Partial detachment is the desired regime for the baseline burning plasma scenario in ITER and next-step devices, as it allows to convert the majority of the energy carried by charged particles through the scrape-off-layer (SOL) is dissipated and thus deposition of localized heat fluxes in the divertor region is avoided. The COMPASS tokamak is equipped with an open divertor and has a relatively short connection length, both factors being unfavourable for access to detachment. As such, it only allows to approach naturally detached operation at very high line-averaged densities (> 10 20 m −3), which are incompatible e.g. with maintaining the ELMy H-mode regime. In order to achieve the detachment at lower densities, impurities (such as nitrogen) should be injected into the plasma in the divertor region. A series of experiments with impurity injection in the range of 1-9×1020 molecules per second at different locations in the divertor were performed with the aim to cool the plasma and influence the particle and heat transport onto the divertor targets and provoke partial detachment. Previously reported results [M. Komm et al, EPS 2017, P1.118] were largely extended by injection of nitrogen at the outer divertor target.

Divertor Thomson scattering (DTS) and laser-induced fluorescence (LIF) are both laser aided diagnostics well suited to combination with common probing and collecting optics that are the most sophisticated and expensive part of any ITER optical diagnostic system. The combination of DTS and LIF are used for simultaneous measurement of local electron (Te, ne), ion (Ti, nHeII) and atom (nHeI, nH(D,T)) parameters and provide basic information on rates of electron and ion processes to allow basic understanding of the physics of divertor plasma detachment. The measured parameters permit the calculation of rates of ionization and recombination using Te, ne, Ti, ni, nHeI and nH(D,T); emission intensity—Te, ne, ni, nHeI and nH(D,T); frictional force of the plasma flow due to collisions with neutrals—Ti, ni, T0, nHeI and nH(D,T) and pressure of the incoming plasma flow—Te, ne, Ti and ni. The paper discusses the benefits of DTS and LIF integration, suggests new approaches to the estimation of DTS capability, LIF implementation and possibilities for further diagnostic development.

The reduction of the incident heat flux onto the divertor will be a necessity for the future thermonuclear reactors. Impurity seeding is recognized as an efficient way to achieve the partial detachment regime, which allows to dissipate a large fraction of power flux by radiation. This paper presents a heat flux real-time feedback system (RTFS) based on impurity seeding controlled by a combined ball-pen and Langmuir probe divertor array in the COMPASS tokamak. A number of features of the system have been studied, such as the type of impurity, seeding location, constants used in the real-time controller and the diagnostic selections. A detailed description of the designed RTFS and the results of the implementation are presented. The findings confirm the applicability of the RTFS for reduction and control of the divertor heat fluxes. Another important implication of this research is the ability of installing such systems in next-step devices.

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