Gas-jet target with online interferometric thickness measurement for nuclear astrophysics

TL;DR

This work presents a newly developed gas-jet target for the Felsenkeller 5 MV underground accelerator, featuring online interferometric thickness measurement to enable precise in-beam thickness control for nuclear astrophysics experiments. The target combines a jet chamber with a five-stage differential pumping scheme, allowing a thick nitrogen jet ($\sim1.5\times10^{18}$ cm$^{-2}$) or a wall-type jet ($\sim7\times10^{17}$ cm$^{-2}$) while preserving the accelerator vacuum. Thickness is determined via three independent methods: CFD simulations, Mach-Zehnder laser interferometry, and alpha-energy-loss measurements, all showing good agreement within about 5%. The interferometer setup yields two-dimensional thickness maps with sub-millimeter resolution, and the thickness scales linearly with inlet pressure up to at least $6$ bar, enabling flexible optimization of target density for radiative capture studies. The system has been demonstrated on the surface and commissioned at the underground site, with plans to complete the windowless static-type gas target behind the jet and to analyze alpha-beam data for future publications.

Abstract

A new jet gas target system has been developed for the Felsenkeller 5 MV underground ion accelerator for nuclear astrophysics. It provides either a 1.5$\times10^{18}$ cm$^{-2}$ thick cylindrical jet or a 7$\times10^{17}$ cm$^{-2}$ thick wall of nitrogen gas, with a surface of 10$\times$10 mm$^2$ to be seen by the ion beam. The system includes a de Laval type nozzle and altogether five pumping stages: In addition to the jet catcher and the jet chamber surrounding it, there are three stages connecting the jet to the ion accelerator. Behind the jet chamber, as seen from the ion beam, a windowless static-type gas target and, subsequently, a beam calorimeter have been installed. This work describes the offline tests of the gas target system prior to its installation on the beam line of the Felsenkeller accelerator. The thickness of the jet has been determined using three different methods: By computational fluid dynamics simulations, with a Mach-Zehnder interferometer, and by $α$-energy loss using a mixed $α$ source. The three methods were shown to be in agreement. For 0-6 bar inlet gas pressure, a linear relationship between inlet pressure and jet thickness has been found. Different shapes of de Laval type inlet nozzles, both circular and slit-type, have been manufactured from fused silica glass or stainless steel and tested using measurements and simulations. The power and stability of the beam calorimeter have been tested. The interferometry has been shown to work reliably and to give two-dimensional projections of the gas jet with sub-mm resolution.

Figures (14)

• Figure 1: Schematic view of the Felsenkeller gas target. Gas flow directions are indicated by arrows. The dimensions of the apertures A-0 to A-4 are given in the upper right corner. Pumps shown are of turbomolecular (ATH, HiPace), Roots (Okta), and multi-stage Roots (ACP) types, vacuum gauges of full range (PKR) or capacitance (CMR) types, gate valves (V), and needle dosing valves (NV). Two planned future upgrades are shown in dashed boxes.
• Figure 2: Photograph of the gas target system. Downward looking on top is the ATH 2303 M turbopump (chamber). On the bottom, partially hidden by Bosch profiles, the Okta 1000 M (Catcher). The structure on the right hosts the ACP backing pumps. The laser and optics are in two black boxes on either side of the center. See text for details.
• Figure 3: Pressure profile as a function of nozzle inlet pressure for nitrogen gas, cylindrical nozzle CF-64-1, and cone-shaped catcher C20 of 20 mm diameter.
• Figure 4: Schematic drawing of the calorimeter inside the static-type gas target. The beam hits the calorimeter from the left side.
• Figure 5: Power tests of the beam calorimeter at hot side temperatures 50-100 $^\circ$C and fixed chiller temperature of 1 $^\circ$C.