UC San Diego

A prototype “top launch” electron cyclotron (EC) heating system has been developed at the DIII-D tokamak that utilizes an in-vessel mirror to reflect an outside-launched microwave beam into the plasma with a nearly-parallel path along the resonance line. The DIII-D EC system currently has eight steerable mirror systems and one of these has been used to test this prototype system. In 2020 and 2022, two top launch systems were installed on DIII-D using in-vessel waveguides through upper ports on the machine. These systems demonstrated a reactor-relevant technology for driving off-axis current in Advanced Tokamak (AT) scenarios. However, one of the in-vessel waveguides failed and both systems were removed in 2023. This new prototype was designed to replace the previous systems with a reliable alternative at 10% the cost. The prototype in-vessel mirror is made of ATJ graphite to match the material of the DIII-D first wall. The mirror’s surface is canted at a fixed angle to reflect the beam at the desired angle and it focuses it inside the plasma. The beam’s propagation was measured to closely match the design via an infrared camera and a target setup. The mirror also has thermocouples embedded in it to monitor its temperature. The mirror was predicted to reflect approximately 95% of the power incident on its surface, which was confirmed via thermocouple readings. The successful demonstration of this prototype system will result in its duplication for all steerable mirror systems on DIII-D, which is predicted to double its off-axis current drive capabilities.