Formation of molecular gas jets for proton beam characterization

Beam profile measurements on the ECR microwave H+ ion source at HUN-REN CER have been conducted using a 45°-inclined Faraday cup and an infrared camera. While this setup allows for current and spatial distribution measurements, its resolution is limited by the IR camera and thermal diffusion on Faraday cup's surface. Emittance has been evaluated through simulations, but no direct measurements are yet available. To address this, a gas-jet-based, non-invasive diagnostic system is being developed to measure both the beam profile and emittance without affecting source performance. Among various diagnostic options-such as scintillating screens, optical transition radiation screen, and residual gas techniques-the gas-jet approach was selected for its robustness under high beam power. The system uses an injector nozzle and skimmers to form a supersonic molecular gas curtain with a thickness of 1-2 mm, a transverse size of 10 × 10 mm2, and a density in the range of 1016-1017particlesm3. As the beam passes through the curtain, it excites the gas atoms (e.g., Ne and Ar), producing photon emission captured by a camera. With proper calibration, this allows reconstruction of the beam current profile. Introducing a mesh before the curtain enables emittance measurement via beamlet formation. This paper presents a semi-analytical model for gas curtain formation and density evolution, along with a design case study tailored to the HUN-REN CER proton ion source.