Image Plane Setup

The Image Plane tab of the Detector setup widget is used to specify the majority of key parameters for the SPECT3D virtual detector.

The Projection, or viewing, model is used to specify a grid of lines-of-sight that emanate from the detector plane through the plasma. SPECT3D supports the following projection models:

• Orthographic Projection (parallel rays)
• Point Projection (converging rays)
• Spherical Crystal Projection
• Crystal spectrometer imaging (this feature is in beta form and not currently available to all users)
  

Point Projection is typically used in cases where small backlighter sources are utilized.

The Vertical alignment combo box is used to specify the alignment axis of the vertical of the detector image grid.

The position of the detector, its center of projection (i.e., the point to which the detector points), and the backlighter position (if applicable) are specified in the coordinate system of the plasma. The Detector Position must be entered for all projection models. When Orthographic Projection is selected, the Center of Projection is also entered. Otherwise, the Backlighter Position is entered for the Point Projection model, while the position of the spherical crystal is entered for the Spherical Crystal Projection model.

Note that backlighter effects can also be included when using Orthographic Projection, in which case the position of the backlighter is defined by the Center of Projection, and the backlighter has an extended size that is equal to the size of the detector.

Positions can be entered in either Cartesian (x,y,z) or spherical polar (r, azimuthal angle, polar angle) coordinates. All coordinates are in the frame of reference of the target plasma.

SPECT3D supports rectangular and disk (circular) detector grids. The latter is particularly useful for calculations of spherically symmetric plasmas, or for cylindrical plasmas when viewing from the top.

For a rectangular detector grid (i.e., image), specify:

• Number of pixels: the number of pixels in each direction on the detector.
• Total size: the horizontal and vertical dimensions of the detector plane (cm).
• Portion to compute: Specify whether to compute the entire image or a part of it. If the plasma target is known to be symmetric, computational time can be reduced if only part of the image is computed. The correspondence between the buttons and the portion of the detector image that will be computed is illustrated below. Note that in setting up the line-of-sight grid, a central row/column of pixels is always computed, resulting in a shift if there is an even number of pixels. For this reason, when computing only a portion of the detector image, it is recommended that users enter an odd number for the number of pixels .

For a disk (circular) detector, specify:

• LOS count: the number of lines of sight to be computed in the radial and azimuthal directions.
• Radius: the radius of the image (cm).
• Portion to Compute: Compute the entire image, or compute an image assuming circular symmetry. The left hand button, shown below, is the option to calculate the image for the entire circle, the right hand button is the option to calculate lines of sight along one radial segment only. The latter choice will reduce computation time considerably, with no loss of accuracy for symmetric images.

When the Spherical Crystal Projection model is chosen, the following parameters should be entered:

• Bragg angle: in degrees
• Crystal radius: in cm.
• The Backlighter -> Spherical Crystal -> Detector Orientation: the two choices are counter clockwise and clockwise. The direction is determined as viewed from the positive z-axis. The spherical crystal configuration illustrated, under projection choices, shows a counterclockwise orientation.