Development of external RF antenna based cusp free high duty factor pulsed negative hydrogen ion source

An external RF antenna based cusp free negative hydrogen (H−) ion source has been designed and developed. This source operates at 10% duty factor and the key experimental results are reported in the paper. The extracted H- ion beam current is 11 mA with 2 ms pulse duration and 50 Hz repetition rate at 50 keV beam energy. Operation of the H- ion source at high duty factor results in temperature rise in components. This may lead to failure of electronic components, vacuum joints of the plasma or igniter chambers, or burn out the extraction electrodes. In order to keep the operating temperature within limits, a water cooling system was designed and incorporated for (i) the 2 MHz RF antenna operating at 90 A RMS at 7 kV AC, (ii) the extraction electrodes, operating at a maximum voltage of 15 kV DC, (iii) the plasma chamber (made of Aluminium Nitride, transparent to RF field with high thermal conductivity), and (iv) the Faraday cup for H- ion current measurement. Forced air cooling was used for the 13.56 MHz RF-based pulsed igniter, plus on the various current-stabilizing electrode biasing networks and RF impedance matching networks.