Efficient plasma heating schemes are a prerequisite for reaching fusion relevant temperatures in fusion machines. On the road to reaching ignition, non-activated scenarios - such as (3He)

This paper presents the system integrating the dedicated measurement and control electronic systems for Gas Electron Multiplierl (GEM) detectors with the Control and Data Acquisition system (CODAS) in the JET facility in Culham, England. The presented system performs the high level procedures necessary to calibrate the GEM detector and to protect it against possible malfunctions or dangerous changes in operating conditions. The system also allows control of the GEM detectors from CODAS, setting of their parameters, checking their state, starting the plasma measurement and to reading the results. The system has been implemented using the Python language, using the advanced libraries for implementation of network communication protocols, for object based hardware management and for data processing.

The W and W 3p-4d inner shell excitation lines in addition to Mo 2p3s lines have been identified from the spectrum taken by an upgraded high-resolution X-ray spectrometer. It is found from analysis of the absolute intensities of the W and Mo lines that W and Mo concentrations are in the range of ∼ 10−5 and ∼ 10−7, respectively. Comparison of the W concentration from the X-ray spectrometer with those from a vacuum-ultra-violet spectrometer and from soft X-ray cameras indicates that the W concentration from the X-ray spectrometer is lower by a factor of ∼ 2 and > 7, respectively. In contrast, comparison of a plasma effective charge determined from the X-ray spectrometer with that from a visible range continuum intensity shows that the plasma effective charge from the X-ray spectrometer is higher by a factor of ∼ 3. Hence it is probable that the W concentration from the X-ray spectrometer is valid within a factor of ∼ 3, while the W concentrations from the vacuum-ultra-violet spectrometer and from the soft X-ray cameras are further higher. Determination of tungsten and molybdenum concentrations from an X-ray range spectrum in JET2 Submitted to: J. Phys. B: At. Mol. Phys. Determination of tungsten and molybdenum concentrations from an X-ray range spectrum in JET3

The high resolution X-Ray crystal spectrometer at the JET tokamak has been upgraded with the main goal of measuring the tungsten impurity concentration. This is important for understanding impurity accumulation in the plasma after installation of the JET ITER-like wall (main chamber: Be, divertor: W). This contribution provides details of the upgraded spectrometer with a focus on the aspects important for spectral analysis and plasma parameter calculation. In particular, we describe the determination of the spectrometer sensitivity: important for impurity concentration determination.

Significant electric fields both parallel and perpendicular to a magnetic field have been observed and modeled self-consistently in an ITER divertor relevant plasma-wall experiment. Due to magnetization, electric current is found to penetrate the plasma beam outside of the cascaded arc plasma source with a length scale proportional to root HeHi, where H-e and H-i are the electron and ion Hall parameters, respectively. Plasma rotation measurements and chemical erosion profiles at a carbon target demonstrate that for a sufficiently well-magnetized plasma, a current through the target causes plasma-wall sheath potentials to significantly increase in a region of net ion collection while for the conditions studied, regions of net electron collection remain unaffected. The plasma-wall sheath profile at the target has been characterized experimentally as a function of negative target potential.

Ion and neutral parameters are determined in the high electron density, magnetized, hydrogen plasma beam of an ITER divertor relevant plasma via measurements of the n=2 excited neutrals. Ion rotation velocity (up to 7 km/s) and temperature (2-3 eV~T_{e}) are obtained from analysis of Hα spectra measured close to the plasma source. The methodology for neutral density determination is explained whereby measurements in the linear plasma beam of Pilot-PSI are compared to modeling. Ground-state atomic densities are obtained via the production rate of n=2 and the optical thickness of the Lyman-α transition (escape factor ~0.6) and yield an ionization degree >85% and dissociation degree in the residual gas of ~4%. A 30% proportion of molecules with a rovibrational excitation of more than 2 eV is deduced from the production rate of n=2 atoms. This proportion increases by more than a factor of 4 for a doubling of the electron density in the transition to ITER divertor relevant electron densities, probably because of a large increase in the production and confinement of ground-state neutrals. Measurements are made using laser-induced fluorescence (LIF) and absorption, the suitability of which are evaluated as diagnostics for this plasma regime. Absorption is found to have a much better sensitivity than LIF, mainly owing to competition with background emission.

At FOM Rijnhuizen, linear plasma generators are used to investigate plasma-material interactions under high-density (<= 10(21) m(-3)), low-temperature (<= 5 eV) plasma bombardment. Research into carbon-based materials has been focused on chemical erosion by hydrogen plasmas. Results from plasma exposure to high-flux (>10(23) H(+)/m(2) s) and low-temperature hydrogen plasma indicate silicon carbide has a lower relative rate of gross erosion than other carbon-based materials (e.g. graphite, diamond, carbon-fiber composites) by a factor of 7-10. Hydrogenic retention is the focus of research on tungsten and molybdenum. For target temperatures of 700-1600 K. the temperature dependence of hydrogenic retention is the dominant factor. Damage to the surface by heavy ion irradiation has shown to enhance retention by a factor of 2.5-4.1. Thermal stressing of W via, e-beam thermal cycling also enhances hydrogenic retention by a factor of 2.1 +/- 0.2, likely due to the introduction of thermal defects, which act as trapping sites for implanted hydrogenic isotopes. (C) 2011 Elsevier B.V. All rights reserved.

Tungsten (W) targets have been exposed to high density (ne⩽4×1019m−3), low temperature (Te⩽3eV) CH4-seeded deuterium (D) plasma in Pilot-PSI. The surface temperature of the target was ∼1220K at the center and decreased radially to ∼650K at the edges. Carbon film growth was found to only occur in regions where there was a clear CII emission line, corresponding to regions in the plasma with Te⩾2eV. The maximum film thickness was ∼2.1μm after a plasma exposure time of 120s. 3He nuclear reaction (NRA) analysis and thermal desorption spectroscopy (TDS) determine that the presence of a thin carbon film dominates the hydrogenic retention properties of the W substrate. Thermal desorption spectroscopy analysis shows retention increasing roughly linearly with incident plasma fluence. NRA measures a C/D ratio of ∼0.002 in these films deposited at high surface temperatures.

Cascaded arc plasma sources with channel diameters between 4 and 8mm were experimentally investigated at discharge currents up to 900A and hydrogen (H-2) flow rates up to 10 slm. Pressure measurements at the arc exit showed that the heavy particle temperature in the discharge channel was about 0.8 eV. The electron temperature was calculated from the electron mass balance, taking into account electron losses due to ambipolar diffusion and convection out of the source channel. This calculation showed that the electron temperature was 1.5-4 eV, increasing with decreasing density in the channel (i.e. with decreasing H2 flow rate and increasing diameter). The results of Thomson scattering measurements at 1 and 5 cm distance from the source exit showed the same trends. Using measurements of the average axial electric field, the effective size of the current-carrying 'active' plasma was calculated, expressed in terms of the filling fraction rho(2) = (reff/R)(2). The data showed that the filling fraction increased linearly with the input power and was independent of the diameter and flow rate. The ionization degree in the active center was estimated to be 20-30% from an evaluation of the electron energy balance, Thomson scattering measurements and H-beta emission measurements. The highest gas efficiency was obtained when the channel was completely filled at a maximum current of 900A (65 kW input power, 8mm channel, 4 slm flow rate) and was 19%. The highest energy efficiency was 7%.

A new cascaded arc containing three separate discharge channels at 15mm distance from each other was constructed to produce intense and wide hydrogen plasma beams and first tests were carried out at Pilot-PSI. Current and voltage measurements as well as calorimetry on the cooling water of the source demonstrated that these channels operated independently. Thomson scattering measurements showed that, depending on the nozzle geometry, the three outputs merge to one beam if the source is operated at argon in magnetic fields up to 1.6T densities. In hydrogen operation, the individual outputs did not merge or interact. Also a first test was performed in argon on the use of a remote ring anode to induce beam mixing due to rotation driven by cross-B currents.

A robust and sensitive Thomson scattering (TS) system has been developed for the high density low temperature plasma in the linear plasma generator Pilot-PSI, which routinely and reproducibly measures electron density and temperature profiles along a detection chord of 25 mm with a spatial resolution of 0.6 mm. The capabilities of the system are illustrated in this paper by a selection of new results from the research program at Pilot-PSI. TS data are presented that demonstrate the present plasma density record in Pilot-PSI: 5 x 10(21) m(-3) at a temperature of 3 eV. TS measurements in front of the target are combined with ion saturation current data to determine plasma velocities of 4-5 km s(-1), which shows that heat convection is dominating over conduction. Single shot operation of TS is also possible, which is demonstrated by measurements revealing a rotating filamentary return current channel to the source anode. Finally, the TS system upgrade that will provide real time feedback of electron density and temperature in the larger plasma generator Magnum-PSI is discussed.

Tungsten targets are exposed to the plasma conditions expected at the strike point of a detached ITER divertor (similar to 10(24) D/m(2)s, Te similar to 2 eV). The surface temperature of the target is similar to 1600 K at the center and decreased radially to similar to 1000 K at the edges. A 2-D spatial scan of the W target using nuclear reaction analysis (NRA) shows an asymmetric D retention profile with the lowest retention values at the center of the target and the highest 6 mm off-center. Even in the regions of larger retention, the D concentrations were <= 5 x 10(15) D/cm(2) as measured by NRA. Thermal desorption spectroscopy (TDS) is used to measure the global D retention. Very low retention with retained fractions ranging from 10(-7) to 10(-5) D-retained/D-incident were measured with TDS. Both NRA and TDS results show no clear dependence of retention on incident fluence possibly indicating the absence of plasma-driven trap production in W under these conditions. (C) 2009 Elsevier B.V. All rights reserved,

We have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (similar to 10(24) m(-2) s(-1)) and low electron temperatures (T-e similar to 1 eV). Optical emission spectroscopy on the CH A-X band was used to characterize the chemical sputtering. Fine grain graphite (R 6650, SGL Carbon Group), ITER-reference carbon fiber composite material (SNECMA NB31 and NB41; Dunlop 3D), nano-and micro-crystalline diamond coatings on molybdenum and SiC (Silit (R) SKD Reaction-Bonded, Saint-Gobain Ceramics) were compared. The chemical sputtering was similar for the different composites under comparable plasma conditions, except for SiC, which produced a ten times lower rate. The CH emission was constant at electron temperatures T-e > 1 eV and ion fluxes ranging between 10(23) and 10(24) m(-2) s(-1), but decreased at lower temperatures. This decrease is possibly due to changes in the excitation of CH and not due to a change in the chemical erosion rate.

Modelling of hydrogen line profiles in the ITER divertor plasma entails several challenges. Because of the high density (N-e > 10(14) cm(-3)) and low temperature (kT(e) < 2eV) expected in the magnetized plasma close to the wall, both Stark broadening and Zeeman splitting have to be taken into account in line shape calculations, [1]. Moreover, the conditions are such that the ion dynamics has to be retained in Stark broadening calculations, and the presence of a high magnetic field (B similar to 5T) introduces couplings between Stark and Zeeman effects, which can significantly alter the profile [2, 3]. We present here Balmer lines obtained by numerical simulations based on a standard Molecular Dynamics technique, of the plasma in an external magnetic field. The simulated profiles are compared to experimental ones obtained in Plasma Surface Interaction (PSI) experiments which are carried out on Pilot-PSI of FOM.

A highly sensitive imaging Thomson scattering system was developed for low temperature (0.1-10 eV) plasma applications at the Pilot-PSI linear plasma generator. The essential parts of the diagnostic are a neodymium doped yttrium aluminum garnet laser operating at the second harmonic (532 nm), a laser beam line with a unique stray light suppression system and a detection branch consisting of a Littrow spectrometer equipped with an efficient detector based on a "Generation III" image intensifier combined with an intensified charged coupled device camera. The system is capable of measuring electron density and temperature profiles of a plasma column of 30 mm in diameter with a spatial resolution of 0.6 mm and an observational error of 3% in the electron density (n(e)) and 6% in the electron temperature (T(e)) at n(e) = 4 x 10(19) m(-3). This is achievable at an accumulated laser input energy of 11 J (from 30 laser pulses at 10 Hz repetition frequency). The stray light contribution is below 9 x 10(17) m(-3) in electron density equivalents by the application of a unique stray light suppression system. The amount of laser energy that is required for a n(e) and T(e) measurement is 7 x 10(20)n(e) J, which means that single shot measurements are possible for n(e)>2 x 10(21) m(-3).

A potential buildup in front of a magnetized cascaded arc hydrogen plasma source is explored via E x B rotation and plate potential measurements. Plasma rotation approaches thermal speeds with maximum velocities of 10 km/s. The diagnostic for plasma rotation is optical emission spectroscopy on the Balmer-beta line. Asymmetric spectra are observed. A detailed consideration is given on the interpretation of such spectra with a two distribution model. This consideration includes radial dependence of emission determined by Abel inversion of the lateral intensity profile. Spectrum analysis is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting.

Introduction The linear device Pilot-PSI is an expanding cascaded-arc plasma system in which high-density magnetized plasma beam of about 1-2 cm diameter interacts with a solid target (so-called end plate). This device is the smaller forerunner of Magnum-PSI, a high-flux linear plasma generator designed to study plasma surface interaction (PSI) at ITER relevant parameters [1]. Density, temperature and velocity components of the Pilot-PSI have been extensively studied and recently reported [2,3]. However, parameters as plasma potential, ion and electron temperature, plasma instabilities and their influence on plasma target energy transfer have to be further studied in order to optimize thermal energy removal from plasma and the life time of the target. In the present contribution, the experimental results obtained by electrical means are reported as a function of gas composition and discharge current in the cascaded arc source. Electrical measurements were performed by a cylindrical probe, a multi-channel analyzer (MCA) and as current-voltage characteristics of the target. Plasma potential and ion saturation current measured by cylindrical probe are used to find the radial distribution of both electric field and gradient of the plasma density. Considerations related to drift instabilities are made and the energy transferred from the plasma column to the end plate is evaluated.