_d (where r_c is the cyclotron radius and
_d the Debye length). The pure ion plasma is contained
in a Penning-Malmberg trap for weeks near thermodynamic equilibrium (i.e. rigid rotor, uniform
density and temperature). Particles losses are suppressed by a "rotating wall," i.e. an
electrostatic potential on the wall rotating faster than the plasma. Test-particle transport is
measured even though there is zero net transport.
The ion spin orientation is used to "tag" the test particles. First, all spins are aligned by direct optical pumping, then the test particles at a particular radius are tagged by reversing their spins. Standard Laser Induced Fluorescence (LIF) techniques are used to non-destructively detect the tagged particles as they move radially. Spin orientation is maintained for 10-1000 sec (for 5 eV > T_i >0.05 eV), which is longer than the measured transport time.
The observed test particle transport is diffusive, i.e. proportional to the gradient of the
test particle density, with a near zero convective contribution. Over a range of 50 in temperature,
20 in density, and 4 in magnetic field, the measured diffusion coefficients are about 10 times
greater than the classical coefficient D_clas 
1.25v_ii*(r_c)². (Here, neutral collsions are negligible.) In contrast, measurement of the
ion-ion collision frequency v_ii using T_parallel to T_perp thermalization
agrees closely with classical theory.
Classical transport theory describes steps in the position of an ion guiding center due to
collisional scattering of the ion velocity vector, arising from impact parameters rho <r_c.
A new theory by O'Neil describes "ExB drift" collisions with impact parameters
in the range of r_c < rho < _d. The colliding
ions travel along a field line with small relative velocity Del_
v and therefore interact for a long time, giving an enhanced diffusion coefficient
D_ExB = D_clas *1.5 *pi^(½) *ln(v_bar/Del_v).
Estimates of the minimum Del_v due to velocity scattering
(or due to rotational shear for high temperature plasmas) predict D_ExB in close agreement
with the measured diffusion coefficients. The experiments thus demonstrate that non-velocity
scattering collisions neglected by classical theory can dominate in plasmas with
_d > r_c.
