Sensitivity of detachment extent to magnetic configuration and external parameters
Divertor detachment may be essential to reduce heat loads to magnetic fusion tokamak reactor divertor surfaces. Yet in experiments it is difficult to control the extent of the detached, low pressure, plasma region. At maximum extent the front edge of the detached region reaches the X-point and can l...
| Main Authors: | , , |
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| Other Authors: | |
| Format: | Article |
| Language: | en_US |
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IOP Publishing
2017
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| Online Access: | http://hdl.handle.net/1721.1/108740 https://orcid.org/0000-0001-9621-7404 |
| _version_ | 1826189357524975616 |
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| author | Lipschultz, Bruce Hutchinson, Ian Horner Parra Diaz, Felix Ignacio |
| author2 | Massachusetts Institute of Technology. Department of Aeronautics and Astronautics |
| author_facet | Massachusetts Institute of Technology. Department of Aeronautics and Astronautics Lipschultz, Bruce Hutchinson, Ian Horner Parra Diaz, Felix Ignacio |
| author_sort | Lipschultz, Bruce |
| collection | MIT |
| description | Divertor detachment may be essential to reduce heat loads to magnetic fusion tokamak reactor divertor surfaces. Yet in experiments it is difficult to control the extent of the detached, low pressure, plasma region. At maximum extent the front edge of the detached region reaches the X-point and can lead to degradation of core plasma properties. We define the 'detachment window' in a given position control variable C (for example, the upstream plasma density) as the range in C within which the front location can be stably held at any position from the target to the X-point; increased detachment window corresponds to better control. We extend a 1D analytic model [1] to determine the detachment window for the following control variables: the upstream plasma density, the impurity concentration and the power entering the scrape-off layer (SOL). We find that variations in magnetic configuration can have strong effects; increasing the ratio of the total magnetic field at the X-point to that at the target, ${{B}_{\times}}/{{B}_{t}}$ , (total flux expansion, as in the super-x divertor configuration) strongly increases the detachment window for all control variables studied, thus strongly improving detachment front control and the capability of the divertor plasma to passively accommodate transients while still staying detached. Increasing flux tube length and thus volume in the divertor, through poloidal flux expansion (as in the snowflake or x-divertor configurations) or length of the divertor, also increases the detachment window, but less than the total flux expansion does. The sensitivity of the detachment front location, z h , to each control variable, C, defined as $\partial {{z}_{h}}/\partial C$ , depends on the magnetic configuration. The size of the radiating volume and the total divertor radiation increase $\propto {{\left({{B}_{\times}}/{{B}_{t}}\right)}^{2}}$ and $\propto {{B}_{\times}}/{{B}_{t}}$ , respectively, but not by increasing divertor poloidal flux expansion or field line length. We believe this model is applicable more generally to any thermal fronts in flux tubes with varying magnetic field, and similar sources and sinks, such as detachment fronts in stellarator divertors and solar prominences in coronal loops. |
| first_indexed | 2024-09-23T08:13:28Z |
| format | Article |
| id | mit-1721.1/108740 |
| institution | Massachusetts Institute of Technology |
| language | en_US |
| last_indexed | 2024-09-23T08:13:28Z |
| publishDate | 2017 |
| publisher | IOP Publishing |
| record_format | dspace |
| spelling | mit-1721.1/1087402023-02-26T02:08:19Z Sensitivity of detachment extent to magnetic configuration and external parameters Lipschultz, Bruce Hutchinson, Ian Horner Parra Diaz, Felix Ignacio Massachusetts Institute of Technology. Department of Aeronautics and Astronautics Massachusetts Institute of Technology. Department of Nuclear Science and Engineering Massachusetts Institute of Technology. Plasma Science and Fusion Center Hutch Lipschultz, Bruce Parra Diaz, Felix I. Hutchinson, Ian Horner Divertor detachment may be essential to reduce heat loads to magnetic fusion tokamak reactor divertor surfaces. Yet in experiments it is difficult to control the extent of the detached, low pressure, plasma region. At maximum extent the front edge of the detached region reaches the X-point and can lead to degradation of core plasma properties. We define the 'detachment window' in a given position control variable C (for example, the upstream plasma density) as the range in C within which the front location can be stably held at any position from the target to the X-point; increased detachment window corresponds to better control. We extend a 1D analytic model [1] to determine the detachment window for the following control variables: the upstream plasma density, the impurity concentration and the power entering the scrape-off layer (SOL). We find that variations in magnetic configuration can have strong effects; increasing the ratio of the total magnetic field at the X-point to that at the target, ${{B}_{\times}}/{{B}_{t}}$ , (total flux expansion, as in the super-x divertor configuration) strongly increases the detachment window for all control variables studied, thus strongly improving detachment front control and the capability of the divertor plasma to passively accommodate transients while still staying detached. Increasing flux tube length and thus volume in the divertor, through poloidal flux expansion (as in the snowflake or x-divertor configurations) or length of the divertor, also increases the detachment window, but less than the total flux expansion does. The sensitivity of the detachment front location, z h , to each control variable, C, defined as $\partial {{z}_{h}}/\partial C$ , depends on the magnetic configuration. The size of the radiating volume and the total divertor radiation increase $\propto {{\left({{B}_{\times}}/{{B}_{t}}\right)}^{2}}$ and $\propto {{B}_{\times}}/{{B}_{t}}$ , respectively, but not by increasing divertor poloidal flux expansion or field line length. We believe this model is applicable more generally to any thermal fronts in flux tubes with varying magnetic field, and similar sources and sinks, such as detachment fronts in stellarator divertors and solar prominences in coronal loops. Wolfson Foundation (Royal Society Wolfson Research Merit Award) Research Councils UK (EPSRC grant number EP/I501045) Research Councils UK (grant number EP/I501045) 2017-05-08T15:35:27Z 2017-05-08T15:35:27Z 2016-04 2016-02 Article http://purl.org/eprint/type/JournalArticle 0029-5515 1741-4326 http://hdl.handle.net/1721.1/108740 Lipschultz, Bruce, Felix I. Parra, and Ian H. Hutchinson. “Sensitivity of Detachment Extent to Magnetic Configuration and External Parameters.” Nuclear Fusion 56, no. 5 (April 8, 2016): 056007. https://orcid.org/0000-0001-9621-7404 en_US http://dx.doi.org/10.1088/0029-5515/56/5/056007 Nuclear Fusion Creative Commons Attribution 3.0 Unported license http://creativecommons.org/licenses/by/3.0/ application/pdf IOP Publishing IOP Publishing |
| spellingShingle | Lipschultz, Bruce Hutchinson, Ian Horner Parra Diaz, Felix Ignacio Sensitivity of detachment extent to magnetic configuration and external parameters |
| title | Sensitivity of detachment extent to magnetic configuration and external parameters |
| title_full | Sensitivity of detachment extent to magnetic configuration and external parameters |
| title_fullStr | Sensitivity of detachment extent to magnetic configuration and external parameters |
| title_full_unstemmed | Sensitivity of detachment extent to magnetic configuration and external parameters |
| title_short | Sensitivity of detachment extent to magnetic configuration and external parameters |
| title_sort | sensitivity of detachment extent to magnetic configuration and external parameters |
| url | http://hdl.handle.net/1721.1/108740 https://orcid.org/0000-0001-9621-7404 |
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