Plasma surface interactions - Operations
Aims and scientific program
The PSI-Operations group is responsible for operation and maintenance of the new linear plasma device Magnum-PSI. This device currently operates in pulsed-state with a 1.8T magnet. First magnetized plasma has been achieved in October 2011. Scientific exploitation of the device has started in the beginning of 2012. This includes operation of several plasma and surface diagnostics techniques. Mid 2012 Magnum-PSI will be upgraded to operate in steady-state with a 2.5T superconducting magnet with a wide plasma beam (diameter up to 10 cm). For hydrogen plasmas, flux densities of ~1024 m-2s-1 should be reachable at low electron temperatures (<10eV).The research program of the group involves technical studies relevant for operation and improvement of the device and for plasma and surface diagnostics. In addition, hydrogenic retention in refractory metals exposed to high-flux plasmas is studied; this research line will include experiments with novel plasma-facing materials.
Personnel
| Function |
Name | |
| Group Leader | P.A. Zeijlmans van Emmichoven | P [dot] A [dot] Zeijlmans [te] differ [dot] nl |
| Research Engineer | H.J. van der Meiden | H [dot] J [dot] vanderMeiden [te] differ [dot] nl |
| Research Technicians | P.H.M. Smeets | P [dot] H [dot] M [dot] Smeets [te] differ [dot] nl |
| H.J.N. van Eck | H [dot] J [dot] N [dot] vanEck [te] differ [dot] nl | |
| S. Brons | S [dot] Brons [te] differ [dot] nl | |
| J. Scholten | J [dot] Scholten [te] differ [dot] nl | |
| R.S. Al | R [dot] S [dot] Al [te] differ [dot] nl | |
| A.R. Lof | A [dot] R [dot] Lof [te] differ [dot] nl | |
| PhD Student | M.H.J. ’t Hoen | M [dot] H [dot] J [dot] tHoen [te] differ [dot] nl |
Magnum PSI
Magnum-PSI will be a world-wide unique facility for plasma-surface interaction research relevant for ITER and reactors beyond ITER. Magnum-PSI will provide steady-state high flux plasma with up to 1024 ions m-2 s-1 at a temperature in the eV range, a large beam area of up to 80cm2 and a continuous magnetic field of 2.5T. The power load on the target will be able to reach densities >10MW m-2 under continuous operation and up to 2GW m-2 in pulsed operation. Magnum-PSI will be the only device so far to enter the strongly coupled regime, in which molecules and dust particles that come off the surface are trapped and remain part of the plasma-surface interaction system. Scientific operation in pulsed operation has started end 2011 and steady-state operation of Magnum-PSI is scheduled to start mid 2012.
Overview of Magnum
Present status of Magnum-PSI
Basic construction of Magnum-PSI has been finished in 2009. The complete vacuum system, targets handling and most plasma and surface diagnostics is now available. Targets can be transported through the system as well as rotated in any given orientation. Target exposures can be made on a routinely basis. All exposures, diagnostics and movements is controlled and observed remotely from the Magnum-PSI control-room. The present focus of the group is on the scientific operation and further commissioning of diagnostics and on reaching all specified parameters of the experiment. In the following, an overview of the major components of Magnum-PSI is given.
Vacuum system
The main vacuum system, where plasma production and exposure takes place, is constructed in modules and placed on a rail-system for easy access. The system is pumped by two turbo molecular pumps with a pump capacity of 4.4 m3/h leading to a base pressure of 10-4 Pa. The plasma source running at full power requires a gas inlet of ~50 Pa·m3/s. Since at the target the pressure has to be reduced to <1 Pa, a differentially pumped system was chosen. The system consists of 3 vacuum vessels separately pumped by 3 roots pumps (accumulated pump-speed of ~55000 m3/h).
Main vacuum vessel
Part of the vacuum system is the TEAC (Target Exchange and Analysis Chamber). The target, which is attached to a 5-m long target manipulator [hyper link], can be withdrawn from the main vacuum system to the TEAC. The chamber will be equipped with a variety of surface analysis techniques. The TEAC has its own turbo molecular pump and can be disconnected from the rest of the experiment by a double gate valve system. With the valves closed, a vacuum of low 10-6 Pa can be reached, and the target can be removed or exchanged without venting the rest of the system.
Magnet
The superconducting magnet consists of 5 solenoids (NbTi cooled by liquid He) wound on a single stainless steel former. It has a warm bore of 1300mm and an axial length of 2450mm. The magnet produces a homogeneous magnetic field of up to 2.5T and covers the main vacuum system. The magnet has 16 radial access ports of 190mm diameter to allow for diagnostic access during plasma exposure. The magnet is now being assembled and tested at the manufacturer site.
2.5T superconducting magnet
Target manipulator
The target is attached to the 5-m long target manipulator allowing transport of the target from the main vacuum system to the TEAC. The manipulator can be rotated by ±120? and allows tilting of the target against the magnetic field in a range of ±90? (with 0? being perpendicular to the magnetic field). This rotation and tilting is particularly challenging due to the water cooling requirements of 50 l/min making the design of a rotating water feed-through necessary.
Target and Target manipulator
Plasma source
The plasma source presently used in Magnum-PSI is a cascaded arc source powered by a 45kW power source. With this source a relatively small (1-2cm) diameter high flux plasma beam can be produced. For large beam dimensions (~10cm diameter), a 270kW source will be needed (>10MW/m2 power density on target). For that reason four power supplies, developed by the Eindhoven University of Technology, have been installed. In addition, a scheme for a pulsed plasma source has been developed to simulate transient heat loads with duration of 0.5–1ms as they occur during so-called ELM instabilities in the tokamak (up to 2GW/m2 power density on target).
Source powersupplies and source manipulator
RF Heating
Experiments on PILOT-PSI show that with a 45kW plasma source, plasma electron temperatures up to 5eV can be achieved. In order to reach electron temperatures up to 10eV, additional heating by means of electromagnetic wave power in the radio frequency (RF) range is planned. Two RF heating methods are proposed: lower hybrid (LH) heating and ion cyclotron resonance (ICR) heating. Both methods are presently under study and will be tested on PILOT-PSI.
Cooling system
An important aspect in the design of Magnum-PSI was thermal effects originating in the excess heat and gas flow from the plasma source and radiation from the target. A total of approximately 500 kW of power and cooling capacity was foreseen. This is based on a total of 50 kW on the target (25 kW from a 270 kW plasma source and 25 kW from a 50 kW RF-heating source). The remaining cooling capacity is mainly needed for cooling the vacuum pump systems and power supplies. Most of the dissipated power is removed by 4 separate water cooling systems: high pressure circuit for cooling of components receiving hig heat flux; low pressure circuit for cooling of vacuum vessel walls; cold water circuit for cooling of vacuum pumps and power supplies; emergency cooling circuit for cooling of heat shield inside superconducting magnet and for cryocoolers.
Diagnostics
Magnum-PSI is equipped with a range of diagnostics for characterizing of plasma conditions, exposure conditions, target surface and retention properties. Basic operation diagnostics comprise of gas-flow, current and voltage, B-field, pressures, water-flow, water-temperature, ect. over 300 variables are recorded in a central time based database. The stored data can be analyzed later. Currently, the control and data acquisition system is being extended to include the diagnostic measurement systems. An accurate timing and fast data acquisition will be build-in to support these diagnostics.
Control-room
The control and monitoring of the installation is realized with PLC based systems. From the Control-room the operator can control Magnum via graphical user interfaces. The control and data acquisition interfaces are being developed mainly in-house together with the Differ software engineering department.
Research Highlights
First magnetized hydrogen plasma and target exposure in Magnum-PSI
Magnum-PSI has produced its first magnetized hydrogen plasma beam on October 4th 2011. Magnum-PSI is the first facility to expose materials to plasma under identical conditions as expected in the divertor of the future fusion reactor ITER. The plasma conditions achieved thanks to the use of the cascaded arc plasma source are up to two orders of magnitude more intense than in other existing laboratory facilities. Thomson scattering was used to show that the electron densities are in the range of 1020 m-3 at electron temperatures of 3-4 eV, leading to power densities on the target of 8 MW m-2 and ion fluxes well in excess of 1023 m-2s-1. A measurement campaign to explore the operational space of Magnum-PSI has been carried out. Thomson scattering, Optical Emission Spectroscopy, calorimetry and fast cameras were used to determine the properties of argon, helium, hydrogen, and deuterium plasmas and their power densities and particles fluxes on target. The results are very promising and enable first scientific operation of the device with pulsed field. In the future, Magnum-PSI will be upgraded with a superconducting magnet producing continuous magnetic fields at higher field strength than produced by the copper coils. Several laser-based diagnostics are commissioned in parallel to analyze the target surface in detail. The result will be a device permitting the investigation of long-term effects in the interplay between plasma and wall material.
Magnetized hydrogen plasma.
Modeling and experiments on differential pumping in linear plasma generators operating at high gas flows.
We have used neutral gas simulations and done experiments to show that differential pumping can be used effectively in linear plasma generators operating at high gas flows. The neutral gas dynamics of the linear plasma generator Magnum-PSI has been modeled with the DSMC code developed by Bird1. This code was chosen because Magnum-PSI will operate in the transitional gas flow regime, with local Knudsen numbers well above 0.1. An efficient way to reach low pressures with large gas flows is differential pumping, where the vacuum vessel is divided by skimmers into separate chambers that are individually pumped. In a two stage differentially pumped system, the optimum shape and position of the first skimmer has been determined. For a good performance of the skimmer, it was found that the tip of the skimmer should be inside the low density region of the expansion since the neutral density increases in the shock region. Therefore the skimmer should be able to penetrate the shock with a minimum influence on the flow. The optimum position thereby depends on the operating conditions of the source (e.g. atomic mass number and the gas flow). The simulation results agree with experimental data obtained on the linear plasma devices Pilot-PSI and PLEXIS. The angle between the skimmer and the gas flow must be kept shallow enough as to not interfere with the expansion, but a skimmer that is too shallow will form a flow restriction for the plasma beam. The optimum inner angle of the skimmer was found to be around 53 degrees. It is shown that differential pumping works in large linear plasma generators operating in the transitional regime. In Magnum-PSI the distance between the source and the skimmer can be varied. This makes it possible to place the skimmer before the shock position in different operating conditions (e.g. gas flow, atomic mass number, background pressure). In the Magnum-PSI operating conditions, a factor 4 pressure reduction in the case of H2 can be achieved with a two stage differential pumping scheme. This factor increases for heavier gasses (e.g. D2 and Ar). In Magnum-PSI a 3 stage differentially pumped vacuum system will be used to keep the neutral pressure in the target chamber below 1 Pa, the limit set by the ITER relevance of PSI studies. 1) A. Bird, Molecular gas dynamics and the direct simulation of gas flows (Clarendon, Oxford, 1994).
Fraction of particles which travel through a 10 cm diameter sampling area with its center on axis, as a function of the distance from the source (lines) and fraction of particles crossing the opening of a 10 cm diameter skimmer at different positions (open symbols). A lower fraction indicates a higher skimmer performance.
Pressure plot of the DSMC calculation where 40 slm D2 gas expands in a three stage differentially pumped vacuum system. Some flow lines are shown for clarity. Neutral pressure below 1 Pa in the target chamber is reached.
Hydrogenic retention in tungsten exposed to ITER divertor relevant plasma flux densities
To better understand the effect of high-flux plasmas on refractory metals, tungsten (W) targets were exposed to high density, low temperature deuterium plasmas in Pilot-PSI. This investigation measured the amount of plasma-implanted deuterium that was trapped and retained in the tungsten target for a range of plasma exposure times (4 – 160 s). The plasma conditions were similar to what is expected in the ITER divertor (ne ~ 1020 m-3, Te ~ 2 eV, heat load ~ MW·m-2) and the W target surface temperature was ~1600 K at the center of the target and decreased to ~1000 K at the edges. Deuterium retention was measured locally in the first 3 µm of the surface by nuclear reaction analysis (NRA). A 2-D NRA scan of the surface revealed significantly higher retention at the cooler edges (6 mm off center, Tw ~ 1000 K) of the target as compared to the center of the target. This indicated that surface temperature was playing a dominant role in determining hydrogenic retention properties as compared to plasma flux density or plasma fluence. Thermal desorption spectroscopy (TDS) measured the global retained deuterium inventory in the exposed W targets. TDS analysis showed very low retained fractions (10-5-10-7 Dretained/Dincident) and overall D inventory (Dretained = 0.5-1.5 x 1016 D). TDS also revealed that polishing the surface of the target enhances retention by a factor of ~2 while annealing the target at 1300 K for 30 minutes has little effect on hydrogenic retention under these conditions. Both TDS analysis and NRA showed no clear dependence of retained D on incident plasma fluence possibly indicating the absence of plasma-driven trap production under these exposure conditions. These results indicate that when operating at surface temperatures of 1000-1600 K, the W strikepoints of the ITER divertor will not retain significant amounts of deuterium (or tritium) due to the bombardment of the surface by the high flux of low energy plasma hydrogenic ions.Read more in: G.M Wright et al., J. Nuc. Mat., accepted for publication.
The left graph shows the results from the 2-D NRA scan of the W target exposed to D plasma for a total of 80 s (~1022 D total fluence). This clearly shows a minimum in retention at the hot center of the target and the highest retention at the cooler edges. The low retention seen at some of the 8 mm off center locations may be a shadowing effect from the target clamping ring. The right graphs show a) the total retained fluence as a function of incident fluence integrated across the entire exposed surface, and b) the retained fraction as a function of total incident fluence. The steep decrease and then flattening of the retained fraction in b) may indicate saturation in the W target.
Annual Report
Read about the 'Plasma Surface Interactions' group in the Annual Report 2010
Recent publications
'Saturation of deuterium retention in self-damaged tungsten exposed to high-flux plasmas', M.H.J. 't Hoen et al., Nuclear Fusion 52 2 (2012)
'A differentially pumped argon plasma in the linear plasma generator Magnum-PSI: gas flow and dynamics of the ionized fraction', H.J.N. van Eck et al., Plasma Sources Sci. Technol. 20 (2011) 045016 (8pp)
'Collective Thomson scattering for ion temperature and velocity measurements on Magnum-PSI: a feasibility study', H.J. van der Meiden, Plasma Physics and Controlled Fusion, 52, 045009, March (2010)
'Hydrogenic retention in irradiated tungsten exposed to high-flux plasma', G.M.Wright et al., Nuclear Fusion 50 (2010) 075006 (8pp)
'Hydrogenic retention of high-Z refractory metals exposed to ITER divertor-relevant plasma conditions', G.M.Wright et al., Nuclear Fusion 50 (2010) 055004 (9pp)
'Carbon film growth and hydrogenic retention of tungsten exposed to carbon-seeded high density deuterium plasmas', G.M. Wright et al., Journal of Nuclear Materials 396 (2010) 176
'Modeling and experiments on differential pumping in linear plasma generators operating at high gas flows', H.J.N. van Eck et al., Journal of Applied Physics 105 (2009) 063307
'Hydrogenic retention in tungsten exposed to ITER divertor relevant plasma flux densities', G.M. Wright et al., Journal of Nuclear Materials 390-391 (2009) 610
'Pre-design of magnum-PSI: A new plasma-wall interaction experiment', H.J.N van Eck et al., Fusion Engineering and Design 82 (2007) 1878