Plasma shaping and its impact on the pedestal of ASDEX Upgrade: edge stability and inter-ELM dynamics at varied triangularity

Abstract

The plasma shape, in particular the triangularity (δ), impacts on the pedestal stability. A scan of δ including a variation of heating power (P heat) and gas puff was performed to study the behaviour of edge localised modes (ELMs) and the pre-ELM pedestal stability for different plasma shapes. Generally, at higher δ the pedestal top electron density (n e) is enhanced and the ELM repetition frequency (f ELM) is reduced. For all δ, the pedestal top n e is already fully established to its pre-ELM value during the initial recovery phase of the n e pedestal, which takes place immediately after the ELM crash. The lowering of the f ELM with increasing δ is related to longer pedestal recovery phases, especially the last pre-ELM phase with clamped pedestal gradients (after the recovery phases of the n e and electron temperature (T e) pedestal) is extended. In all investigated discharge intervals, the pre-ELM pedestal profiles are in agreement with peeling-ballooning (PB) theory. Over the investigated range of δ, two well-separated f ELM bands are observed in several discharge intervals. Their occurrence is linked to the inter-ELM pedestal stability. In both kinds of ELM cycles the pedestal evolves similarly, however, the 'fast' ELM cycle occurs before the global plasma stored energy (W MHD) increases, which then provides a stabilising effect on the pedestal, extending the inter-ELM period in the case of the 'slow' ELM cycle. At the end of a 'fast' ELM cycle the n e profile is radially shifted inwards relative to the n e profile at the end of a 'slow' ELM cycle, leading to a reduced pressure gradient. The appearance of two f ELM bands suggests that the pedestal becomes more likely PB unstable in certain phases of the inter-ELM evolution. Such a behaviour is possible because the evolution of the global plasma is not rigidly coupled to the evolution of the pedestal structure on the timescales of an ELM cycle.

Numerical survey of predicted peeling response in edge localised mode mitigated and suppressed phases on ASDEX upgrade

“…This work is primarily concerned with testing the initial hypothesis for the requirement of high triangularity, which proposes that increasing triangularity causes an increase in the pressure pedestal, enhancing the peeling response, thereby facilitating suppression access [2]. This hypothesis implies a positive correlation between δ U and the pedestal pressure (consistent with previous work [29]), however figure 3 shows that this is not conclusively detected in this work. The hypothesis also requires a positive correlation between pedestal pressure and peeling response, which is shown in figure 6 to be likely to exist, although the peeling response is only very weakly sensitive to the pedestal pressure.…”

“…The plots do not refute this, but the scatter is large enough that they provide only weak evidence in support of it. The effect of triangularity on pedestal pressure is well documented [29], suggesting that confounding factors are obscuring the trend in this result. In both subplots, β N is restricted to within 1.7 < β N < 2.1.…”

“…b) The operational points in (j, α) space for the database are plotted for high and low triangularity. Using the MISHKA code [51], finite-n peeling ballooning stability boundaries are computed for two representative high and low δ U points from the database, demonstrating that the peeling-ballooning stability is significantly enhanced by increased shaping as expected [29]. The approximate co-location of the high and low δ U points suggests that the dependence of (j, α) on δ U is weakened or broken by RMP induced confinement degradation.…”

“…To ensure consistency between the T e and n e profiles, this same shift was then applied to the density profile as measured using the same diagnostic, as performed in [29]. An ELM synchronisation technique is commonly used for measuring pedestal profiles of ELM-y plasmas [29], in order to isolate the pedestal profile immediately preceding the ELM crash. However, for this work measurements of the ion temperature and rotation velocity by the CXRS diagnostic were required [30], which has a time resolution comparable to the ELM frequencies of interest to this study, making ELM synchronisation infeasible.…”

Nonlinear evolution characteristics of peeling-ballooning mode under negative triangularity

“…† 通讯作者.E-mail: mlmou@scu.edu.cn 第一作者.E-mail: 291878886@qq.com 1 引言 高约束运行模式(high confinement regime，H 模) [1] 因其较高的约束参数已 成为未来聚变堆稳态运行的基本模式之一， 但高约束条件下伴随的Ⅰ型边界局域 模(edge localized mode，ELM) [2] 会引起等离子体边缘区域台基的周期性崩塌， 崩塌过程中释放的强粒子流和热流会导致偏滤器热负荷过载。ELM 现象通常被 认为由剥离气球模(peeling-ballooning mode，P-B 模) [3] 激发产生，该不稳定 性由 H 模放电时台基区电流驱动的剥离模和压强梯度驱动的气球模耦合而成。 三角形变是等离子体形状参数之一，在 DIII-D [4] 、JET [5] 、ASDEX Upgrade [6] 及其他托卡马克装置 [7] 上的 H 模放电实验发现， 正三角形变 (positive triangularity， PT)位型放电时约束水平显著提高。相关模拟表明，PT 可以解耦 P-B 模，使台 基获得更大压强梯度和电流，从而有助于进入气球模第二稳定区 [3,8] 。然而，在 实验和模拟中，负三角形变(negative triangularity，NT)位型具有截然不同的特 性 [9] 。一方面，NT 可以增加低约束模式(low confinement regime， L 模)向 H 模转换的阈值， 进而可在 L 模实现类似 H 模的高比压放电， 同时不会出现 ELM [9,10] 。另一方面，TCV 实验发现，NT 位型下 H 模放电的台基压强高度低于 PT， 但其 ELM 频率更高，幅值更小 [11] 。此外，NT 也有其独特的放电优势，如：更 大的外分界面、更灵活的偏滤器配置空间、更 大的刮削层空间、更低的内部极向 线圈磁场需求，以及偏滤器室的更大泵送电导 [12] 等。相关模拟工作表明，NT 可 使气球模失稳，并且导致气球模第二稳定区通道完全关闭 [12,13] ，但已有研究并 未给出 NT 位型下 P-B 模的非线性演化特征， 而 P-B 模的非线性演化直接关系到 ELM 爆发产生的热流和粒子流大小，故本文对此进行了详细研究。 本文通过 Corsica 的 TEQ 平衡模块构建了具有不同三角形变的托卡马克平衡 [14] ，并使用 BOUT++代码 [15,16]…”