Setting Direct Laser Light Power Calculation Parameters

VISRAD utilizes a hybrid analytic/ray-trace model to compute the spatial distribution of the laser power deposited onto a target grid. The analytic model provides the spatial profile for the intensity at a surface element, while the ray-trace component is used to test occultation by other surface elements and energy loss due to light escaping the mesh through holes. This hybrid model avoids "speckled" distribution patterns that can occur when using a purely ray-trace model.

The ray-trace model sub-divides the light of each laser beam cone into a number of rays that pass through a grid of points within the cone. This grid is defined by N_r, the number of nodes in the radial direction, and N_θ, the number of azimuthal nodes. Increasing the number of points can increase the accuracy of the calculation, but also increases the CPU time required for the laser energy deposition calculation.

To adjust the parameters used for the ray-trace calculation, select the Lasers | Set Power Calculation Params menu item, and set the number of radial and azimuthal points in the Power Calc. tab.

Randomized Beam Power Multipliers

For VISRAD simulations, it is possible to apply a randomized beam power multiplier to each beam in the laser beam system. The motivation for this is to support studies that address the sensitivity of results to statistical variations in actual beam powers that occur in experiments. A single multiplier is generated for each beam. For time-dependent calculations, the same multiplier is used throughout the beam pulse.

The randomized multiplier option is turned On or Off in the Power Calc tab of the Set Laser Power Calculation Parameters dialog (accessed with the Lasers | Set Power Calculation Params menu item). If On, the user will be prompted with a warning message when updating various quantities under the Display menu.

User input includes the standard deviation (in percent) for the beam powers, and a seed value for the random number generator (optional). To re-generate multipliers, the Setup Randomized Beam Powers button must be pressed.

The values of the multipliers are listed in the Laser Beams Summary dialog (in the column just after the beam power).

Laser Power Deposited For Beams Using Off-Axis Parabolas (OAPs)

An option is available to compute the laser power deposited in target components from the pre-reflection portion of short-pulse laser beams; i.e., the portion of a short pulse beam between its entrance port and its off-axis parabolic reflecting mirror (OAP) in the OMEGA-EP, OMEGA, and ORION chambers. To include this in the laser power deposition calculations, check the box in the Short-Pulse Beams tab of the Set Laser Power Calculation Parameters dialog (accessed with the Lasers | Set Power Calculation Params menu item).

Results for the power deposited by the pre-OAP portion of the beams can be viewed in the Power Summary Table and the Surface Elements Table (under the "Pre-OAP Las Pwr" columns).

The power in the post-OAP portion of the beam is reduced by the power deposited by the pre-OAP portion of the beam. The precise nature of the reflection of light off the OAP is not modeled in detail.

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