Proceedings Article | 14 May 2008
KEYWORDS: Pulsed laser operation, Laser ablation, Wind energy, Aluminum, Error analysis, Calibration, Ultrafast phenomena, Oscillators, Curium, Vehicle control
An impulse measurement device and analysis package was conceived, designed,
constructed, tested, and demonstrated to be capable of: measuring nanoNewton-seconds to
milliNewton-seconds of impulse due to laser-ablation; being transported as carry-on baggage;
set-up and tear-down times of less than an hour; target exchange times of less than two minutes
(targets can be ablated at multiple positions for thousands of shots); measurements in air and in
vacuum; error of just a few percent; repeatability over a wide range of potential systematic error
sources; and time between measurements, including ring-down and analysis, of less than 30
seconds. The instrument consists of a cantilever (i.e. leaf spring), whose time-dependent
displacement/oscillation is measured and analyzed to determine the impulse imparted by a laser
pulse to a target. These shapes are readily/commercially available, and any target material can
be used, provided it can be fashioned in the form of a cantilever, or as a coating/film/tape,
suitable for mounting on a cantilever of known geometry. The instrument was calibrated both
statically and dynamically, and measurements were performed on brass, steel, and Aluminum,
using laser pulses of ~7ns, ~500ps, and ~500fs. The results agree well with those published in
the literature, with surface effects, atmosphere, and
pre-/post-pulses demonstrating interesting
effects and indicating areas for further study. In addition to exploring space-propulsion
applications, measurements were performed to explore the strong beneficial effects of depositing
lines of energy ahead of supersonic and hypersonic vehicles. This deposition creates a low-density
channel, through which a vehicle can travel with dramatically reduced drag.
Temperature and pressure are both also reduced on the front surfaces of the vehicle, while density and pressure are increased at the vehicle base. When applied off-center, this technique can be used to control the vehicle, employing the entire body as the control surface and eliminating the need for actuators. Numerical results for
drag-reduction, temperature-reduction, and control forces are indicated here.
