## A few questions to consider while working through xspace's Particle
Motion modules:

On the ``Particle Motion: Uniform...'' page, look at the default orbit.
Note that the Field Strength is 200 nano-Tesla (this is typical for space
physics, not laboratory fusion plasmas). What does that imply must be spatial
units of the graphs. Try various values for the slider bars, verify that
the gyroradius scales proportional to v_perp/B.

On the ``E x B Drift'' page. Can you verify that the E x B drift is
independent of the particle's velocity, and scales only as E/B? (To be
quantitative look at the counters on the right showing the time and the
max Y position.) What choice of parameters causes the orbit to cross its
previous trajectory only once every gyro-period? What choice of initial
parameters will lead to orbits which look like half-ovals which do not
overlap at all, but whose slopes are discontinuous where the half-ovals
connect? What choice of parameters will give rise to smooth orbits which
don't overlap? (Note: I should eventually include pictures of these various
possible orbits, and leave it to you to figure out how to make them.)

``Gradient B Drift'': explore on your own.

``Curvature Drift'': Do the plots of the magnetic field lines make sense?
(Is there a bug in the code?)

``Mirror'': Do particles bounce even if they go out of view in the z-direction?
How well is magnetic moment mu conserved? Does this improve as rho/R is
made smaller, and get worse as rho/R gets bigger? (rho is gyroradius, and
R is the scale length over which B varies).

``Dipole'': The magnetic field around many planets can be approximated
as a dipole magnetic field, if one is close enough to the planet to ignore
the distortion caused by the solar wind (see the start-page of xspace).
Space physicists have observed that these planetary dipole magnetic fields
can trap a very high pressure plasma fairly efficiently, with beta approaching
unity! (beta is the ratio of the plasma pressure to the magnetic field
pressure B^2/(8 pi).) In the late 1980's or early 1990's, this fact led
Hasegawa and Chen to propose a fusion reactor concept based on a dipole
field (this is being investigated experimentally by Mauel at Columbia).