Matthew Carr Supervised by Dr Joe Khachan First steps towards a Polywell fusion device prototype

Research in to the polywell IEC device at University of Sydney started in February 2009 Two research students working on the project Outline the steps taken to design and construct a polywell prototype Discuss preliminary experimental results Discuss necessary upgrades and improvements needed for next round of experiments Overview

Invented by R Bussard in Patented in 1989 and Combines elements of IEC and magnetic confinement fusion. Polywell Concept Image: "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, 57th International Astronautical Congress, 2006

Like other IEC systems, the principle uses a virtual cathode to accelerate positive ions in the background plasma in to the centre of a deep potential well of electrons Roots in Traditional IEC Image: “The Polywell: A Spherically Convergent Ion Focus Concept”, N Krall, Fusion Technology, Reformated in colour by Mark Duncan, 2007 Converging ions can undergo fusion reactions, or pass through the well and fall back in again

Use large magnetic fields instead of electrostatic grids to create virtual cathode Field created by pairs of oposing current loops, each creating a cusp about the origin. Differences to IEC Magnetic fields vanish in centre due to symmetry creating a null point. Particles in the well see a magnetic mirror on all sides Image: “Forming and Maintaining a Potential Well in a Quasispherical Magnetic Trap”, N Krall, Physics of Plasmas 2, Reformated in colour by Mark Duncan, 2007

Between 1989 and 2006 he built 13 prototype machines. Using experimental data he found that Power loss to fusion power production will always decrease with: – increasing drive voltage – increasing B field – increasing size “Because of this, it is always possible to reach a condition of power breakeven... with any fusion fuel combination”[1] Scaling Laws [1] "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, 57th International Astronautical Congress, 2006

Stronger B fields will enhance the magnetic bottle effect and lead to greater confinement Large B fields achieved through both large coil currents and high numbers of turns in the coils Competing relationship between rise time and inductance. A large inductance makes the time constant longer. Adiabatic heating of electrons requires a fast B field rise time. Magnetic Field Strength [1] "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, 57th International Astronautical Congress, 2006

Limited to cylindrical bell jar, 42cm in diameter WB6 sized machine would not fit because of e-gun space and large volume for electron recirculation Our version must be scaled down so as to still test and observe recirculation effects Size limitations enforce practical limits on the number of turns we can use Recirculation

Size limitations and inductance argument dictate we should use a small number of turns and provide a large base load current. To achieve 1 Tesla of magnetic field in polywell faces of 6cm diameter, we require a current of 10kA. 10 turns is a middle ground between size and inductance limitations, and the need for a more practical base current of 1kA. Current Requirements

Size limitations and inductance argument dictate we should use a small number of turns and provide a large base load current. To achieve 1 Tesla of magnetic field in polywell faces of 6cm diameter, we require a current of 10kA. 10 turns is a middle ground between size and inductance limitations, and the need for a more practical base current of 1kA. 10 turns reduces asymmetry effects from feed lines. Magnetic Field Requirements

Critically damped current pulse from a capacitor in near short circuit conditions. Our capacitors are 1.5mF Aluminuim electrolytic Voltage rating of 450Vdc By using only ten turns per coil, we have kept our resistance and inductance low, close to critical damping conditions. Simplest Approach

Mechanically Strong enough to withstand coil repulsion Should not obstruct any of the face cusps or corner cusps Polywell Structure Image: “Should Google Go Nuclear?”, Mark Duncan, 2008,

Polywell Structure [1] "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion", Robert W. Bussard, 57th International Astronautical Congress, 2006 Metal structures will have substantial eddy currents induced across their surface area. Creates two problems: – Energy wasted to heating the metal former – Back emf generated will partially shield the interior, possibly leading to unusual field configurations outside the design criteria. Therefore all metal surfaces should be conformal to the magnetic field lines.[1]

Current Calibration

Experimental Setup

Forming potential wells with the polywell is definitely possible Achieved floating potentials as deep as 250V Time scales vary over 3-5ms Explore; pressure, field strength, electron injection energy, electron injection current, and injection angle. Preliminary Results

Expected an upward trend with respect to increasing B Have observed a near parabolic turning point in our data Increasing B field

10kV 15mTorr 7.5kV 25mTorr

Pressure Effects 15mTorr25mTorr Orange => 10kV Blue => 12.5kV Green => 15kV Potential disapears at 35mTorr

Potential well Comparisons Image: “Forming and Maintaining a Potential Well in a Quasispherical Magnetic Trap”, N Krall, Physics of Plasmas 2, Reformated in colour by Mark Duncan, 2007 N Krall reports observing potential wells as deep as the energy of the e-gun Too early for us to make comparisons to their findings Our e-gun currents are a factor of 1000 smaller than the e-gun used by Krall

Zoo of potential well Structures

Where to from here? e -guns need to be upgraded for higher currents. Most likely a pulsed cylindrical cathode. Expand to lower pressure ranges Test scaling laws claimed by Bussard in regards to larger sizes, and magnetic fields

Another student is building a numerical model based on single particle motion Fields are generated at point of iteration using vector potentials, giving B and E due to changing current B and E are assumed constant at this point. Then the trajectory is derived from E cross B drift Recirculation is ignored and electrons that leave the polywell are considered lost. Numerical Model

Any questions? Thank You