Topic/Type: 1.2 Fusion Plasmas (magnetic & inertial confinement), Oral

Particle Tracking in Complex Geometry

D.B. King, W. Arter

UKAEA Fusion, Abingdon, OX14 3DB, UK

Neutral beams are regularly used on fusion devices to provide additional heating. The JET neutral beams regularly provide more than 20MW of power for up to 10s. Background gas in the neutral beam duct can cause high energy neutral particles to re-ionise through collisions. These ionised, high energy particles are affected by the stray tokamak fields in the duct region, causing them to collide with the duct walls. The power deposited by these re-ionised particles can be strongly focused, leading to significant power densities on the duct wall. This effect can be a major limiting factor in neutral beam operation, so accurate knowledge of re-ionised power is vital for the design of beamline components.

Re-ionised power models are currently required for MAST Upgrade, JET EP2 and post-ITER systems. These projects have very different and complex geometries, background gas densities and magnetic fields. Beam properties such as energy, fractional energy components, total beam power and number of beams vary widely.

To calculate these power models a 3D particle tracking code has been developed at Culham. SMARDDA is a Monte Carlo code that can import data from CAD assemblies to allow representation of complex geometries. This allows easy portability of the code to different machines and an accurate representation of the beamline. Neutral particles are generated within a beam envelope and tracked along the beamline. Charged particles are created at each neutral tracking step with a power dependent on the reaction cross section and gas density. The charged particles are tracked in the magnetic field using a leapfrog method.

The particle interesection with the duct wall is found using the SMARDDA particle tracking algorithm. This algorithm is a variation of the DDA schemes [1], which uses the exclusive \'OR\' of the SMART algorithm of Spackman and Willis [2]. Points are located with respect to an octree using bit level manipulation. The octree here is an hierarchical data structure designed to address efficiently triangulated surfaces in 3-D space. The final stage of calculating track intersection with the duct wall is performed by Badouel\'s algorithm [3].

The final code is an efficient, generalised tool for calculating total power density on the duct walls from any combination on beam properties. The algorithms developed at Culham and the first results from the code will be presented.

[1] A.Y. Chang. A survey of geometric data structures for ray tracing. Technical
report, Polytechnic University, Brooklyn, 2001.

[2] J. Spackman and P. Willis. The SMART navigation of a ray through an
oct-tree. Computers and Graphics, 15(2):185?194, 1991.

[3] D. Badouel. An efficient ray-polygon intersection. In A.S. Glassner,
editor, Graphics Gems, page 390. Academic Press Professional, Inc.
San Diego, CA, USA, 1990.