Neutral Particle Analyzer

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NPA Geometry

Detector Aperture

An NPA detector is defined by an aperture for which neutral particles must pass through and an detector. The aperture/detectors are defined by three points and a shape as shown in the figures above. It is assumed that between the aperture and the detector that particles travel in straight lines i.e. there is no stripping foil at the aperture.

The full definition of the NPA detector is given below (right and left as implied by looking through the aperture at the detector)

Variable Type Rank Dimensions Units Description
nchan Int32 0 NA NA Number of channels
system String 0 NA NA Name of the NPA system(s)
data_source String 0 NA NA Source of the NPA geometry data
id String 1 [nchan] NA Channel ID
radius Float64 1 [nchan] cm Line of sight radius at midplane or tangency point
a_shape Int16 1 [nchan] NA Shape of the aperture (1=rect, 2=circ)
d_shape Int16 1 [nchan] NA Shape of the detector (1=rect, 2=circ)
a_cent Float64 2 [3,nchan] cm Position of the center of the aperture
a_redge Float64 2 [3,nchan] cm Position of the apertures right edge
a_tedge Float64 2 [3,nchan] cm Position of the apertures top edge
d_cent Float64 2 [3,nchan] cm Position of the center of the detector
d_redge Float64 2 [3,nchan] cm Position of the detectors right edge
d_tedge Float64 2 [3,nchan] cm Position of the detectors top edge

Monte Carlo NPA calculation

The Monte Carlo method of calculating the NPA flux (MC-NPA) is as follows

  1. Sample Fast-ion distribution function and get initial position, energy, and pitch
  2. Determine the range of gyro-angles ( ) that would allow a neutral particle to go through the NPA aperture and hit the detecting region. If the range is zero increase counter and goto 1 else goto 3
  3. Choose the gyro-angle to be in the middle of the range calculated in step 2. Charge exchange the ion (set initial state) and solve the collisional radiative model along particle track. Scale by the gyro-range .
  4. Sum the final state of the neutral and bin the particle by its energy. Increase counter.
  5. Repeat N times

An alternative approach is to fire the particles directly at the NPA detector and then scale the resultant flux by the probability of that trajectory occuring. This approach is taken in the weight function method (WF-NPA) detailed here.

An example of the calculated NPA flux for the two different methods are shown below.

NPA Flux

Relevant Namelist Settings

  • n_npa: Number of Monte Carlo particles used in MC-NPA calculation
  • calc_npa: Calculate NPA flux using the Monte Carlo Method
  • calc_npa_wght: Calculate NPA weight function and flux using the weight function method
  • ne_wght: Number of energies in weight function calculation
  • np_wght: Number of pitches in weight function calculation
  • emax_wght: Maximum energy in weight function calculation

Fortran References