On the NBI system for substantial current drive in a fusion power plant: Status and R&D needs for ion source and laser neutralizer

• NBI is a candidate for a cw tokamak DEMO due to its high current drive efficiency.
• The plug-in efficiency must be improved from the present 20–30% to more than 50%.
• A suitable candidate is a photo neutralizer with almost 100% neutralization efficiency; basic feasibility studies are underway.
• Cw operation with a large availability puts rather high demands on source operation with some safety margins, especially for the components with high power density loads (source back plate and extraction system).
• Alternatives to the present use of cesium are under exploitations.

The requirements for the heating and current drive systems of a fusion power plant will strongly depend on the DEMO scenario. The paper discusses the R&D needs for a neutral beam injection system — being a candidate due to the highest current drive efficiency — for the most demanding scenario, a steady state tokamak DEMO. Most important issues are the improvement of the wall-plug efficiency from the present ∼25% to the required 50–60% by improving the neutralization efficiency with a laser neutralizer system and the improvement of the reliability of the ion source operation. The demands on and the potential of decreasing the ion source operation pressure, as well as decreasing the amount of co-extracted electrons and backstreaming ions are discussed using the ITER requirements and solutions as basis. A further concern is the necessity of cesium for which either the cesium management must be improved or alternatives to cesium for the production of negative ions have to be identified.