[en] Highlights: • A non-evaporable getter pump using the ZAO NEG alloy was installed on TCV and tested. • The pump performance was determined from in-situ pumping speed measurements. • The results of the tests indicate that current getter pump technologies are well suited to tokamak operations. A non-evaporable getter pump using the ZAO alloy was installed on TCV and operated during the 2019 experimental campaign. The pump performance, determined from in-situ pumping speed measurements, indicates that current getter pump technologies are well suited to tokamak operations with high power exhaust plasmas, and provide an interesting alternative for applications in fusion experiments.

[en] Turbulent fast ion transport has been investigated in astrophisycal, laboratory and fusion plasmas. When gyro- and drift-averaging across plasma structures manifest as non-markovian and non-local effects, this results in generally non-diffusive transport. The intermittency of the formation of these plasma structures, such as blobs, can potentially be reflected in the transport of the fast ions as well. In the TORPEX basic plasma device, a toroidal beam of suprathermal Li-6 ions is injected into electrostatic plasma turbulence. Conditional sampling techniques confirm turbulent E × B-drifts as physical driving mechanism of radial fast ion transport, which features sub-, super- or quasi-diffusive regimes depending on the fast ion energy and propagation time. To address the question of how far local intermittency is associated with each regime, we analyze characteristics of time-intermittency on an extensive set of local fast ion time-series across all observed regimes. Modeling the time-average fast ion profiles as the result of a meandering smaller instantaneous beam allows us to predict the skewness of such time-series based on their time-average. Comparisons with the skewness of simple two-valued time-series can yield relative indications towards certain transport regimes in our specific system, based on the differences in the size of the instantaneous fast ion beam. (paper)

[en] Assuming hydrogen is charge neutral, CPT invariance demands that antihydrogen also be charge neutral. Quantum anomaly cancellation also demands that antihydrogen be charge neutral. Standard techniques based on measurements of macroscopic quantities of atoms cannot be used to measure the charge of antihydrogen. In this paper, we describe how the application of randomly oscillating electric fields to a sample of trapped antihydrogen atoms, a form of stochastic acceleration, can be used to place experimental limits on this charge. (paper)

[en] The operational range and the reactor relevance of the TCV experiments are being enhanced by two sets of major upgrades. The first includes the installation of neutral beam injection (NBI) and new electron cyclotron (EC) auxiliary heating sources, to reach ITER relevant beta values and vary the electron to ion temperature ratio. A 15–30 keV, 1 MW tangential NBI system has been operational on TCV since 2015. A second beam of 1 MW, 50–60 keV ion energy, also aligned tangentially but opposite to the first beam, is foreseen to approach beta limits, vary the applied torque through zero and probe suprathermal ion physics. For the EC power, two 0.75 MW gyrotrons at the second harmonic have been installed. The next step will add two 1 MW dual frequency gyrotrons, one of which is currently being commissioned. These heating upgrades will increase the total available power for high-density plasmas from 1.25 MW to 5.0 MW. The rest of the upgrade consists of installing an in-vessel structure to form a divertor chamber of increased closure, to reach higher neutral divertor densities and impurity compression and thereby extend TCV divertor regimes toward more reactor relevant conditions for conventional and advanced divertor configurations. Graphite gas baffles will be installed inside the TCV vessel to delineate divertor and main chamber regions. The first set of baffles features 32 tiles on the high and 64 tiles on the low-field side, with geometry guided by simulations performed using the SOLPS-ITER code. The baffles are expected to be effective for a wide range of divertor configurations, including snowflake and super-X divertors, yet maintain plasma close to the inner wall for improved passive stabilization. The baffle dimensions may be varied in the future to modify the divertor closure. Control of the plasma, neutral and impurity densities will be achieved by a combination of toroidally distributed gas injection valves and impurity seeding, and a possible addition of cryo-condensation pumps. Significant diagnostic developments will be undertaken, to better characterize the divertor plasma, measure power and particle deposition at the strike points, and, specifically, improve our physics understanding of the detachment process. (paper)

[en] In magnetically confined fusion devices, the use of millimeter waves (mmw) at the electron cyclotron (EC) frequencies ranges from plasma diagnostics to plasma heating, current drive and core confinement preservation. For large tokamaks such as ITER, numerical simulations and analytical estimates suggest that plasma edge turbulence could significantly broaden the EC-beam, possibly preventing tearing modes stabilization at the designed power levels. We report measurements of mmw-beam scattering by plasma turbulence in the TCV tokamak. A mmw-Gaussian beam is injected from the top of the device and the transmitted power is measured at the bottom. We show that the measured plasma density fluctuations in the upper part of the scrape-off layer (SOL) are the cause of fluctuations of the transmitted mmw-power. A full-wave model based on COMSOL multiphysics is presented and compared against the wave-kinetic-equation solver WKBeam in a TCV case. Using the SOL turbulence simulations from the GBS code, comparison between the scattering effect on the mmw-beam with both the full-wave simulations and the experiments are ongoing. We also present experimental observations of rapid changes in the transmitted power caused by ELMs in ELMy H-mode plasma. (paper)

[en] Precision symmetry tests at low energies have played important roles in our understanding of fundamental interactions. ALPHA (Anti-hydrogen Laser Physics Apparatus) is an international project located at CERN, whose prime goal is to perform tests of CPT symmetry on anti-hydrogen. By precise spectroscopic comparisons of well-studied atomic hydrogen with its antimatter counterpart, we hope to probe indirectly physics at or beyond the Planck scale. High precision tests with anti-hydrogen would likely require samples of trapped anti-hydrogen atoms. While substantial numbers of anti-hydrogen atoms have been produced in several experiments, their trapping has not yet been achieved. It is the initial goal of ALPHA to demonstrate stable trapping of anti-hydrogen. In the ALPHA apparatus, cold plasmas of typically 3x10⁴ antiprotons and of 4x10⁶ positrons are mixed in a Penning trap to form cold anti-hydrogen. A multipolar magnetic trap of depth 0.5 Kelvin is superimposed on the Penning trap to confine the anti-atoms. The ALPHA experiment features a 30,000 channel silicon vertex detector in order to identify annihilations of the expected small number of trapped atoms. Since its first beam in 2006, ALPHA has been making substantial progress towards trapping of anti-hydrogen. In 2009, we have reached, for the first time, the detection sensitivity and trap conditions for which observation of anti-hydrogen trapping can be realistically expected. In this talk, we will present results from that run, where we have observed first candidate events. The details of the detector analysis, as well as possible sources of background will be discussed. Prospects for precision CPT tests with trapped anti-hydrogen will also be discussed. (authors)

[en] The Tokamak à Configuration Variable (TCV) tokamak is in the midst of an upgrade to further its capability to investigate conventional and alternative divertor configurations. To that end, modular and removable gas baffles have been installed to decrease the coupling between the divertor and the plasma core. The baffles primarily seek to suppress the transit of recycling neutrals to closed flux surfaces. A first experimental campaign with the gas baffles has shown that the baffled divertor remains compatible with a wide range of configurations including snowflake and super-X divertors. Plasma density ramp experiments reveal an increase of the neutral pressure in the divertor by up to a factor ×5 compared to the unbaffled divertor and thereby qualitatively confirm simulations with the SOLPS-ITER code that were used to guide the baffle design. Together with a range of new and upgraded divertor diagnostics, the baffled TCV divertor is now used to validate divertor models for ITER and next step devices with particular emphasis on geometric variations. (letter)

[en] Fast-particle driven Alfvén Eigenmodes have been observed in low-collisionality discharges with off-axis neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and a reduced toroidal magnetic field. During NBI and ECRH, toroidicity induced Alfvén Eigenmodes (TAEs) appear in frequency bands close to 200 kHz and energetic-particle-induced geodesic acoustic modes (EGAMs) are observed at about 40 and 80 kHz. When turning off ECRH in the experiment, those beam-driven modes disappear which can be explained by a modification of the fast-ion slowing down distribution. In contrast, coherent fluctuations close to the frequency of the beam-driven TAEs are present throughout the experiment. The modes are even observed during ohmic plasma conditions, which clearly demonstrates that they are not caused by fast particles and suggests an alternative drive, such as turbulence. The mode-induced fast-ion transport has been found to be weak and marginal in terms of the fast-ion diagnostic sensitivities. Measurements of the plasma stored energy, neutron rates, neutral particle fluxes and fast-ion D-alpha spectroscopy show good agreement with neoclassical modelling results from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and without ECRH and therefore with and without the modes. Instead, a significant level of charge exchange losses are predicted and observed which generate a bump-on-tail fast-ion distribution function that can provide the necessary free energy to EGAMs. (paper)

[en] We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise CPT test on trapped antihydrogen is a long-standing goal.

[en] We demonstrate controllable excitation of the center-of-mass longitudinal motion of a thermal antiproton plasma using a swept-frequency autoresonant drive. When the plasma is cold, dense, and highly collective in nature, we observe that the entire system behaves as a single-particle nonlinear oscillator, as predicted by a recent theory. In contrast, only a fraction of the antiprotons in a warm plasma can be similarly excited. Antihydrogen was produced and trapped by using this technique to drive antiprotons into a positron plasma, thereby initiating atomic recombination.