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Pedestal structure and stability in JET-ILW and comparison with JET-C
KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics. (Fusion Plasma Physics)
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios.

The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW.

The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used to predict the pedestal height in JET.

The experimental analysis has shown several differences in the pedestal structure of comparable JET-ILW and JET-C discharges. One of the key differences introduced in this work is the pedestal relative shift (a separation between the middle of the pedestals of the electron density and temperature) that plays a major role in the difference in the pedestal performance between JET-C and JET-ILW. The work shows that the relative shift can vary significantly from pulse to pulse and that, on average, JET-C plasmas have lower relative shift than JET-ILW plasmas. The pedestal relative shift tends to increase with increase in the gas fuelling and the heating power. Furthermore, the increase in the relative shift has been empirically correlated with the degradation of the experimental normalized pressure gradient αexp.

To understand the differences in the JET-C and JET-ILW pedestal stability, parameters that affect the pedestal stability and that tend to vary between comparable JET-C and JET-ILW discharges have been identified. These parameters are the pedestal relative shift, pedestal density neped, effective charge number Zeff, pedestal pressure width wpe, and normalized pressure βN. The modelling performed with the predictive Europed code has shown that these five parameters are sufficient to explain the difference in the pedestal performance between JET-C and JET-ILW.

Furthermore, the modelling has shown that the relative shift and neped play a major role in affecting the critical normalized pressure gradient αcrit (normalized pressure gradient expected by the model comparable to αexp), while the relative shift, wpe and Zeff have a major impact on the pedestal pressure height. Finally, a possible mechanism that has led to the degradation of the pedestal pressure from JET-C to JET-ILW is proposed.

Abstract [sv]

Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier.

Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW.

Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET.

Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförbara förhållanden. En nyckelskillnad i piedestalens struktur som introduceras i detta arbete är piedestalens relativa skift, d.v.s. separering av piedestalens mittpunkt för elektrondensiteten och temperaturen. Det relativa skiftet kan variera avsevärt mellan pulser och i genomsnitt har JET-C-plasma ett lägre relativt skift än JET-ILW-plasma. Piedestalens relativa skift tenderar att öka med ökande gasinflöde och uppvärmningseffekt. Vidare korrelerar ökningen i det relativa skiftet empiriskt med försämring av den experimentella normaliserade tryckgradienten αexp.

För att förstå skillnaderna i JET-C och JET-ILW-piedestalstabiliteten identifierades parametrarna som påverkar piedestalstabiliteten och som tenderar att variera mellan jämförbara JET-C- och JET-ILW-plasmasurladdningar. Dessa parametrar är det relativa skiftet för piedestalen, piedestalens densitet neped, det effektiva laddningstalet Zeff, piedestalens bredd wpe och det normaliserade trycket βN. Modelleringen med den prediktiva Europed-koden visar att dessa fem parametrar är tillräckliga för att förklara piedestalens skillnader mellan JET-C och JET-ILW. Vidare visar modelleringen att det relativa skiftet och neped påverkar den kritiska normaliserade tryckgradienten αcrit (normaliserad tryckgradient som i modellen antas vara jämförbar med αexp) medan det relativa skiftet, wpe och Zeff har en stor påverkan på piedestalens höjd. Slutligen föreslås en möjlig mekanism som har lett till försämring av piedestaltrycket vid övergången från JET-C till JET-ILW.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 89
Series
TRITA-EECS-AVL ; 2020:24
Keywords [en]
JET, Thomson scattering, pedestal, pedestal position, pedestal stability, Europed modelling
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-273543ISBN: 978-91-7873-521-1 (print)OAI: oai:DiVA.org:kth-273543DiVA, id: diva2:1430817
Public defence
2020-06-09, https://kth-se.zoom.us/webinar/register/WN_qgz5ej_sQVOiEwrSoar3Ew, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20200518

Available from: 2020-05-18 Created: 2020-05-18 Last updated: 2020-05-18Bibliographically approved
List of papers
1. Confinement and pedestal structure in high performance scenarios in JET-ILW and comparison with JET-C
Open this publication in new window or tab >>Confinement and pedestal structure in high performance scenarios in JET-ILW and comparison with JET-C
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2015 (English)In: 42nd European Physical Society Conference on Plasma Physics, EPS 2015, European Physical Society (EPS) , 2015Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
European Physical Society (EPS), 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-194722 (URN)2-s2.0-84979716198 (Scopus ID)
Conference
42nd European Physical Society Conference on Plasma Physics, EPS 2015, 22 June 2015 through 26 June 2015
Note

Funding Details: EC, European Commission

QC 20161121

Available from: 2016-11-21 Created: 2016-10-31 Last updated: 2020-05-18Bibliographically approved
2. Effect of relative shift on the pedestal stability in JET-ILW and comparison with JET-C
Open this publication in new window or tab >>Effect of relative shift on the pedestal stability in JET-ILW and comparison with JET-C
2016 (English)In: 43rd European Physical Society Conference on Plasma Physics, EPS 2016, 2016Conference paper, Published paper (Refereed)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-203355 (URN)
Conference
43rd European Physical Society Conference on Plasma Physics, EPS 2016, 4 July 2016 through 8 July 2016
Note

QC 20170317

Available from: 2017-03-15 Created: 2017-03-15 Last updated: 2020-05-18Bibliographically approved
3. Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C
Open this publication in new window or tab >>Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C
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2018 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 58, no 5, article id 056010Article in journal (Refereed) Published
Abstract [en]

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (d) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
JET, pedestal, pedestal position, pedestal stability, EUROPED, Thomson scattering
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-225691 (URN)10.1088/1741-4326/aab216 (DOI)000427700800001 ()2-s2.0-85045951206 (Scopus ID)
Funder
Swedish Energy Agency, 40146-1
Note

QC 20180411

Available from: 2018-04-11 Created: 2018-04-11 Last updated: 2020-05-18Bibliographically approved
4. Pedestal structure in high current scenarios in JET-ILW and JET-C
Open this publication in new window or tab >>Pedestal structure in high current scenarios in JET-ILW and JET-C
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2017 (English)In: 44th EPS Conference on Plasma Physics, EPS 2017, European Physical Society (EPS) , 2017Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
European Physical Society (EPS), 2017
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-246610 (URN)2-s2.0-85055049989 (Scopus ID)9781510849303 (ISBN)
Conference
44th European Physical Society Conference on Plasma Physics, EPS 2017, 26 June 2017 through 30 June 2017
Note

QC 20190614

Available from: 2019-06-14 Created: 2019-06-14 Last updated: 2020-05-18Bibliographically approved
5. Comparison of Peeling-Ballooning limited JET-C and JET-ILW plasmas
Open this publication in new window or tab >>Comparison of Peeling-Ballooning limited JET-C and JET-ILW plasmas
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2020 (English)Other (Other academic)
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-273398 (URN)
Note

Article in preparation, to be submitted to Nuclear Fusion journal

QC 20200518

Available from: 2020-05-16 Created: 2020-05-16 Last updated: 2020-05-18Bibliographically approved

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