Negative ion characterization in a helicon plasma source for fusion neutral beams by cavity ring-down spectroscopy and Langmuir probe laser photodetachment

Abstract

Negative ions are characterized in the helicon plasma source RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center by means of Cavity Ring-Down Spectroscopy (CRDS) and Langmuir Probe (LP)-assisted laser photodetachment. A high density and axially homogeneous plasma column is produced via a RF antenna able to sustain the propagation of helicon waves in a steady state regime. An electron density n e ∼ = 2.0 × 10 18 m −3 in H 2 plasma at 0.3 Pa and 3 kW of input power is measured in the center of the plasma column by LP and microwave interferometry. The electron temperature profile is peaked on axis reaching T e ≈ 5 eV and decreasing to 1.5 eV at r = 0.05 m. Thus, a hot core region forms where H 2 molecules are rovibrationally excited (H 2 (ν)), and a cold edge, where low energy electrons can attach to H 2 (ν) and produce H − ions by dissociative attachment. In this work we use LP-assisted laser photodetachment and CRDS diagnostics to measure H − and D − radial density profiles and how they depend on source parameters. We show that negative ions are distributed on a shell of 0.06 m radius with a peak value of ∼ 2.0 × 10 16 m −3 in H 2 plasma. These results suggest that, although substantial technical development is needed, helicon plasmas could be considered as a possible candidate as sources of negative ions for future NBIs.

Direct measurements of electronic ground state ro-vibrationally excited D2 molecules produced on ECR plasma-facing materials by means of VUV-FT absorption spectroscopy

“…In this work, the unique combination of a SR source and Vacuum Ultra Violet Fourier Transform Spectrometer (VUV-FTS) allows a direct and accurate measurement of the density of the ro-vibrationnal components of the D 2 molecule in its ground state. Recent results have shown that a caesium-free (Cs-free) Helicon plasma reactor, named RAID, produces at 3 µbar and 5 kW a negative ion density of ∼ 2 × 10 16 m −3 in H 2 plasma and ∼ 1.75 × 10 16 m −3 in D 2 plasma 75 . In comparison with BATMAN 25 ion source, the one eighth scale prototype of the future negative ion source of ITER tokamak, the same density of H − is obtained with an input power of 40 kW when using caesium 76 .…”

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…If the negative ion to electron n i -/n e density ratio is considered, results obtained at similar plasma conditions and position in PROMETHEUS-I reactor (see Sect. 3.1) [67] (a network of five identical ECR dipolar sources) and in the RAID reactor [76] are comparable: 0.18 and 0.225, respectively. In consequence, the plasma features, produced by an ECR dipolar source, can be considered similar enough to the one obtained in the RAID reactor to allow an investigation of the ECR plasma rather than the helicon plasma when plasma diagnostic cannot be set up for RAID.…”

“…2.2.1 Dipolar plasma source to scrutinize the production of ro-vibrationally excited molecules at lower scale Actual neutral beam injector (NBI) design shows a global efficiency below 50% (The total or global efficiency of the injector, called wall-plug efficiency is the ratio of the neutral power at the entrance of the tokamak and the total electric power consumed in the injector (ion source + accelerator + recovery electrode + auxiliary systems). A higher aspect ratio (height over width) could increase the efficiency to more than 60% [76]. In particular, the photoneutralization could be employed on blade-like beams.…”

“…In the RAID reactor (see Sect. 3.3), a helicon antenna (bird cage type) has proved its ability to produce, at 0.3 Pa and 5 kW, a negative ion density of 2 × 10 16 m −3 and 1.75 × 10 16 m −3 in H 2 and D 2 Cs-free plasmas [76], respectively. Similar density of H − (3.5 × 10 16 m −3 ) is obtained in the BATMAN ion source with an input Fig.…”

“…The results obtained in the RAID reactor at EPFL [76,79] demonstrate, at full scale, the efficiency of dissociative electron attachment (DA) to produce a large density of negative ions in Cs-free plasmas. The yield of DA strongly depends on the vibrational level of the ground state molecules [19]: the cross section increases from 3.11×10 −28 to 1.08×10 −21 m 2 for increasing vibrational level from v = 0 to 6.…”