Advanced vertical control for tokamak reactors with double-null divertors

对于紧凑型托卡马克反应堆来说，双零偏滤器是一种很有吸引力的排气解决方案，但它的成功取决于实现垂直等离子体位置控制，这种控制既稳健又精确，并使用了一些非磁状态估计。一个自然而然的选择是感应两个偏滤器之间的热流不平衡，因为这正是我们希望最小化的量，以确保平等的功率共享。在这一控制概念中，等离子体垂直运动和被检测到的热流之间的时间延迟是一个重要的参数。如果信息沿SOL快速平移，那么热通量就成为等离子体位移的一个很好的估计器，但是如果它很慢(与垂直振荡时间尺度相比)，那么闭环控制就变得更具挑战性。目前尚不清楚反应堆中的这一特征溶蚀时间将是什么，但似乎谨慎的做法是通过设计一种在这种情况下所需的高级垂直控制方案来缓解它“缓慢”的可能性。我们建议创建一种模型预测控制算法，它利用与时间相关的SOL行为的快速缩减/替代模型，基于最近的测量来实时预测系统的未来演变，并相应地在垂直控制线圈上执行。该模型应根据MAST-U数据和仿真进行验证，该算法在硅片上测试表明，该方法可以实现对MAST-U和/或反应堆偏滤器功率平衡的实时控制。

The double-null divertor is an attractive exhaust solution for compact tokamak reactors, but its success is contingent on achieving vertical plasma position control which is both robust and precise, and uses some non-magnetic state estimation. A natural choice for this would be sensing the heat flux imbalance between the two divertors, since this quantity is exactly what we wish to minimise to ensure equal power sharing. The time delay between plasma moving vertically and the heat flux being detected - a part of which is accounted for by transience in the scrape-off layer (SOL) - is an important parameter in this control concept. If the information advects along the SOL quickly, then heat flux becomes a good estimator of the plasma displacement, but if it is slow (compared to the vertical oscillation timescale) then closed-loop control becomes much more challenging. It is unknown what this characteristic SOL time will be in a reactor, but it seems prudent to mitigate against the possibility it is 'slow' by devising an advanced vertical control scheme of the kind which would be required in this scenario. We propose to create a model predictive control algorithm which makes use of a fast reduced/surrogate model for the time-dependent SOL behaviour to predict, based on recent measurements, the future evolution of the system in real time and actuate on the vertical control coils accordingly. This model should be validated against MAST-U data and simulations, and the algorithm tested in-silico to demonstrate that real-time control of MAST-U and/or a reactor divertor power balance could be achieved by this method.