This paper summarizes the status of the COMPASS tokamak, its comprehensive diagnostic equipment and plasma scenarios as a baseline for the future studies. The former COMPASS-D tokamak was in operation at UKAEA Culham, UK in 1992-2002. Later, the device was transferred to the Institute of Plasma Physics of the Academy of Sciences of the Czech Republic (IPP AS CR), where it was installed during 2006-2011. Since 2012 the device has been in a full operation with Type-I and Type-III ELMy H-modes as a base scenario. This enables together with the ITER-like plasma shape and flexible NBI heating system (two injectors enabling co-or balanced injection) to perform ITER relevant studies in different parameter range to the other tokamaks (ASDEX-Upgrade, DIII-D, JET) and to contribute to the ITER scallings. In addition to the description of the device, current status and the main diagnostic equipment, the paper focuses on the characterization of the Ohmic as well as NBIassisted H-modes. Moreover, Edge Localized Modes (ELMs) are categorized based on their frequency dependence on power density flowing across separatrix. The filamentary structure of ELMs is studied and the parallel heat flux in individual filaments is measured by probes on the outer mid-plane and in the divertor. The measurements are supported by observation of ELM and inter-ELM filaments by an ultra-fast camera.

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.

Axisymmetric geodesic acoustic mode (GAM) oscillations of the magnetic field, plasma potential and electron temperature have been identified on the COMPASS tokamak. This work brings an overview of their electromagnetic properties studied by multi-pin reciprocating probes and magnetic diagnostics. The n = 0 fluctuations form a continuous spectrum in limited plasmas but change to a single dominant peak in diverted configuration. At the edge of diverted plasmas the mode exhibits a non-local structure with a constant frequency over a radial extent of at least several centimeters. Nevertheless, the frequency still reacts on temporal changes of plasma temperature caused by an auxiliary NBI heating as well as those induced by periodic sawtooth crashes. Radial wavelength of the mode is found to be about 1-4 cm, with values larger for the plasma potential than for the electron temperature. The mode propagates radially outward and its radial structure induces oscillations of a poloidal E × B velocity, that can locally reach the level of the mean poloidal flow. Bicoherence analysis confirms a non-linear interaction of GAM with a broadband ambient turbulence. The mode exhibits strong axisymmetric magnetic oscillations that are studied both in the poloidal and radial components of the magnetic field. Their poloidal standing-wave structure was confirmed and described for the first time in diverted plasmas. In limited plasmas their amplitude scales with safety factor. Strong suppression of the magnetic GAM component, and possibly of GAM itself, is observed during co-current but not counter-current NBI.

Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n  =  2 RMP maintaining good confinement . Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes.

No abstract

This contribution presents the experimentally observed edge plasma evolution during limit cycle oscillations (LCO) measured with a new Langmuir and ball-pen multi-pin probe head at the COMPASS tokamak. The observed LCO regime modulates the intensity of density fluctuations , radial electric field Er and intensity of emission with a frequency 3–5 kHz. The density fluctuations grow after Er decreases in strength which appears to be strongly correlated with the evolution of the pressure gradient . The magnetic signature of the LCO shows a left–right asymmetry with propagation from the low to high field side. High-frequency (above 100 kHz) precursor-like oscillations are observed as well.

Runaway electrons (REs) as one of the yet unsolved threats for ITER and future tokamaks are a topic of intensive research at most of the European tokamaks. The experiments performed on COMPASS are complementary to the experiments at JET and MST (Medium-Size Tokamaks), building on the flexibility of the diagnostics setup and low safety constraints at this smaller device. During the past couple of years two different scenarios with the RE beam generation triggered by gas injection have been developed and investigated. The first one is based on Ar or Ne massive gas injection (MGI) into the current ramp-up phase leading to a disruption accompanied by runaway plateau generation [1], while the second uses smaller amounts of gas in order to get runaway current dominated plasmas [2]. The successful generation of the beam in the first scenario depends on various parameters, including the toroidal magnetic field. The generated beam is often radially unstable, and the stability seems to be a function of various parameters, including the value of current lost during the CQ. The second scenario is much more quiescent, with no observable fast current quench and it is highly reproducible. This allows to reasonably diagnose the beam phase and also to apply secondary injections or resonant magnetic perturbations (RMP) to assist the decay of the beam. In this regard, interesting results have been achieved using secondary deuterium injection into a runaway electron beam triggered by Ar or Ne. The current of the RE beam can be controlled at a fixed value, however only using relatively high loop voltage. The radial position is not fully controlled and the request for the stabilising field is changing independently on plasma current which implies that the RE energy plays a key role in correct approach to beam position control. The experiments with elongated beams were also carried out. Last but not least it seems that Ar and Ne behave differently in terms of radiated power and HXR intensity during the beam decay. 1.

No abstract