Upgraded Test Facility Research Articles

Beam optics of RF ion sources in view of ITER’s NBI systems

A low beamlet divergence is crucial for the efficiency of the ITER-NBI systems, since it affects the transmission of the beam through the duct. There is a requirement of 7 mrad e-folding divergence for the ITER Heating Neutral Beam. Significantly higher divergences (10–15 mrad) have been observed in RF-source based experiments albeit at low beam energy. This could be the consequence of a broad perpendicular velocity distribution of the H−/D− particles before extraction. This paper explores this hypothesis and its implications for ITER. To estimate H−/D− perpendicular temperatures in the RF-driven BATMAN Upgrade test facility, spatially resolved measurements of the beam power density are compared with IBSimu calculations. The estimated perpendicular temperatures show a strong dependence on the source filling pressure, decreasing from approximately 4 eV at 0.3 Pa to 2 eV at 0.4 Pa. Ion-optics calculations of the ITER-HNB grid system are performed to evaluate whether the temperatures estimated in the BATMAN Upgrade test facility are tolerable in view of beam-grid interaction and beamline transmission. The beamline transmission is fairly insensitive to the perpendicular temperature, but the heat loads at the downstream grids increase with the perpendicular temperature.

Work function of the caesiated converter surface at the BATMAN Upgrade H− ion source at different operational scenarios

Since negative hydrogen ion sources for neutral beam injection (NBI) systems rely on the surface production of negative hydrogen ions, Cs is injected to lower the work function of the extraction electrode surface. The adsorbed Cs layers are affected by residual gases from the given non-UHV conditions as well as by reactive hydrogen species during plasma phases, which leads to a complex surface chemistry and the occurrence of temporal changes of the work function. To control the work function and get insight into its temporal dynamics, an absolute work function diagnostic has been developed for ion sources with which measurements can be performed in vacuum phases between pulses. The diagnostic is applied at the BATMAN Upgrade test facility, which is equipped with the prototype RF ion source for the ITER NBI. It is shown that the Cs conditioning of the ion source leads to a dramatic decrease in the work function to ultra-low values < 1.5 eV. First measurements after the application of 1000 s pulses indicate that the ultra-low work function layer is not stable upon long-term plasma exposure and it is revealed that high dynamics of the Cs surface properties are given right after the pulse.

Overview of recent and upcoming activities at the BATMAN Upgrade test facility

The BATMAN Upgrade (BUG) test facility at IPP is contributing to the development of RF-driven H- sources towards the ITER neutral beam injection and beyond. BUG is equipped with an 1/8 size of the ITER NBI ion source and thus is highly flexible for setup changes or diagnostic access. The present strategy for BUG is aligned along two paths: (i) BUG is continuing to be upgraded upgrading for long pulse operation (up to 1 hour, both in H and D) to identify measures for the stabilization of long pulses. For pulses > 100 s a cooling of the plasma grid (first grid of the extraction system) is required in order to avoid the dominance of thermal Cs desorption. A Cs evaporation concept with evaporation close to the extraction system is being tested, showing a stabilization of co-extracted electrons in 100 s H pulses, (ii) BUG is used for investigations of whole beam and beamlet optics for which the beam diagnostics have been massively upgraded. The beam diagnostics have been used to characterize a newly installed MITICA-like extraction system including asymmetric deflection compensation magnets.

ITER-HNB plasma and extraction grids in BUG: Characterization of magnetic deflection correction

The BATMAN Upgrade test facility aims to extract H-/D- at high current densities. The inevitably co-extracted electrons are deflected out of the beamlet by permanent magnets before acceleration. These magnets induce a row-wise zig-zag deflection of the beamlets. In the new MITICA-like extraction system, this deflection is compensated by additional deflection correction magnets. IBSimu modeling studies indicate that the deflection correction system not only acts on the horizontal beamlet deflection, but also causes aberrations to the vertical angular distribution. Dependencies of the beamlet position and size are studied in several parameter scans with the available diagnostic tools (CFC tile calorimetry, Beam Emission Spectroscopy). The horizontal beamlet deflection is robustly compensated within the systematic uncertainty of the experiment. The triangular beamlet shape predicted on basis of the magnetic field structure is visible in the CFC calorimetry.

Computational design of magnetic beamlet deflection correction for NNBI

Coextracted electrons are inevitable for negative ion based NBI systems. To avoid wastefully accelerating these electrons to the full energy, they are magnetically deflected out of the beamlet onto the extraction grid. The deflection of the electrons creates an unwanted deflection of the ions, which is magnetically corrected in the ITER heating neutral beam. In order to gain experience with magnetic deflection correction, IPP has designed a magnetic deflection correction system in collaboration with the ITER organization, which will be characterized in the BATMAN Upgrade test facility. The ion-optics code IBSimu is used for the design of the magnetic deflection correction system. A comparison of the design candidates shows that the magnetic deflection correction system inherently leads to an increased divergence as result of the magnetic field structure. The divergence increase manifests itself as an increased fraction of a broader halo component.

First direct comparison of whole beam and single beamlet divergences in a negative ion source with simultaneous BES and CFC tile calorimetry measurements

Neutral beam injection (NBI) systems are required for heating and current drive in the next generation fusion experiment ITER, and strict requirements are placed on the beamlet core divergence (&amp;lt;7 mrad) for transmission into the tokamak. The measurement of single beamlet divergence is challenging due to the multi-beamlet nature of the negative ion sources that are required for such systems; diagnostic systems compatible with large high power ion beams can only provide spatially averaged measurements, leading to mixing of beamlet signals within diagnostic results. To improve the understanding of this effect, a direct comparison has been made between the single beamlet and multi-beamlet divergence by a combination of both Beam Emission Spectroscopy (BES) and 1D carbon fiber composite tile calorimetry, in a joint campaign by IPP and Consorzio RFX. The measurements performed so far in this campaign at the BATMAN Upgrade Test Facility have led to two major results. First, an excellent agreement is found for single beamlet divergences from the two diagnostics, showing that the results from these diagnostic systems can be compared for single beamlets. Second, the contribution of beamlet deflection, caused by an alternating magnetic field at the extract grid, to the divergence as measured using BES has been quantified with up to a factor of 3 increase when compared with the single beamlet value. This demonstrates that further investigation is needed into how mixing of information from a beamlet affects diagnostic results with a combination of both simulation and experiment, which will be performed in a next step.

Beam formation and transport in the BATMAN Upgrade test facility

In the BATMAN Upgrade test facility, an RF-driven negative hydrogen ion source is developed in support of the ITER NBI. Its 5 × 14 apertures represent approximately one ITER beamlet group. Diagnostics include a tungsten wire calorimeter, a copper calorimeter, and two arrays of beam emission spectroscopy lines of sight.When the RF-power is increased at fixed extraction potential, the extracted current density saturates. This is due to scraping of the beamlet by the extraction grid; a method is developed to characterize the injected current density. This current density can vary locally, e.g. due to the E × B drift induced by the filter field. A procedure is introduced to infer the vertical injected current density profile from the behaviour of the different beam emission spectroscopy lines of sight during an RF-power scan.The injected current density profile is used to calculate aperture specific beamlet properties with IBSimu. The beamlet properties are projected towards the calorimeters with the ABC3D code, and compared with measurements to benchmark the current density profile and IBSimu calculations. Beamlet-to-beamlet variations of power and width in the tungsten wire calorimeter are well reproduced. It is crucial to include heat transfer in the copper calorimeter to reproduce the measured profiles.

Beamlet scraping and its influence on the beam divergence at the BATMAN Upgrade test facility

For the ITER fusion experiment, two neutral beam injectors are required for plasma heating and current drive. Each injector supplies a power of about 17 MW, obtained from neutralization of 40 A (46 A), 1 MeV (0.87 MeV) negative deuterium (hydrogen) ions. The full beam is composed of 1280 beamlets, formed in 16 beamlet groups, and strict requirements apply to the beamlet core divergence (<7 mrad). The test facility BATMAN Upgrade uses an ITER-like grid with one beamlet group, which consists of 70 apertures. In a joint campaign performed by IPP and Consorzio RFX to better assess the beam optics, the divergence of a single beamlet was compared to a group of beamlets at BATMAN Upgrade. The single beamlet is measured with a carbon fiber composite tile calorimeter and by beam emission spectroscopy, whereas the divergence of the group of beamlets is measured by beam emission spectroscopy only. When increasing the RF power at low extraction voltages, the divergence of the beamlet and of the group of beamlets is continuously decreasing and no inflection point toward an overperveant beam is found. At the same time, scraping of the extracted ion beam at the second grid (extraction grid) takes place at higher RF power, supported by the absence of the normally seen linear behavior between the measured negative ion density in the plasma close to the extraction system and the measured extracted ion current. Beside its influence on the divergence, beamlet scraping needs to be considered for the determination of the correct perveance and contributes to the measured coextracted electron current.

Design and Fabrication of the 1.9 K Magnet Test Facility at BNL, and Test of the First 4-m-Long MQXF Coil

The future high luminosity upgrade of the Large Hadron Collider (LHC) at CERN will include 20 4.2-m-long Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn high gradient quadrupole magnets, which will be components of the triplets for two LHC insertion regions. In order to test these and four preproduction models, the vertical superconducting magnet test facility of the Superconducting Magnet Division (SMD) at Brookhaven National Laboratory (BNL) has been upgraded to perform testing in superfluid He at 1.9 K and 1 bar, the operational condition at the LHC. This has involved extensive modification of the 4.5 K cryogenics plant, including piping, compressors, and other upgraded components; a new vertical test cryostat, which can accept larger diameter magnets; a modernized power supply system upgraded with IGBT switches and fast shutoff capability, and that can supply 24 kA to test high field Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnets; and completely new data acquisition, signal analysis, and control software and hardware, allowing for fast, high precision, large volume data collection. This paper reports on the design, assembly, and commissioning of this upgraded test facility, and presents results of the first magnet test performed.

Upgrading the NIFS superconductor test facility for JT-60SA cable-in-conduit conductors

The superconductor test facility at the National Institute for Fusion Science (NIFS) was upgraded to test cable-in-conduit (CIC) conductors for the JT-60SA equilibrium field (EF) coil. Supercritical helium (SHe) lines were assembled with transfer tubes and a heat exchanger. The CIC conductor was covered with a thermal insulation vessel, filled with gas helium at atmospheric pressure. The temperature of the conductor was varied using a film heater attached to an inlet pipe. Critical current (Ic) and current sharing temperature (Tcs) measurements of the prototype CIC conductor were carried out successfully in the upgraded test facility. During the measurements, the conductor temperature was precisely controlled.

FEM calculation of eddy current losses and forces in thin conducting sheets of test facilities for fusion reactor components

An efficient finite element formulation of the eddy current field in thin conducting sheets has been developed coupled to a description of the magnetic field by a gauged magnetic vector potential. The numerical problems associated with the thin conductors have thereby been eliminated. Results of computations on a model of the TOSKA Upgrade test facility are presented. The aluminium radiation shields are not in danger; the stresses acting upon them are negligible. This is due to their being located relatively far from the superconducting coil. Only parts of the supply lines near the connecting flange of the vacuum vessel experience considerable stress. Their reinforcement under rapid changes may be necessary. The vacuum vessel is not endangered, since its wall of 20 mm thickness easily withstands the stresses due to the induced eddy currents. Normal pressures of up to about 50 t/m/sup 2/ occur at the LN2 shield. This makes modifications of its structure necessary.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>