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«Association EURATOM / IPP.CR I N S T I T U T E O F P L A S M A P H Y S I C S, v.v. i. ACADEMY OF SCIENCES OF THE CZECH REPUBLIC ANNUAL REPORT ...»

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The so-called ball-pen probe (BPP) was used for a direct determination of the plasma potential (see Fig. 1, probe head after measurements). Such measurements, combined with the results of other probes, would allow the reliable and fast determination of transport parameters and would support and further facilitate the development of a more comprehensive picture of edge plasma phenomena, in particular with respect to the profile of the plasma potential and thence the electric field in this region.

–  –  –

Part II - PHYSICS As we can see (Fig. 1), the surface of the BN screen around and in between the probes was damaged probably due to high energy radiation. For the next campaign a small modification of the carbon shield is suggested to protect the front part of the boron nitride of the BPP probe from radiation and plasma flux. In the campaign, during several shots the floating potential and its fluctuations (with sampling frequency f = 0.5 MHz) of the four collectors was determined and compared, see Fig. 2.

Fig. 2: The temporal evolution of the potential of two ball-pen probes (h = - 0.5, -2 mm). All ball-pen probes Achieved milestones have been inserted four times to the deuterium edge plasma under different conditions.

(h=-3, -2, -1, -0.5 mm) :

The probes have measured potential in H-mode and ohmic regime as seen from Dα signal. The reduction of the fluctuations of Dα signal around 2.7s is caused by change of the gas puffing.

It was found that the four values of the floating potential are almost equal, which is a strong indication that the floating potential of all four collectors is indeed practically equal to the plasma potential, while the retraction depth of the collector did not matter much for the determination of this important parameter. However, all probes have provided different positive signals when the probe head has been located deep in the limiter shadow. These signals can be caused by the presence of high energetic ions due to ion losses during the application of NBI (5 MW).

Unfortunately, the two Langmuir pins which were also mounted on the probe head (see Fig. 1) did not deliver any results. The first Langmuir probe should record the current-voltage characteristic to determine the electron temperature Te. The other Langmuir pin should have been negatively biased to record the ion saturation current from which the density fluctuations could have been derived. Another possibility envisaged was to record the cold floating potential Vfl, so that for comparison Te could have been determined also by turning around the above mentioned relation Te = (Φpl – Vfl)/ln(Ies/Iis), where Φpl would have been taken from one of the BPPs.

–  –  –

In collaboration with:

V. Rohde, P. Lang, Association EURATOM IPP, IPP Garching, Germany Experiments in many fusion devices like have shown an ability of the edge plasma biasing to influence plasma turbulences and edge density profile via mechanism of the E×B velocity shear.

A reduction in poloidal electric field, temperature, and density fluctuations across the shear layer leads to a reduction of the anomalous conducting and convection heat fluxes resulting in an energy transport barrier. On the other hand, such temporarily and spatially well-defined induced effect would be used for an edge plasma perturbation as a potential candidate for the ELM triggering. Some experimental indications have been found for a potential of the edge plasma biasing impacting on the ELM behaviour [1]. The intention of this explorative approach is to establish a well-defined “perturbation” by the pulsed edge plasma biasing and to search for correlations with the ELM behaviour.

The ELM pacing experiments were realized on the ASDEX-Upgrade tokamak in several shots at BT ~ - 2.1-2.5 T, Ip ~ 0.6-0.8 MA, ne ~ 4-8*1019 m-3 and with a different auxiliary heating. As a biasing electrode, the midplane manipulator (MEM) with several carbon tips inserted into the massive carbon envelope, usually serving for the probe measurements, was used, see Fig.1.

The tips were biased using four KEPCO sources triggered by a single signal generator. Biasing voltage was set to -100 V and applied as short rectangular pulses (millisecond time scale) at different frequencies close to an ELM natural frequency. A depth of the MEM insertion was chosen to be acceptable for the MEM according to an expected heat load, and set stepwise to be from the limiter position up to 5 mm far from the separatrix. In the realized shots, only a later part of each shot was used for a single MEM insertion, lasting Fig. 1. Biasing carbon tips of the typically 200 ms. There an ELM activity was monitored midplane manipulator head (without using the Hα signals from the inner and outer divertor a carbon envelope) on the ASDEXlegs and using magnetic coil signals. At the same time, Upgrade tokamak.

changes of the stored energy in plasma were measured.

The first analysis of measured data shows a change of natural ELM frequency during a period of the biased MEM insertion and an occurrence of new smaller satellite ELMs. The data evaluation is in progress.

References:

[1] M. Tsalas, et al., J. Nuc. Mater. 337-339 (2005) 751-755 Part II - PHYSICS Determination of reflection characteristics of small hydrocarbon ions of low incident energies in collisions with with room-temperature and heated carbon and tungsten surfaces Z. Herman, J. Žabka, A. Pysanenko J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the CR





In collaboration with:

T. D. Märk and his research group (Institute for Ion and Applied Physics, University of Innsbruck, Innsbruck), Association EURATOM-ÖAW, Austria W. Schustereder, MPI Garching, Association EURATOM, Germany The aim of this research is investigation of interactions between slow ions and surfaces of materials relevant to plasma-wall interactions in fusion devices. The objective is - on the one hand - to obtain data on the survival of slow ions in surface collisions, product ion energy losses in ion-surface interactions, dissociative and chemical processes at surfaces, and information on collision kinematics, using the special ion-surface scattering apparatus available in Prague, and

- on the other hand – to determine reflection characteristics of hydrocarbon ions, namely the sticking coefficient of hydrogen (deuterium) on various surfaces.

–  –  –

occurred at these low incident energies partly because of initial internal energy of the projectile ion and it took place after the interaction with the surface, as manifested in similar velocity distributions of both CD5+ and CD3+. Analogous data were obtained for the incident ions CD3+.

Because of a low survival probability of the incident ion, the angular data had to be corrected for a contribution from background gas-phase scattering of the projectile ion and for a fraction of fast ions deflected in front of the surface by local surface micro-charges (this effect was visible on HOPG only at very low incident energies). The scattering data for the reactive radical cation CD4+ showed contributions of inelastic and dissociative scattering of the incident projectile ion and also of reactive scattering from interaction of CD4+ with the surface material. The main reaction was H-atom transfer to CD4+ from hydrocarbons, adsorbed on room-temperature carbon surface, and at 10 eV formation of C2 hydrocarbon ions in reactions of CD4+ with terminal CH3groups of surface hydrocarbons. The scattering results were used to estimate the effective surface mass involved in the inelastic collisions. The values of it correspond to the mass of one or several terminal hydrocarbon groups of the hydrocarbons adsorbed on the room-temperature carbon surface [1,3].

Analogous data for C2D4+ interaction with room-temperature HOPG are being analyzed and will be published soon.

Fragmentation of polyatomic cations and dications (C7Hn+/2+, n=6,7,8) with hydrocarbon-covered surface was investigated over the incident energy range 5-50 eV to estimate the role of the projectile charge on energy partitioning in surface collisions [4].

2. Reflection properties of deuterated hydrocarbon ions Surfaces relevant to fusion devices (plasma-sprayed tungsten, carbon fiber composite, beryllium) were bombarded by deuterated hydrocarbon ions of energies up to 100 eV. Sticking coefficients to the surface of one of the collision products, deuterium molecules, were determined using the nuclear reaction analysis combined with the Rutherford back-scattering method (MPI Plasma Physics, Garching). Upon bombardment with CD3+ ions, the sticking coefficient of D2 to plasmasprayed tungsten (PSW) was found to be about 0.2, and 0.2-0.3 on carbon fiber composite (CFC), in both cases little dependent on the incident energy between a few and 100 eV [5].

References:

[1] A. Pysanenko, J. Žabka, F. Zappa, T.D. Märk, Z. Herman, Int. J. Mass Spectrom.

(accepted) [2]. J. Roithová, J. Žabka, Z. Dolejšek, Z. Herman, J. Phys. Chem. B, 106 (2002) 8293).

[3]. A. Pysanenko, J. Žabka, Z. Herman: XVI Symposium on Atomic, Cluster, and Surface Physics (SASP 2008), Les Diablerets, (CH), January 2008 (Extended abstract) [4]. L. Feketeová, T. Tepnual, V. Grill, P. Scheier, J. Roithová, Z. Herman, T.D. Märk, Int. J. Mass Spectrom. 265, 337-346 (2007) [5] B. Rasul, F. Zappa, N. Endstrasser, W. Schustereder, J. D. Skalny, Z. Herman, P. Scheier, T.D. Märk, 18th International Conference on Plasma-Surface Interactions, Toledo (E), 2008 (Book of Abstracts).

Part II - PHYSICS Experiments and modeling of fast particle generation at LH and ICRF heating

–  –  –

In collaboration with:

J. P. Gunn, L. Colas, A. Ekedahl, M. Goniche, M. Kočan, F. Saint-Laurent, Assoc. EURATOMCEA Cadarache A retarding field analyzer (RFA) was used during lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) experiments in the Tore Supra tokamak to measure the flux of supra-thermal particles emanating from the near field region in front of the antennas.

As it is well known, fast electrons can be generated in front of the LH grills, and they cause high thermal loads (hot spots) in the locations at which they impact. When these fast electrons escape from the space in front of the grill mouth, they leave the ions behind, and a region of a positive charge is created. This positive charge then possibly can accelerate also ions to energy of several hundreds eV, which might cause sputtering in locations magnetically connected to the grill mouth. According to a novel electron acceleration process recently proposed [1], fast electrons might be possibly generated also near the ICRF antennas: The electrons can gain energy in passing the near ICRF antenna rf field inhomogeneity, because of the temporal phase changes of the field, which do not average out on the electron quiver motion time scale. This mechanism can enhance the RF sheath potential, and consequently also the energy of ions, which are accelerated by this sheath potential.

Fig. 1. Scheme of the RFA fast particle measurements in front of the LH grill.

–  –  –

To find the fast ions, we considered as beneficial to measure in locations, where the fast electron beam is not too strong. Therefore, during the measurements, the detection and energy distribution measurements of the locally generated fast electrons and ions in front of LH and possibly also in front of the ICRF antennas was attempted to carry out.

For this, we estimated the fast ion drift in the above mentioned electrostatic field in front of the LH antenna, to find such possible locations, where the fast ions can be present, and in the same time, the fast electron energy is as low as possible. No fast ions were found, and we will not describe these measurements in detail. We concluded that, if the fast ions are really generated, they are masked by the fast electrons, and it is necessary to screen these electrons more strongly than it is now possible by the biasing of the RFA grids – the available voltage is too low.

By varying q from about 4 to about 7, a detailed poloidal – radial mapping of the fast electron beam were done, Fig. 1. The first RFA grid was biased to UG1=+200 V to repel thermal ions, 2nd grid voltage was UG2=-200 V to repel thermal electrons, PC2=1.5 MW (maximum available power). Similar mapping was also done for 0.8 MW. It can be seen that fast electrons extend up to the LCFS (“2nd beam”), what can have important consequences for the thermal loads and for computations of the LH SOL dissipated power. The next Fig. 2 shows growth of the radial beam intensity distribution with power. As the comparison of the 1st and 2nd beam (more near to the LCFS) shows, the mean collector current in the two beams is comparable at higher PLH.

Fig. 2. Variations of the beam intensity (collector current) as a function of LH power.

The 2nd beam (i.e., the unexpected part of the beam radially more distant from the grill mouth) amplitude varies much more strongly with power. The 2nd beam generation might perhaps be connected with the nonlinear production of higher spatial harmonics, studied in exploration of the so called spectral gap problem [2].

Part II - PHYSICS Fig. 3. Variations of the collector current with UG1 up to -1000 V.

Fig. 3 shows that the fast electrons with energy larger then 1keV remain to be present up to the LCFS for medium LH powers.

Further, new RFA experiments were also performed devoted to exploration of the flux tubes connected to ICRH Q5 antenna to determine sheath potential in presence of ICRF as a function of SOL density, and to determine whether there are any suprathermal electrons or ions using retarding field analyzer (RFA). Strong variations of the RFA slit and collector current were found in locations magnetically connected to the vicinity of the Q5 antenna, in correspondence with the variations of the plasma density and temperature near the antenna. Detailed mapping of the floating potential in the flux tubes connected to an ICRH antenna was done.



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