) Li(p 1 ) Energy (kev) C Sn

29 Lithium segregation in Li-Sn alloy Li is preferentially sputtered or evaporated from the Li-Sn surface. This may change the local stoichiometry of the alloy and therefore its properties. The BackScattered Electrons (BSE) detector gives insight on the microstructure of the transversal cut and reveals the existence of 3 different phases (LiSn, LiSn2, Sn). From paper by K. Natesan and W. E. Ruther, J. Nuc. Mat (2002) March Slide 29

30 Future work The radial impurity content across the plasma column of ISTTOK characterized by visible spectroscopic measurements is envisioned. Commissioning of the broadband spectrometer to monitor the impurity signatures on the visible range. Utilization of the fast camera with the 10 chord fiber bundle to study the Sn-II line emission across the radial direction (5mm spatial resolution). Reparation of the bolometer March Slide 30

31 Summary The experimental setup and relative experimental procedure for the exposure of tin and lithium-tin alloy samples were described. Samples of Sn and Li-Sn where exposed at ISTTOK to deuterium plasmas for comparable exposure times. The samples were irradiated in liquid and solid states. Deuterium retention was observed and quantified in all samples. Retention was inferred with nuclear reaction analysis (NRA) technique. Retention for these materials is low which reinforces their potential usage as PFC March Slide 31

32 Education 2017 March Slide 32

33 Line ratio method to determine electron temperature: Ratios of He I lines 4 3S 2 3P at nm and 4 1S 2 1P at nm High repeatability over several shots 2017 March Slide 33

34 Comparison of vapor pressure 2017 March Slide 34