L. Schiesko

Magnetic filter field dependence of the performance of the RF driven IPP prototype source for negative hydrogen ions

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m2. An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.

Magnetic field dependance of the plasma properties in a negative hydrogen ion source for fusion

Axially resolved measurements of plasma parameters were performed by two Langmuir probes moving in parallel from the exit of the driver (where the plasma is generated) up to the extraction region neighbourhood of the IPP RF negative hydrogen ion source prototype. At the driver exit, the plasma parameters show an unexpected inhomogeneity in the presence of the magnetic field: a cold and dense plasma flows out of the top part of the driver while a hot and low density plasma flows from the bottom part. A local relation between the top and bottom parameters is derived from the conservation of the energy flux.

Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source

Systematic Langmuir probe measurements were performed during the caesium (Cs) conditioning of the IPP prototype negative hydrogen ion source. Conditioning consists of slowly injecting caesium into the source in order to increase the extracted negative ion current and minimize the co-extracted electrons. This process usually takes several days. Measurements were taken near the plasma grid (PG) where the majority of the extracted negative ions are created. A clear transition between a poorly conditioned (volume dominated H− production) and a well conditioned (surface dominated H− production) source was seen. By following the plasma parameter variations, an overall decrease in the plasma potential, the floating potential and also the electron saturation current was observed. However, the ion saturation current remained constant. Consequently, near the PG where negative ions are generated and extracted, a transition occurred in the space charge balance (quasineutrality): the space charge is balanced by electrons when the source is poorly conditioned and by negative ions when the source is well conditioned.

On the proton flux towards the plasma grid in a RF-driven negative hydrogen ion source for ITER NBI

The transport of protons in a RF-driven negative hydrogen ion source for ITER NBI was investigated by means of a 3D Monte Carlo transport code. As input for the code a consistent axial profile of the electrostatic potential for the complete ion source was constructed. The potential difference between the plasma generation volume and the plasma grid is in the range 10?20?V and depends on the RF power, the filling pressure and the plasma grid bias voltage. The mean free path for proton collisions with the background gas is in the range of several centimeters which means that the protons are noticeably decelerated and scattered. The velocity distribution function of the protons impinging the plasma grid resembles much more a low-temperature Maxwell distribution function than the high energetic proton beam which would be expected by looking only at the maximum value of the potential in the plasma generation volume. This finding is of high relevance for further analysis of the negative hydrogen ion production on the plasma grid surface as well as the ion transport and beam formation.

Plasma expansion across a transverse magnetic field in a negative hydrogen ion source for fusion

High power negative hydrogen ion sources operating at 0.3 Pa are a key component of the neutral beam injection systems for the international fusion experiment ITER. To achieve the required large ion current at a tolerable number of co-extracted electrons the source is equipped with a magnetic filter field (up to 10 mT). The IPP prototype source (1/8 of the area of the ITER source) has been equipped with a flexible magnetic filter frame to perform filter field studies (position, polarity, strength). Axial profiles of the plasma parameters are measured with two Langmuir probes, positioned in the upper and the lower half of the expansion chamber. In addition to the expected decrease in electron temperature and density a vertical drift develops the direction depending on the polarity of the field. Without field no drift is observed. The drift is less pronounced in caesium seeded discharges and almost vanishes in deuterium, indicating an influence of the ion mass on the drift. A comparison with results from a half-size ITER source reveals that the plasma is much more uniform in the large source.

Investigation of the magnetic field influence on the plasma parameter homogeneity in a large H− / D− RF ion source relevant for ITER

A study was conducted on the large half-size ITER source RADI testbed in order to determine the influence of the magnetic field strength on the plasma parameters in the neighbourhood of the grid. The magnetic field at RADI is created by a current flowing in the plasma grid, in contrast to the small IPP prototype source where permanent magnets are used. The investigations show a plasma homogeneity within 10% near the grid along the direction of the magnetic field lines as well as a reduction of the electron temperature. In the direction perpendicular to the magnetic field lines, and for field strengths higher than 2.4 mT, the density shows a minimum due to the reduced transport coefficient across a perpendicular magnetic field. A form of effective enhanced confinement is achieved for low magnetic field intensity.

Production of negative ions on graphite surface in H2/D2 plasmas: Experiments and srim calculations

In previous works, surface-produced negative-ion distribution-functions have been measured in H2 and D2 plasmas using graphite surfaces (highly oriented pyrolitic graphite). In the present paper, we use the srim software to interpret the measured negative-ion distribution-functions. For this purpose, the distribution-functions of backscattered and sputtered atoms arising due to the impact of hydrogen ions on a-CH and a-CD surfaces are calculated. The srim calculations confirm the experimental deduction that backscattering and sputtering are the mechanisms of the origin of the creation of negative ions at the surface. It is shown that the srim calculations compare well with the experiments regarding the maximum energy of the negative ions and reproduce the experimentally observed isotopic effect. A discrepancy between calculations and measurements is found concerning the yields for backscattering and sputtering. An explanation is proposed based on a study of the emitted-particle angular-distributions as ca...

Size scaling of negative hydrogen ion sources for fusion

The RF-driven negative hydrogen ion source (H−, D−) for the international fusion experiment ITER has a width of 0.9 m and a height of 1.9 m and is based on a ⅛ scale prototype source being in operation at the IPP test facilities BATMAN and MANITU for many years. Among the challenges to meet the required parameters in a caesiated source at a source pressure of 0.3 Pa or less is the challenge in size scaling of a factor of eight. As an intermediate step a ½ scale ITER source went into operation at the IPP test facility ELISE with the first plasma in February 2013. The experience and results gained so far at ELISE allowed a size scaling study from the prototype source towards the ITER relevant size at ELISE, in which operational issues, physical aspects and the source performance is addressed, highlighting differences as well as similarities. The most ITER relevant results are: low pressure operation down to 0.2 Pa is possible without problems; the magnetic filter field created by a current in the plasma gri...

Negative ion beam characterisation in BATMAN by mini-STRIKE: Improved design and new measurements

The ITER project requires additional heating provided by two injectors of neutral beams resulting from the neutralisation of accelerated negative ions. To study and optimise negative ion production, the SPIDER test facility (particle energy 100keV; beam current 50A) is under construction in Padova, with the aim of testing beam characteristics and to verify the source proper operation. The SPIDER beam will be characterised by the instrumented calorimeter STRIKE, whose main components are one-directional carbon fibre carbon composite tiles. Some prototype tiles have been employed in 2012 as a small-scale version (mini-STRIKE) of the entire system to investigate the features of the beam from BATMAN at IPP-Garching. As the BATMAN beamlets are superposed at the measurement position, about 1m from the grounded grid, an actively cooled copper mask is located in front of the tiles; holes in the mask create an artificial beamlet structure. Recently the mini-STRIKE has been updated, taking into account the results obtained in the first campaign. In particular the spatial resolution of the system has been improved by increasing the number of the copper mask holes. Moreover a custom measurement system has been realized for the thermocouple signals and employed in BATMAN in view of its use in SPIDER. The present contribution gives a description of the new design of the system as well as of the thermocouple measurements system and its field test. A new series of measurements has been carried out in BATMAN. The BATMAN beam characterisation in different experimental conditions is presented.

First experiments with Cs doped Mo as surface converter for negative hydrogen ion sources

A study was conducted on the properties of molybdenum implanted with caesium as an approach to reduce the Cs consumption of negative hydrogen ion sources based on evaporated Cs. The depth profiles of the implanted Cs were simulated by SDTrimSP and experimentally determined by X-ray photoelectron spectroscopy depth profiling. In particular, one year after implantation, the depth profiles showed no signs of Cs diffusion into the molybdenum, suggesting long term stability of the implanted Cs atoms. The H− surface generation mechanisms on the implanted samples in hydrogen plasma were investigated, and the stability of the H− yield during four hours low power hydrogen plasma discharges was demonstrated. An estimation of the work function reduction (−0.8 eV) by the Cs implantation was performed, and a comparison of the relative negative ion yields between the implanted samples and highly oriented pyrolitic graphite showed that the Cs doped Mo negative ion yield was larger.