Proton Linac Development Division
External RF Antenna Based Negative Hydrogen Ion Source

An external RF antena based pulsed negative hydrogen ion source has been developed in-house. It has been successfully tested for extraction of 16 mA negative hydrogen ion beam current of 2 ms pulse duration at 50 keV and 2 Hz repetition rate, and 6 mA at 50 Hz repetition rate. It consists of pulsed RF ignitor, main plasma generator, three electrode extraction system, extraction chamber, hydrogen gas purging system, RF and DC power supplies, high voltage deck and vacuum system. The plasma generating and extraction equipments are mounted on the high voltage deck (80 kVDC rated). The vacuum and accelerating system (50 kVDC) are at ground potential. Differential vacuum is created by using 3 turbo molecular pumps having 3000 lit/sec capacity connected to 6-port extraction chamber. During the operation hydrogen gas is purged at 25 SCCM generating 5.5X10-2 mbar gas pressure in plasma chamber and 4.5X10-4 mbar in extraction chamber. A low density, low temperature seed plasma is generated using an ignitor, based on 13.56 MHz, 1 kW pulsed RF source. For this, wide band 1 kW amplifier is developed in-house. High density plasma generated using 2 MHz, 60 kW pulsed RF power through external antenna. Pulsed 2 MHz RF power source is developed using SiC MOSFET along with RF transformer and matching network. The RF power is coupled to the main plasma by an external RF antenna through inductive coupled mechanism. As soon as seed hydrogen plasma is created, ignition antenna peak current reduces from 60 A to 30 A. Just before ignition current pulse ends, the main 2 MHz RF source starts delivering power to the plasma. Negative hydrogen ion extracted using three electrode (plasma electrode, extraction electrode and ground electrode) extraction geometry. The co-extracted electrons from plasma electrode with ~12 keV energy are steered and dumped in the extraction electrode by permanent magnet dipoles kept inside extraction electrode. Heavy negative hydrogen ions have very little bending due to these dipoles and travels straight towards ground electrode. The extracted negative hydrogen ion beam current measured using Faraday cup. This RF based negative hydrogen ion source will be used for the 1 GeV negative hydrogen Linac for IFSR applications.

RF based Negative Hydrogen Ion Source

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