A horizontal powder injector for W7-X

Abstract

Injection of low-Z powders into fusion plasma has been used to improve wall conditions, similar to the standard boronization process using diborane. Powder injection has the advantage of being much simpler, non-toxic, and efficient. The W7-X stellarator is planning on utilizing powder injection in long pulse discharges; a proof-of-principle test for horizontal injection into the plasma was conceived, a Probe Mounted Particle Injector (PMPI). The PMPI concept is developed using a polyetheretherketone (PEEK) paddle wheel hub that is driven by a piezo motor, due to the high magnetic fields, that rotates at 100 deg/s. This small PMPI fits into an envelope of 120 mm diameter x 150 mm long, a standard size for Multi-Purpose Manipulator (MPM). The device is housed in a carbon cup mounted on a retractable probe that can be placed near the plasma edge, enabling powder injection ~4-8 cm radially into the boundary plasma. The feed for the paddle is via piezo electric actuator that vibrates a funnel filled with powder into a trough for the paddle to push. The 8 paddle arms, 35 mm long and 10 mm wide, are made from 0.38 mm thick PEEK which drag slightly along the powder-filled trough bottom, becoming a spring-loaded paddle which accelerates the powder upon release. Design challenges are the high ambient magnetic field, vacuum compatible materials, high temperature environment, limited rotary-drive options, and compact space. The design and testing of this new device will be presented.

Mitigation of plasma–wall interactions with low-Z powders in DIII-D high confinement plasmas

“…Top powder injection facilitates dissipation in upper divertor configuration but might be less effective in lower divertor configurations. Additional devices have been developed and used for horizontal material, and powder injection, also suitable for lower divertor applications [52,[55][56][57]. Bottom-up injection of materials into the lower divertor has recently been demonstrated at DIII-D and will be further developed.…”

Characterization of the W7-X scrape-off layer using reciprocating probes

“…OP1.2b 18 samples for material exposition IPP-LBO1 [21] OP1.2a/b Holds four coated glass targets for laser ablation PPPL-PMPI1 [22] OP1.2b Horizontal powder flinger for boron impurity injection As an interesting note, the various patches of slightly different hues in the island region in Figure 2 b) are all intersected by either the upper divertor in module 1 or the lower divertor in module 3. However, the number of toroidal orbits around the entire machine (major radius R ∼ 5.5 m) until the field line hits the target varies, resulting the in the connection length distribution being in Figure 2 b) being more complex than the divertor target map in Figure 2 c).…”

“…It is installed at the outboard mid-plane and can perform fast reciprocating plunges through the island chain up to the LCFS of the confined plasma. While the MPM was employed for a wide variety of applications (such as magn etic probes, material studies, impurity injection, gas fueling and more), we here focus on electric probe measurements which FZJ-COMB1 [13,14] OP1.1 Nine electric probes, magnetic pick-up probe FZJ-COMB2 [15,16] OP1.2a/b Nine electric probes, magnetic pick-up probe, ion sensitive probe, material exposition, gas pipe IPP-FLUC1 OP1.2a/b 28 Electric probes (poloidal array, parallel + poloidal Mach probe) FZJ-MACH1 [17] OP1.2a Polar (Gundestrup) + radial Mach probe array (28 electrodes) FZJ-RFA1 [18] OP1.2a Six retarding field analyzers, two electric probe pins FZJ-GAS1 OP1.2a Four electric probe pins, gas pipe FZJ-GAS2 OP1.2b Four electric probe pins, piezo valve for gas injection FZJ-MACH2 OP1.2b Polar (Gundestrup) + radial Mach probe array (28 electrodes) FZJ-RFA2 OP1.2b Six retarding field analyzers, four electric probe pins, gas pipe RFX-HRP1 [19] OP1.2b Three magnetic pick-up probes, eight electric probe pins, three Mach probes NIFS-FILD1 [20] OP1.2b Eight Faraday films for fast ion loss detection FZJ-MAT1 OP1.2a Eight samples for material exposition FZJ-MAT2 OP1.2b 18 samples for material exposition IPP-LBO1 [21] OP1.2a/b Holds four coated glass targets for laser ablation PPPL-PMPI1 [22] OP1.2b Horizontal powder flinger for boron impurity injection provide profiles of key plasma param eters: electron temperature, density and electric field are determined from triple probes and swept Langmuir probes, while plasma turbulence characteristics are inferred from spatially distributed arrays of probes operating in floating potential or ion satur ation current mode. This paper is structured as follows: after introducing the MPM and the magnetic configuration space in section 2, the IPP-FLUC1 probe head and probe analysis techniques are presented in section 3.…”

Plasma–surface interaction in the stellarator W7-X: conclusions drawn from operation with graphite plasma-facing components

“…Iron (Fe) is caused by first wall erosion during He GDC prior to the first boronisation which is also linked to O release due to erosion of surface oxides. Note that directly prior to the third boronisation, an in situ B deposition via B 4 C was performed by a powder injector system located at the midplane manipulator during plasma operation [36,37]. About 1.5 g B was introduced into W7-X and deposited primarily on the target plates; this B is embedded the third broad B layer in figure 7(c).…”