Blower Gun pellet injection system for W7-X

Abstract

Foreseen to perform pellet investigations in the new stellarator W7-X, the former ASDEX Upgrade Blower Gun was revised and revitalized. The systems operational characteristics have been surveyed in a test bed. The gun is designed to launch cylindrical pellets with 2 mm diameter and 2 mm length, produced from frozen Deuterium D 2 , Hydrogen H 2 or a gas mixture consisting of 50% H 2 and 50% D 2 . Pellets are accelerated by a short pulse of pressurized helium propellant gas to velocities in the range of 100-250 m/s. Delivery reliabilities at the launcher exit reach almost unity. The initial pellet mass is reduced to about 50% during the acceleration process. Pellet transfer to the plasma vessel was investigated by a first mock up guiding tube version. Transfer through this S-shaped stainless steel guiding tube (inner diameter 8 mm; length 6 m) containing two 1 m curvature radii was investigated for all pellet types. Tests were performed applying repetition rates from 2 Hz to 50 Hz and propellant gas pressures ranging from 0.1 to 0.6 MPa. For both H 2 and D 2 , low overall delivery efficiencies were observed at slow repetition rates, but stable efficiencies of about 90% above 10 Hz. About 10% of the mass is eroded while flying through the guiding tube. Pellets exit the guiding tube with an angular spread of less than 14°.

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

“…Consequently, the 3.2 MW of ECRH used during this phase produces plasmas that have T e ≫ T i centrally as the core temperature measurements from Thomson scattering and the X-ray imaging crystal spectrometer (XICS) diagnostics show (third plot in the figure). A density ramp is initiated 1.86 s into the discharge by injecting a series of 28 frozen hydrogen pellets into the plasma at a frequency of 30 Hz using a so-called blower gun 42 . Although the fuelling efficiency of the first few pellets is poor, noticeable improvement occurs thereafter 26 and increments in the line-integrated density caused by individual pellets become clearly discernible with ∫dℓn e slightly exceeding 12 × 10 19 m −2 when the series ends at t = 2.8 s. At this time point, the ECRH power is increased to 4.5 MW to maintain T e at values sufficient for good O2 absorption.…”

Overview of first Wendelstein 7-X high-performance operation

“…The average duration of the pellet ablation is known from the ≈1 ms extent of the H α ablation emission originating from excited neutrals of the ablated pellet material. Since the blower-gun injector [32] cannot provide precise control over the pellet velocity, the injection timing suffers from a jitter which is an order of magnitude larger than the ablation duration. This made an event synchronization necessary.…”

High-performance plasmas after pellet injections in Wendelstein 7-X

“…In the recent experimental campaign OP1.2 that took place in 2017 and 2018 [30], a pellet injector with a restricted number of pellets [33] was available to verify the fuelling performance, to compare the outboard and inboard side injections and to test operational scenarios. In this paper, we study the properties of plasmas after pellet injections and compare them to similar gas-fuelled discharges.…”