Neutron diagnostics for the study of fast ion confinement and scenario developments in the high-field compact spherical tokamak ST40

在聚变等离子体中，快离子的能量远远高于热等离子体背景。快离子是由外部辅助加热产生的，如中性束注入(NBI)和离子回旋共振加热(ICRH)，或者由聚变反应本身产生。在前一种情况下，快离子是氢同位素，能量范围从几十个凯夫到几个梅夫。聚变反应除了产生氢同位素外，还产生能量在MeV范围内的阿尔法粒子。快离子在加热等离子体、维持维持聚变反应所需的高温方面发挥着重要作用，对实现燃烧的等离子体至关重要。NBI加热对电流驱动也很重要，也就是对超出感应区域的托卡马克的长脉冲运行，从而实现聚变反应堆。因此，在等离子体中限制快离子足够长的时间，以便它们能够将能量传递到背景等离子体，对于实现基于热核聚变反应的发电厂的目标至关重要。然而，快离子约束被等离子体不稳定性降低，其中一些不稳定性是由快离子本身触发的。在这种情况下，快离子与不稳定性之间的能量交换导致快离子的重新分布和损失，最终降低聚变堆的性能。此外，等离子体中快离子的损失可能会导致反应堆第一壁的损坏，这对将在DT聚变反应中产生的非常高能量的阿尔法粒子来说尤其是一个问题。高场紧凑型球形托卡马克ST40是一种紧凑型装置，设计用于在高达3T(远高于目前的STS)和2 mA等离子体电流的环向磁场下运行，从而确保NBI系统产生的快离子受到限制。由于聚变功率与磁场的4次方成比例，这些独特的能力允许实现性能的改进，并解决与在高场中达到盈亏平衡条件(即，当产生的聚变功率等于输入功率时)的能力有关的关键问题，如最近的结果所示，这在目前的其他STS中是无法实现的。快离子的限制预计会因等离子体的不稳定性而减少，例如Alfvén模，而第一轨道损失估计会导致DT反应中相当大一部分粒子的损失，这两者都会导致所产生的聚变功率的减少。为了开发减少快速离子再分配和损失的场景，测量它们在ST40中的限制程度是至关重要的。目前，ST40配备了一个用于监测总中子产额的激活系统和一个用于准直观测快离子群的液体闪烁体。将安装2.45 MeV和14.1 MeV中子(分别来自DD和DT反应)的钻石探测器。该项目将分两个阶段进行。第一阶段将侧重于所有三种中子诊断方法(激活系统、液体闪烁体和钻石)的特性，以综合描述具有NBI加热的ST40的不同运行状态。在这一阶段，将使用NRESP、MCNP、FLUKA和GEANT4等辐射传输程序对探测器的响应函数进行正确建模。然后，将在能够提供相关中子场的校准设施中评估响应函数，例如高产额、14 MeV Nessa(UppSAla中子源)设施和NPL。这一阶段的关键也是发展基于现场可编程门阵列的在线数据采集和处理。第二阶段将专门使用TRANSP/NUBEAM、ASCOT/AFSI和DRESS等等离子体传输程序模拟中子发射率，然后将其与实验观测结果进行比较。这将允许验证和约束预测

In fusion plasmas, fast ions have energies much higher than the thermal plasma background. Fast ions are generated by external auxiliary heating such as Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating(ICRH) or by the fusion reactions themselves. In the former cases, fast ions are hydrogen isotopes with energies in the range from tens of keVs up to a few MeVs. Fusion reactions produce, in addition to hydrogen isotopes, alpha particles with energies in the MeV range. Fast ions play an important role in heating the plasma, maintaining the high temperatures necessary to sustain the fusion reactions and crucial in achieving a burning plasma. NBI heating is also important for current drive, that is for long pulse operation of tokamaks beyond the inductive regime and therefore for the realization of a fusion reactor. Confining fast ions in the plasma for time long enough so that they can transfer their energy to the background plasma is therefore crucial for achieving the goal of a power plant based on thermonuclear fusion reactions. However, fast ion confinement is degraded by plasma instabilities some of which are triggered by the fast ion themselves. In this case, energy exchange between the fast ions & the instabilities results in the redistribution and loss of fast ions, ultimately reducing the performances of fusion reactors. Furthermore, the loss of fast ions in the plasma can result in the damage of the reactor first wall, an issue particularly for the very energetic alpha particles that will be produced in DT fusion reactions.The high-field compact spherical tokamak ST40 is a compact device designed to operate at high toroidal magnetic fields up to 3 T (much higher than present day STs) and 2 MA plasma current thus ensuring the confinement of the fast ions generated by the NBI system. These unique capabilities allow to achieve improvement in the performances thanks to the scaling of the fusion power with the 4th power of the magnetic field and to address key questions relative to the capability of reaching break-even conditions (i.e. when the amount of produced fusion power equals the input power) in high fields that are not achievable in other present day STs as demonstrated in recent results. The confinement of the fast ions is expected to be reduced by plasma instabilities such as the Alfvén modes while first orbit losses are estimated to lead to the loss of a significant fraction of particles from DT reactions and both causes the reduction in the generated fusion power. In order to develop scenarios with reduced fast ions redistribution & losses it is essential to measure how well they are confined in ST40. At present,ST40 is equipped an activation system to monitor to the total neutron yield and a liquid scintillator for the collimated observation of the fast ion population. Diamond detectors for 2.45 and 14.1 MeV neutrons (from DD and DT reactions respectively) will be installed.This project will be carried out in two phases. The first phase will be focused on the characterization of all three neutron diagnostics (activation system, liquid scintillators and diamonds) to provide an integrated characterization of different operating regimes in ST40 with NBI heating. In this phase, the correct modelling of the detectors' response functions will be carried out using radiation transport codes such as NRESP, MCNP, FLUKA and GEANT4. The response functions will then be evaluated in calibration facilities able to provide the relevant neutron fields such as the high yield, 14 MeV NESSA (NEutron Source inUppSAla) facility and NPL. Crucial in this phase is also the development of on-line data acquisition and processing based on FPGAs. The second phase will be dedicated to the modelling of the neutron emissivity using plasma transport codes such as TRANSP/NUBEAM, ASCOT/AFSI and DRESS which can then be compared to the experimental observations. This would allow to verify & constrain the predictions