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，快离子具有比热等离子体背景高得多的能量。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）或由聚变反应本身产生。在前一种情况下，快离子是氢同位素，其能量范围从几十keV到几MeV。聚变反应除了产生氢同位素外，还产生能量在MeV范围内的α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以及实现燃烧等离子体方面起着重要作用。NBI加热对于电流驱动也是重要的，即对于托卡马克的长脉冲操作超过感应制度，因此对于实现聚变反应堆。因此，将快速离子限制在等离子体中足够长的时间，以便它们可以将其能量转移到背景等离子体，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，快离子约束退化的等离子体不稳定性，其中一些是由快离子本身触发。在这种情况下，快离子和不稳定性之间的能量交换导致快离子的重新分布和损失，最终降低聚变反应堆的性能。此外，等离子体中快离子的损失会导致反应器第一壁的损坏，高场紧凑型球形托卡马克ST 40是一种设计用于在高达3 T的高环向磁场下运行的紧凑型装置（比现在的ST高得多）和2 MA等离子体电流，从而确保了由NBI系统产生的快离子的约束。这些独特的能力允许实现性能的改善，这要归功于聚变功率与磁场的四次方的缩放，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）的能力相关的关键问题，如最近的结果所示，这在其他当今ST中是无法实现的。快离子的约束预计将减少等离子体的不稳定性，如阿尔文模式，而第一轨道损失估计导致损失的DT反应的粒子的显着部分，都导致减少产生的聚变功率。为了开发减少快速离子再分布和损失的方案，必须测量它们在ST 40中的限制程度。目前，ST-40配备了一个用于监测中子总产额的活化系统和一个用于准直观测快离子布居数的液体闪烁体。将安装探测2.45和14.1 MeV中子（分别来自DD和DT反应）的金刚石探测器。第一阶段将集中在所有三个中子诊断（活化系统，液体制冷剂和金刚石）的特性，以提供在NBI加热ST 40不同的操作制度的综合特性。在这一阶段，将使用NRESP、MCNP、FLUKA和GEANT 4等辐射输运代码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（如高产率、14 MeV NESSA（UppSAla中子源）设施和NPL）中对响应函数进行评估。这一阶段的关键也是基于FPGA的在线数据采集和处理的发展。第二阶段将致力于使用等离子体传输代码，如TRANSP/NUBEAM，阿斯科特/AFSI和DRESS的中子发射率的建模，然后可以与实验观测进行比较。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