Development of a dual $\bf v$ and $\dot{\bf B}$ diagnostic for fast reconnection

The Terrestrial reconnection experiment (TREX) under construction at the Wisconsin Plasma Astrophysics Facility will study magnetic reconnection in a low-$\beta$, collisionless plasma. A probe to simultaneously measure the velocity and magnetic fields during a fast reconnection event is being constructed. An array of 3D $\dot{\bf B}$ probes and 2D Mach probes will measure at a $\sim$2 cm spatial resolution and MHz frequencies. Using a digitally controlled drive, the probe will be able to sweep the full radial length (1.5 m) of the experiment and through an angle of $\sim$1 radian, thus providing significant coverage of the anticipated event region. Measurements and results from the probe will be presented.