Negative Hydrogen ion density measurement in a permanent magnet based Helicon Ion source (HELEN-I) using cavity ring down spectroscopic technique
- Mr. Debrup MUKHOPADHYAY
- Mr. Debrup MUKHOPADHYAY (Institute for Plasma Research)
Negative ion source based neutral beam injection system is one of the most efficient method of fusion plasma heating. Negative ion source is used instead of positive ions in case of higher power NBIs,due to high neutralization factor in the beam energy range useful for ITER type fusion reactors.To understand the efficiency of negative ion source, negative ion density measurement is a prerequisite. In the present work, a highly sensitive cavity ring down spectroscopy(CRDS) system has been developed and employed in a in house developed permanent magnet based Helicon plasma source (HELEN-I) operated in volume production mode, to measure the line integrated negative ion density in it. The present CRDS system uses a Nd-YAG laser of wavelength 1064 nm. A stable optical cavity is created by using highly reflecting mirrors of reflectivity ~99.99%. The stable cavity is part of the plasma chamber and is created in such a way that hydrogen plasma is present within the cavity volume. When the laser enters the cavity through the first mirror, it suffers multiple reflections on the inner surfaces of the cavity mirrors and therefore passes through the plasma volume multiple times.The transmitted intensity through the second mirror is measured with a photo detector. During the travel time (~100 microsec), the laser photon photo detach the loosely bound electrons from negative hydrogen ions in the plasma volume within the line of sight of the laser beam. As a result, the laser pulse intensity goes down exponentially with time. The characteristic of the temporal decay of the laser pulse intensity,known as ring down time(RDT) is a function of line of sight integrated negative ion density. The experimental result of the negative ion density measurement by CRDS system for different working pressure, axial magnetic field and RF (13.56 MHz) power are discussed. It is seen that at high power range (800-950 Watt), when the plasma source is working in Helicon(w) mode, having plasma density of order of 10^18 per meter cube and a plasma temperature of ~2 eV. The corresponding line integrated negative ion density of ~10^16 per meter cube in volume mode without any application of transverse filter magnetic field.