Speaker
Description
From the point of view of diagnostics developments, for many years JET diagnostics has been upgraded in order to provide adequate support for the scientific exploitation of the upcoming deuterium-tritium campaign, DTE2, with particular attention to the experimental and operational conditions expected during deuterium-tritium campaigns. Diagnostic capabilities relevant to burning plasmas conditions have been specifically targeted with the focus mainly on fast ions, instabilities, neutron, gamma, ion temperature and operations support.
The imaging systems at JET have become critical for operations support but the neutron yield during 50-50 D-T operation is likely to cause irreversible damage to the cameras or in the best case scenario white out the image during plasma operation. Shielding these cameras from the neutrons is the only way to keep these diagnostics functional during D-T operation. This has been achieved by relocating the cameras to a low radiation environment behind the biological shield by relaying the emitted light out of the torus hall (which involves path lengths of the order of 40m) through a carefully designed optical system. This has been done for a wide-angle view{1} and one divertor view from the top of the machine{2}, carrying information in the visible, near-Infrared and mid-Infrared wavelengths. A wide angle view image obtained using this optical relay and acquired by a visible camera can be seen in Figure 1. The same system is also used for protection of JET’s inner wall, by providing detection capabilities for dangerous plasma events that can lead for instance to melting of Be tiles (Figure 2). In ITER several optical diagnostics will have relays with similar lengths making the operation of such system on JET especially relevant.
The effects of the different isotopic composition on the plasma turbulence and associated effects (such as the changes in ELM behaviour) will be one of the main topics of the D-T scientific campaigns. Reflectometry measurements in the plasma core are paramount to better understand the mechanisms driving the turbulence and the related transport in JET advanced plasma scenarios. A diagnostic upgrade project has delivered two additional frequency bands to the correlation reflectometer to allow the measurement of density fluctuations inner in the plasma, i.e., in the core region and even up to the high field side region in the most favourable cases. This is useful to study the radial electric field (Er) which plays an important role in critical areas of tokamak science such as: L-H transition physics and H-mode pedestal structure, turbulence suppression through equilibrium E×B shear and understanding of core rotation, momentum transport and intrinsic rotation. Fast Er measurements also allow the characterization of zonal flows and geodesic acoustic modes. During past scientific campaigns and in certain vertical target plasma shapes, Doppler backscattering measurements have been achieved, but a new microwave access front-end now enables Er measurements on a routine basis{3}.
Accurate acquisition of the currents flowing in the 9 poloidal field circuits of JET has been upgraded and is now obtained with an open-loop, open-path, Hall effect system. The poloidal field currents acquisition system, better temporal resolution now available is useful since a strong coupling between the plasma and the coils means that high currents can be induced during a disruption. The early phase of the current quench lasts less than 10 milliseconds at JET and sees the rise of electromagnetic forces as well as runaway electron beams. The better time resolution for P1 current measurement improves understanding of the conditions in which runaway electrons are created. Another important motivation to acquire the poloidal field power supplies current, with high precision and good temporal resolution (>2.5kHz), is to understand how the central solenoid and the other poloidal field currents drive the recovery of the plasma current between ELMs and to understand better how the plasma current suddenly changes as a consequence of an ELM.
On JET the study of Toroidal Alfvén Eigenmodes (TAEs), has been for the last two decades of high interest. Instabilities in the Alfvén frequency range can be driven by fast ions (including fusion generated alpha particles) and can lead to their spatial redistribution and eventually fast radial transport that can affect the fusion performances and could damage the first wall of future fusion reactors. The understanding of the mechanisms of the mode stability is therefore of paramount importance for ITER and can also be used to control the alpha particle population itself. AEs can be excited by means of in-vessel antennas and fast ions can be produced by additional heating like ICRH or NBI injections. A unique, state of the art, detection system allows in real-time the detection of TAEs of specific toroidal mode number(s) in the range n=0-15, the measurement of their damping rate and amplitude and their tracking. A new generation of amplifiers{4} allows a more reliable operation and provides the diagnostic with the potential to further increase the antenna current, hence TAE modes excitation (see Figure 3){5}.
Furthermore as JET technical characteristics makes it a unique device worldwide to address specific ITER research plan priorities, a set of diagnostic enhancements is currently in the design phase, covering the development of a state of the art Laser Induced Desorption (LID) system in support to the ITER diagnostic programme and upgrading capabilities which are required for ITER relevant JET scientific exploitation. Specifically, these efforts intend to provide tritium retention monitoring using laser induced desorption combined with mass spectrometry, increased imaging and bolometric coverage for a second shattered pellet injector. An overview of status and scope of these projects will also be presented.
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission
*See the author list of E. Joffrin et al. accepted for publication in Nuclear Fusion Special issue 2019,
https://doi.org/10.1088/1741-4326/ab2276
{1} M. Clever et al., Fusion Eng. Des. 88, 1342 (2013).
{2} I. Balboa et al., Rev. Sci. Instrum. 83, 10D530 (2012).
{3} Hillesheim J. C. et al 2016 Phys. Rev. Lett. 116 065002.
{4} P. Puglia et al., Nucl. Fusion 56, 112020 (2016).
{5} V. Aslanyan et al., EPS Conf. (2017)
| Affiliation | Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa |
|---|---|
| Country or International Organization | Portugal |
