The path to a higher performing source is well known: increase the magnetic fields within the source.

However, VENUS represents the maximum magnetic field that can be reached with a traditional ECR ion source geometry using the superconductor NbTi, so a field increase will require either new superconducting materials or optimized geometry.

Replacing NbTi with Nb₃Sn in a VENUS-like geometry is being attempted elsewhere, but years of effort have shown that this material is extremely difficult to use in ECR ion sources. A more reliable path to a higher-performing source has been proposed by Dan Xie of NSD: the MARS-D (Mixed Axial and Radial System-Demonstrator) ECR ion source. This source replaces the six racetrack sextupole coils of a VENUS-like source with a single closed-loop-coil (see Figure 1) that, alone, provides both radially-confining sextupole fields and axially-confining solenoid fields.  This single coil requires smaller additional solenoids than for a racetrack sextupole, allowing for higher coil current operation.

Figure 2. Expected beam energy vs. accelerated species mass for different ion sources, showing four sources used at the 88″ cyclotron, in blue, orange, green and red.  The proposed MARS-D, shown in purple, will lead to a significant further increase in performance.

The MARS-D magnet system centered around this closed-loop-coil will operate at resonant magnetic fields 60% higher than VENUS (45 GHz vs. 28 GHz heating), while still using well-tested NbTi.  These higher fields and heating frequencies increase the performance: MARS-D should produce U⁴²⁺ beams with the same intensity that VENUS produces for U³³⁺.  The ability to produce high current beams with higher charge states can significantly reduce the cost of a future accelerator or dramatically increase the capabilities of an existing one, as the kinetic energy of a linac scales linearly with charge state while for a cyclotron it scales as the square.

MARS-D will be the fourth ECR ion source designed and constructed at LBNL.  The construction of this source will reaffirm LBNL as the world-leader in ECR ion source technology, while coupling it to the 88″ Cyclotron will once again broaden this accelerator’s reach in terms of heavy-ion beam energy and intensity (Figure 2).  This source will serve as cost-effective and straight-forward means of upgrading existing facilities’ capabilities while simultaneously providing a powerful starting point for future accelerators to design around.