Off-axis neutron camera for MAST Upgrade

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发挥着重要作用。NBI加热对于电流驱动也很重要，即托卡马克在感应区之外的长脉冲运行，因此对于实现聚变反应堆也很重要。因此，将快离子限制在等离子体中足够长的时间，使它们能够将能量转移到背景等离子体中，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，快离子之间的能量交换和不稳定性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堆第一壁的损坏，这对于ITER和DEMO中将产生的高能α粒子来说尤其是一个问题。MAST升级配备了轴上和离轴中性束注入器。轴上NBI在MAST Upgrade赤道平面上沿切向注入快离子，导致在等离子体核心中具有峰值的快离子沉积轮廓。已知由轴上NBI系统产生的大的快离子压力梯度驱动等离子体不稳定性，其降低快离子约束并因此降低等离子体性能。离轴NBI也被切向注入，但在赤道平面上方约60 cm，从而加宽了快离子沉积轮廓并减小了抑制这些等离子体不稳定性的驱动的快离子压力梯度。这一概念的验证在MAST上得到了证明，并得到了MAST升级的初步结果的证实。在致力于研究快离子的广泛诊断中，中子诊断提供了关于受限快离子的信息，这要归功于在MAST升级上，中子发射主要由束热核反应控制的事实。MAST升级项目上的中子发射是通过最近安装的6通道准直中子通量监测器测量的，该监测器沿赤道轴的切线方向观察。该项目将重点关注多通道准直中子通量监测器的设计、开发、建造、安装和调试，该监测器专用于测量离轴中子发射率，其中快离子由离轴NBI沉积。对初步观察结果的分析预计将是该项目的一部分。

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 and the instabilities result 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 ITER and DEMO.MAST Upgrade is equipped with an on-axis and an off-axis neutral beam injector. The on-axis NBI, injects fast ions on MAST Upgrade equatorial plane in tangential direction resulting in fast ion deposition profile that is peak in the plasma core. The large fast ion pressure gradient resulting from the on-axis NBI system is known to drive plasma instabilities that reduce the fast ion confinement and thus the plasma performances. The off-axis NBI is also injected tangentially but approximately 60 cm above the equatorial plane, thus broadening the fast ion deposition profile and reducing the fast ion pressure gradient suppressing the drive of these plasma instabilities. Proof-of-concept of this was demonstrated on MAST and confirmed by preliminary results on MAST Upgrade.Among the wide range of diagnostics dedicated to the study of fast ions, neutron diagnostics provide information on the confined fast ions thanks to the fact that on MAST Upgrade, neutron emission is dominated by beam-thermal nuclear reactions. Neutron emission on MAST Upgrade is measured by a recently installed 6-channels collimated neutron flux monitor looking in tangential direction on the equatorial axis.The project will focus on the design, development, construction, installation and commissioning of a multi-channel, collimated neutron flux monitor dedicated to the measurement of the neutron emissivity off-axis, where the fast ions are being deposited by the off-axis NBI. Analysis of initial observations are expected to be part of this project.