Multipole Ion-beam Experiment (MIX)


Theory:

With a few changes and additions to the IEC concept, some dramatic improvements in the performance (fusion rates and efficiency) of these machines is achievable. Replacing the ion-accelerating wire grid with a powerfull electromagnet facilitates the trapping of a cloud of electrons in its center. The number of ions able to meet and fuse there subsequently is no longer limited by the ions’ mutual repulsion, and so, at least in theory, a useful amount of net power can be produced. With the correct shape and geometry of the hardware, ions are still accelerated towards a common focus, but they are constrained to travel perpetually along unobstructed paths, allowing them to recirculate hundreds of thousands of times.  Of course, this requires very high vacuum levels (P < 10-6 Torr), and dedicated ion sources to produce ions at the top of the potential well.

While electrons do find themselves in the core of a MIX magnet (via ionization or secondary emission from ion impact on metal surfaces), injecting electrons with special biased emitters has the effect that the electric potential of the electron cloud in the magnet interior can be driven modestly negative, relative to the magnet surface. The shape of the cloud is such that electrons congregate near the openings of the magnet, and this causes a modification of the local potential- a very effective focusing lens is thus created for the ion beams, known as a Gabor lens.

MIXschematic

Schematic of MIX-8 device using Octahedral Electromagnet


The magnetic field produced by the MIX magnet is known as a multi-pole field, effectively an alternating arrangement of north and south poles arranged about each other to produce a field null in the center. This type of field produces a plasma confinement topology that is stable to magneto-hydrodynamic (MHD) perturbations, and is an effective means to trap not only electrons, but also a cold plasma. This core-trapped plasma can reach fairly high densities and may serve as a target for fusion collisions with the recirculating ion beams.


MAGtopology

Cross-section of mod-B surfaces for octahedral MIX magnet. Magnetic topology is cusped multipole, with field ~ 0 at device center


ScalaBeam

OPERA Simulation results for recirculating ion beam, trapped current several Amperes. Green areas: ions with kinetic energy of 100 kV, red areas are the turning points for deuterium ions


Experiment:

In 2009, we constructed a truncated cube prototype (MIX-14), water cooled (it consumed 6 kW of resistive power!). It was capable of 0.1 Tesla peak fields on the beam axes. This structure was biased up to 100 kV (negative) with respect to several ECR and duoplasmatron ion sources, and incorporated a number of openings for diagnostic access.

MIXphoto

MIX-14 cathodic magnet upon assembly 

MIXchamber

MIX device assembled and running


The MIX device was located in a 3ft thick concrete radiation bunker, with a large number of diagnostics systems and safety interlocks.  Up until 2011, it constituted the main experiment in our laboratories. MIX seemed initially to produce very encouraging results (such as I2 scaling of fusion rates, shown below), however it was soon determined that a fundimental problem to do with the physics of the interface between recirculating beams and external ion sources would always severely restrict the achievable power levels. This discovery, along with several ion source incarnations, ultimately led to the innovation we call the MARBLE


neutrons

Neutron rate vs. ion current for MIX single beam line as determined by He3 detector, Feb 2011


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