This block contains information about bremsstrahlung radiation. See EPOCH input deck for more information on the input deck.

EPOCH is capable of simulating bremsstrahlung radiation using the teqhniques described by Morris et al ¹ and Vyskočil et al ². In order to run this module, the compiler flag -DBREMSSTRAHLUNG must be switched on in the Makefile. There should also be a corresponding “bremsstrahlung” block for the input deck which uses a similar format to the “qed” block. Bremsstrahlung cross sections may be calculated for both electrons and positrons. Photons may also undergo pair production according to the Bethe-Heitler model.

An example bremsstrahlung block is shown below:

begin:bremsstrahlung
   enable = T
   start_time = 0
   produce_photons = T
   photon_energy_min = 1 * kev
   photon_weight = 1.0
   photon_dynamics = F
   use_plasma_screening = F
   use_brem_scatter = T
   use_bethe_heitler = F
   use_positron_brem = F
end:bremsstrahlung

• enable - Logical flag to turn bremsstrahlung on or off. “use_bremsstrahlung” is accepted as an alias. The default is “F”.

• start_time - Floating point value specifying the time after which bremsstrahlung radiation is modelled. The default is 0.

• produce_photons - Logical flag to allow population of a photon species. If “F”, then only the energy loss of the electrons will be simulated. The photon species can be specified by including the line identify:brem_photon in the corresponding species block. If the compiler flag -DPHOTONS is active, QED and bremsstrahlung will both populate the first species with identify:photon if no brem_photon species is specified. The default is “F”.

• photon_energy_min - Floating point value specifying the minimum energy of produced photons. Electron energy loss is still calculated for lower energy photon emissions, but these photons are not added to the photon species. The default is 0.

• photon_weight - Floating point value which applies a multiplier to the weight of produced macro-photons, in order to increase the number of overall emissions and obtain better spectra. Must be less than or equal to 1 and greater than 0. For example, 0.1 would make emission 10 times more likely, but for macro-photons only 10% the weight of the generating macro-electrons. Electron recoil would be reduced accordingly. The default is 1. Note that only one emission is possible per macro-electron per timestep, so setting this too low will saturate emissions.

• photon_dynamics - Logical flag to specify whether or not to push photons. If “F”, then the generated photons are immobilised at the point of emission. The default is “F”.

• use_plasma_screening - Logical flag to specify whether a cross section enhancement due to heated ionised targets is considered, based on theory described by Wu et al³. It is expected that for high energy electrons passing through low density, ionised plasmas with electron temperatures over $\sim$100 eV ($\sim8\times 10^{5}$ K), the bremsstrahlung emission rate could increase by a factor of 2-3. This has not been tested experimentally, and so the default value is set to “F”.

• use_radiation_reaction - Logical flag to specify whether the electrons experience energy loss when emitting photons or not. “use_bremsstrahlung_recoil” is accepted as an alias. Debugging flag, default “T”.

• table_location - String specifying the location of the emission look-up tables for bremsstrahlung. The default path is set to src/physics_packages/TABLES/br.

• use_brem_scatter - Samples photon ejection angle from a differential cross section. Default is “F”, where photons are emitted in the direction of the incident particle (ultra-relativistic approximation).

• use_bethe_heitler - Allows photons to undergo Bethe-Heitler pair production. Default is “F”. This requires both electron and positron species to be defined.

• use_positron_brem - Samples bremsstrahlung radiation from positrons. Default is “F”. If “T”, electrons and positrons share the same parameter values set in the bremsstrahlung block.

Bremsstrahlung, like QED, requires the code to know which species are electrons and which are photons, so uses the same identify system (with identify:brem_photon for a bremsstrahlung-only photon species). Additionally, the atomic numbers of the atom/ion species are required in the species block. For example, atomic aluminium (charge = 0) could be specified as:

begin:species
   name = Aluminium
   atomic_number = 13
   charge = 0

   mass = 49218
   number_density = 6.022e28
   fraction = 0.5
   dump = T
end:species

If the atomic number is not specified then it will be assumed that the ion is fully ionised and the atomic number would be set to the charge (the nearest integer to the ion charge when expressed in units of elementary charge). If ionisation is considered, the atomic number must be specified once, and all child species will retain the same atomic number.

If Bethe-Heitler pair production is considered, the user may identify specific species to populate with Bethe-Heitler electrons and positrons using the identity aliases:

• identify:bethe_heitler_electron - bh_electron is also accepted.

• identify:bethe_heitler_positron - bh_positron is also accepted.

If these species are unspecified, EPOCH will populate the first electron and positron species present read in from the input deck. Additional identity aliases are provided in the QED section.

References