| 1. |
- Abel, I, et al.
(författare)
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Overview of the JET results with the ITER-like wall
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104002-
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Tidskriftsartikel (refereegranskat)abstract
- Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma-facing materials. The large reduction in the carbon content (more than a factor ten) led to a much lower Z(eff) (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that the fuel retention rate with the new wall is substantially reduced with respect to the C wall. The re-establishment of the baseline H-mode and hybrid scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type-I ELMy H-mode regimes with H-98,H-y2 close to 1 and beta(N) similar to 1.6 have been achieved using gas injection. ELM frequency is a key factor for the control of the metallic impurity accumulation. Pedestal temperatures tend to be lower with the new wall, leading to reduced confinement, but nitrogen seeding restores high pedestal temperatures and confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on Be wall plasma-facing components due to lower radiation made the routine use of massive gas injection for disruption mitigation essential.
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| 2. |
- Widdowson, A., et al.
(författare)
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Removal of beryllium-containing films deposited in JET from mirror surfaces by laser cleaning
- 2011
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 415:1, s. S1199-S1202
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Tidskriftsartikel (refereegranskat)abstract
- A set of stainless steel (SS) and molybdenum mirror samples located in the divertor and at the outer mid-plane of the vessel were exposed in JET from 2005 to 2007. A selection of these mirror samples with well adhered deposits (i.e. not flaking) of up to a few hundred nanometers in thickness and with Be/C ratios ranging from 0 to similar to 1 have been cleaned using a laser system developed at CEA, Saclay. Following laser cleaning the recovered reflectivity was generally better in the infrared than the visible spectrum, with recovery of up to 90% of the initial reflectivity being obtained at 1600 nm for both Mo and SS mirrors falling as low as 20-30% of initial reflectivity at a wavelength of 400 nm for some SS mirrors, rising to similar to 80% for Mo mirrors. Some deposit remained on the mirrors after the cleaning trials.
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| 3. |
- Rubel, Marek, et al.
(författare)
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Comprehensive First Mirror Test for ITER at JET with Carbon Walls
- 2010
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Ingår i: Proceedings of the23rd IAEA Fusion Energy Conference.
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Konferensbidrag (refereegranskat)abstract
- Metallic mirrors will be essential components of all optical spectroscopy and imaging systems forplasma diagnosis that will be used on the next-step magnetic fusion experiment, ITER. Any change of the mirrorperformance, in particular reflectivity, will influence the quality and reliability of detected signals. On therequest of the ITER Design Team, a First Mirror Test (FMT) has been carried out at JET during campaigns in2005-2007 and 2008-2009. To date, it has been the most comprehensive test performed with a large number oftest mirrors exposed in an environment containing both carbon and beryllium; the total plasma time (in 2005-2007 period) over 35 h including 27 h of X-point operation. 32 stainless steel and polycrystalline molybdenumflat-front and 45oangled mirrors were installed in separate channels of cassettes on the outer wall and in the MkII HD divertor: inner leg, outer leg and base plate under the load bearing tile. Post exposure studies comprisedreflectivity measurements and surface analyses with microscopy, secondary ion mass spectrometry, ion beamanalysis and energy dispersive X-ray spectroscopy.. The essential results are: (i) on the outer wall highreflectivity (~90%) is maintained for mirrors close to the channel entrance but it is degraded by 30-40 % deeperin the channel (ii) reflectivity loss by 70-90% is measured for mirrors placed in the divertor: outer, inner andbase; (iii) deuterium and carbon are the main elements detected on all mirror surfaces and the presence ofberyllium is also found; (iv) thick deposits show rough columnar structure and thickness is 1-20 μm; (v) bubblelike structures are detected in deposits; (vi) the deposition in channels in the divertor cassettes is pronounced atthe very entrance; (vii) photonic cleaning with laser removes deposits but the surface is damaged by laser pulses.In summary, reflectivity of all tested mirrors is degraded either by erosion with CX neutrals or by the formationof thick deposits. The implications of results obtained for first mirrors in next-step device are discussed andcritical assessment of various methods for in-situ cleaning of mirrors is presented. The conclusion is thatengineering solutions should be developed in order to install shutters or to implement a cassette with mirrors toreplace periodically the degraded ones
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| 4. |
- Coenen, J. W., et al.
(författare)
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Long-term evolution of the impurity composition and impurity events with the ITER-like wall at JET
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:7, s. 073043-
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Tidskriftsartikel (refereegranskat)abstract
- This paper covers aspects of long-term evolution of intrinsic impurities in the JET tokamak with respect to the newly installed ITER-like wall (ILW). At first the changes related to the change over from the JET-C to the JET-ILW with beryllium (Be) as the main wall material and tungsten (W) in the divertor are discussed. The evolution of impurity fluxes in the newly installed W divertor with respect to studying material migration is described. In addition, a statistical analysis of transient impurity events causing significant plasma contamination and radiation losses is shown. The main findings comprise a drop in carbon content (x20) (see also Brezinsek et al (2013 J. Nucl. Mater. 438 S303)), low oxygen content (x10) due to the Be first wall (Douai et al 2013 J. Nucl. Mater. 438 S1172-6) as well as the evolution of the material mix in the divertor. Initially, a short period of repetitive ohmic plasmas was carried out to study material migration (Krieger et al 2013 J. Nucl. Mater. 438 S262). After the initial 1600 plasma seconds the material surface composition is, however, still evolving. With operational time, the levels of recycled C are increasing slightly by 20% while the Be levels in the deposition-dominated inner divertor are dropping, hinting at changes in the surface layer material mix made of Be, C and W. A steady number of transient impurity events, consisting of W and constituents of inconel, is observed despite the increase in variation in machine operation and changes in magnetic configuration as well as the auxiliary power increase.
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| 5. |
- Rubel, Marek, et al.
(författare)
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Overview of the second stage in the comprehensive mirrors test in JET
- 2011
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Ingår i: Physica Scripta. - : Institute of Physics Publishing (IOPP). - 0031-8949 .- 1402-4896. ; T145, s. 014070-
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Tidskriftsartikel (refereegranskat)abstract
- The first mirror test for ITER in JET with carbon walls has been completed. Thirty polycrystalline Mo mirrors including four coated with a 1 μm rhodium (Rh) film were exposed to plasma in the divertor region and in the main chamber. The mirrors were installed in eight cassettes of pan-pipe shape. The reflectivity of all mirrors exposed in the divertor has been degraded by 80–90% because of the formation of thick (>20 μm) flaking co-deposits on surfaces. Only small reflectivity losses (5–10%) occurred on mirrors located at the channel mouth of the cassettes from the main chamber wall. This is due to the in situremoval of deposited species by charge exchange neutrals. Deuterium, 12C and 9Be are the main isotopes detected on surfaces, but other isotopes (13C) are also found in some locations, thus indicating differences in the material migration. Rhodium coatings with an initial reflectivity that is 30% better than that of pure Mo survived the test without detachment, but their resultant reflectivity was the same as that of the exposed Mo surfaces.
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| 6. |
- Rubel, Marek, et al.
(författare)
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Tungsten migration studies by controlled injection of volatile compounds
- 2013
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 438, s. S170-S174
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Tidskriftsartikel (refereegranskat)abstract
- Volatile tungsten hexa-fluoride was locally injected into the TEXTOR tokamak as a marker for material migration studies. The injection was accompanied by puffing N-15 rare isotope as a nitrogen tracer in discharges with edge cooling by impurity seeding. The objective was to assess material balance by qualitative and quantitative determination of a global and local deposition pattern, material mixing effects and fluorine residence in plasma-facing components. Spectroscopy and ex situ ion beam analysis techniques were used. Tungsten was detected on all types of limiter tiles and short-term probes retrieved from the vessel. Over 80% of the injected W was identified. The largest tungsten concentration, 1 x 10(18) cm(-2), was in the vicinity of the gas inlet. Co-deposits contained tungsten and a mix of light isotopes: H, D, He-4, B-10, B-11, C-12, C-13, N-14, N-15, O-16 and small quantities of F-19 thus showing that both He and nitrogen are trapped following wall conditioning (He glow) and edge cooling.
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| 7. |
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| 8. |
- Wauters, T., et al.
(författare)
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Self-consistent application of ion cyclotron wall conditioning for co-deposited layer removal and recovery of tokamak operation on TEXTOR
- 2013
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Ingår i: Nuclear Fusion. - : Institute of Physics (IOP). - 0029-5515 .- 1741-4326. ; 53:12, s. 123001-
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a demonstration experiment of ion cyclotron wall conditioning (ICWC) on TEXTOR covering all ITER wall conditioning aims and discusses the implications for ITER. O-2/He-ICWC applied to erode carbon co-deposits removed 6.6x10(21) C-atoms (39 pulses, 158 s cumulated discharge time). Large oxygen retention (71% of injected oxygen) prevented subsequent ohmic discharge initiation. Plasma operation was recovered by a 1h47 multi-pulse D-2-ICWC procedure including pumping time between pulses with duty cycle of 2 s/20 s, cleaning the vessel from oxygen impurities, followed by a 23 min He-ICWC procedure (2 s/20 s), applied to desaturate the deuterium-loaded walls. A stable ohmic discharge was established on the first attempt right after the recovery procedure. The discharges showed improved density control and only slightly increased oxygen characteristic radiation levels (1-1.5 times). After the recovery procedure 36% of the injected O-atoms remained retained in the vessel, derived from mass spectrometry measurements. This amount is in the estimated range for storage in remote areas obtained from surface analysis of locally exposed samples. The removed amount of oxygen by D-2 and He-ICWC obtained from mass spectrometry corresponds to the retention in plasma-wetted areas estimated by surface analysis. It is concluded that most of the removed oxygen stems from plasma-wetted areas while shadowed areas, e. g. behind poloidal limiters, may feature net retention of the discharge gas. On ITER, designed with a shaped first wall, the ICWC plasma-wetted area will approach the total surface area, reducing consequently the retention in remote areas. A tentative extrapolation of the carbon removal on TEXTOR to tritium removal from co-deposits on ITER in the 39 x 4 s O-2/He-ICWC discharges, including pumping time between the RF pulses, corresponds on ITER to a tritium removal in the order of the estimated retention per 400 s DT-burn (140-500 mgT (Shimada and Pitts 2011 J. Nucl. Mater. 415 S1013-6)).
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| 9. |
- Widdowson, A., et al.
(författare)
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Comparison of JET main chamber erosion with dust collected in the divertor
- 2013
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 436:Suppl., s. 827-832
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Tidskriftsartikel (refereegranskat)abstract
- A complete global balance for carbon in JET requires knowledge of the net erosion in the main chamber, net deposition in the divertor and the amount of dust and flakes collecting in the divertor region. This paper describes a number of measurements on aspects of this global picture. Profiler measurements and cross section microscopy on tiles that were removed in the 2009 JET intervention are used to evaluate the net erosion in the main chamber and net deposition in the divertor. In addition the mass of dust and flakes collected from the JET divertor during the same intervention is also reported and included as part of the balance. Spectroscopic measurements of carbon erosion from the main chamber are presented and compared with the erosion measurements for the main chamber.
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| 10. |
- Wisse, M., et al.
(författare)
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Laser-assisted cleaning of beryllium-containing mirror samples from JET and PISCES-B
- 2014
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Ingår i: Fusion engineering and design. - : Elsevier BV. - 0920-3796 .- 1873-7196. ; 89:2, s. 122-130
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Tidskriftsartikel (refereegranskat)abstract
- A set of seven polycrystalline mirror samples retrieved from the JET tokamak has been cleaned in vacuum using a pulsed laser system. The surfaces of samples exposed to plasma during 2008-2009 campaigns as part of the second phase of a comprehensive first mirror test contained a mixture of carbon, beryllium and tritium. For this reason, the samples were treated in a vacuum chamber constructed specially for this purpose. In some cases mirrors show an increase of the specular reflectivity after cleaning, though beryllium and carbon deposits were not fully removed. Additionally, three samples coated in PISCES-B with a 110-120 nm beryllium layer were subjected to laser cleaning tests as well.
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