Generation of Hydrogen Negative Ions via an Electron Cyclotron Resonance Large-Area Plasma Source for Fusion Applications

Hydrogen negative ions (H⁻) are vital in hydrogen plasmas, with key applications in plasma-based material processing, accelerator physics, and, most prominently, thermonuclear fusion. In fusion systems, H⁻ ions enable efficient plasma heating and current drive, particularly in magnetically confined reactors. Conventional H⁻ ion sources for fusion typically employ inductively coupled plasma (ICP) systems powered by ~1 MW of radio frequency (RF) energy. These rely on collisional power coupling, which is inherently less efficient than resonant methods such as Electron Cyclotron Resonance (ECR). Despite ECR's potential for higher efficiency, its use in large-area, high-current negative ion sources remains relatively unexplored.

This study investigates an ECR-based large-volume plasma system designed to optimize both plasma generation and magnetic confinement for efficient volume production of H⁻ ions. Experiments were conducted using the ECR-based Large Negative Ion Beam Source (ELNIBS)—a cylindrical expansion chamber approximately 1 meter in height and diameter. Plasma was generated by a Compact ECR Plasma Source (CEPS) mounted at the chamber's top dome. The CEPS magnetic field decayed exponentially into the chamber, promoting plasma expansion. To further reduce plasma-wall interactions and enhance density, a three-dimensional magnetic confinement structure using permanent ring magnets was implemented around the chamber walls. Plasma diagnostics were carried out across microwave powers of 400–600 W and gas pressures ranging from 1–3 mTorr. Langmuir probes, positioned axially and radially, measured key parameters such as electron density and temperature. Positive and negative ion species were analyzed using a plasma sampling module (PSM) and a Hiden HPR-60 quadrupole mass spectrometer (MBMS). The system produced a spatially uniform plasma with electron densities on the order of n_e∼10¹¹ cm⁻³ and low electron temperatures (T_e​∼1 eV)—conditions favorable for H⁻ ion formation. The presence of H⁻ ions was directly confirmed via MBMS measurements. Detailed mass and energy spectra of the extracted ions will be presented.