Controlling marginally detached divertor plasmas
Controlling marginally detached divertor plasmas
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DOI：
10.1088/1741-4326/aa6b16
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发表时间：
2017-06-01
期刊：
影响因子：
3.3
通讯作者：
Stangeby, P. C.
中科院分区：
文献类型：
--
作者：
Eldon, D.;Kolemen, E.;Stangeby, P. C.
DIII-D的一个新的控制系统已经稳定了ELM间分离偏滤器的H模式等离子体状态，接近再附着的阈值，从而证明了以最小的气体抽吸保持分离的能力。当相同的控制系统被命令将等离子体保持在阈值（这里定义为偏滤器靶板附近的T-e = 5eV）时，所得的Te分布分成两组，一组与边缘脱离一致，另一组与边缘附着一致。当控制系统试图保持在阈值时，等离子体在附着和分离状态之间抖动。控制系统从Kolemen等人（2015 J.Nucl.Mater. 463 1186），并且它通过使用真实的时间D-α测量来处理ELMing等离子体，以从源自偏滤器汤姆森散射的真实的时间Te测量中去除ELM期间切片。测量的和请求的ELM间Te之间的差被传递到PID（比例-积分-微分）控制器以确定喷气命令。虽然某种程度的分离对于ITER偏滤器的健康是必不可少的，但是更深分离的等离子体具有更大的辐射损失，并且在极端情况下具有约束退化，使得期望将分离限制到保护靶板所需的最低水平（Kolemen等人，2015 J. Nucl. Mater. 463 1186）。然而，在外部撞击点处观察到的等离子体条件中的分叉，其中离子B x del B漂移到偏滤器中，使得这是一个重大挑战。如果偏滤器等离子体在ELM之间重新附着，则在分离可以重新建立之前，将有一个长的（取决于喷气系统中的延迟）高热通量窗口。因此，需要很好地理解重新附着阈值附近的分离行为，以适当地调整主动控制系统，从而在不重新附着的情况下保持理想的偏滤器性能。顶部的基座电子密度在抖动的分叉和稳定的边缘分离操作过程中是相同的不确定性，显示需要本地实时测量偏滤器的条件。
A new control system at DIII-D has stabilized the inter-ELM detached divertor plasma state for H-mode in close proximity to the threshold for reattachment, thus demonstrating the ability to maintain detachment with minimal gas puffing. When the same control system was instead ordered to hold the plasma at the threshold (here defined as T-e = 5 eV near the divertor target plate), the resulting Te profiles separated into two groups with one group consistent with marginal detachment, and the other with marginal attachment. The plasma dithers between the attached and detached states when the control system attempts to hold at the threshold. The control system is upgraded from the one described in Kolemen et al (2015 J. Nucl. Mater. 463 1186) and it handles ELMing plasmas by using real time D-alpha measurements to remove during-ELM slices from real time Te measurements derived from divertor Thomson scattering. The difference between measured and requested inter-ELM Te is passed to a PID (proportional-integral-derivative) controller to determine gas puff commands. While some degree of detachment is essential for the health of ITER's divertor, more deeply detached plasmas have greater radiative losses and, at the extreme, confinement degradation, making it desirable to limit detachment to the minimum level needed to protect the target plate (Kolemen et al 2015 J. Nucl. Mater. 463 1186). However, the observed bifurcation in plasma conditions at the outer strike point with the ion B x del B drift into the divertor makes this a significant challenge. If the divertor plasma were to reattach between ELMs, there would be a long (depending on delays in the gas puff system) window of high heat flux before detachment could be re-established. Thus, good understanding of detachment behavior near the threshold for re-attachment is required to properly tune an active control system to maintain ideal divertor performance without reattaching. The top-of-pedestal electron densities during dithering across the bifurcation and during stable marginally detached operation are the same within uncertainty, showing the need for local real-time measurements of the divertor conditions.