Neutral Beam Penetration and Photoemission

Scope and motivation

This proposed Workshop is intended to assess the sensitivity of predictions of hydrogen beam penetration and of beam emissions in relevant fusion plasma conditions to uncertainties in atomic data. It was conceived in order to support the aims of the Neutral Beams CRP which will provide evaluated and recommended data for the principal atomic processes relevant to heating and diagnostic neutral beams in fusion plasmas.

The intended scope of the workshop is codes that simulate neutral beam penetration and beam-based photoemission. The photoemissions are used in diagnostics, including beam emission spectroscopy (BES) and the motional Stark effect (MSE) technique, which both rely on photoemissions from the beam neutrals, and charge exchange recombination spectroscopy (CXRS or CHERS), which relies on photoemissions from plasma impurities.

We think of beam penetration (or, equivalently, beam attenuation) as the most fundamental process for the comparison due to its importance for beam heating and for all beam-based diagnostics. Beam-based spectroscopy provides more refined tracers of the atomic physics (excitation, ionization and charge transfer) that determines the beam penetration. With respect to emissions we expect to focus the code comparison on the neutral particle photoemissions used in BES and MSE diagnostics – the simulation of CXRS/CHERS is an optional extra.

The workshop should bring together neutral beam modellers that may use a variety of atomic models and data. Relevant processes include excitation and deexcitation of beam neutrals by electron and ion impact (including impact by impurity ions), ionization from the ground state or from relevant excited states by electron and ion impact, and charge transfer resolved with respect to the excitation state of the pre-collision neutral H and the post-collision impurity ion. The comparison exercise would help modellers to identify possible errors in the atomic data and to identify important sensitivities of the simulations of beam penetration and beam-based emissions to uncertainties in the atomic data.

The workshop is concerned with processes involving the beam neutrals and, optionally, halo neutrals. The fast ion slowing down and thermalization is not studied in this code comparison exercise. Also we are not concerned with instrumentation issues, realistic machine geometry, or similar concerns that would be pre-eminent in a comparison of synthetic diagnostic signals for a real fusion experiment.

Proposed test cases

Two established series of code comparison workshops serve as models for the proposed event. The Non-Local Thermodynamic Equilibrium (NLTE) Code Comparison Workshop series started in 1996, with 10th NLTE Workshop (NLTE10) held in 2017. The Spectral Line Shapes in Plasmas (SLSP) Code Comparison Workshop is a similar, biennial event started in 2013. For each instance of the NLTE and SLSP code comparison workshops a set of cleanly defined test cases is specified and distributed to the expected participants several months before the event. The researchers do their best calculations and produce results in an agreed format. The code results are assembled before the meeting into a database (accessible to the participants only) and then the contributors come together for 4 or 5 days of intense work during which they seek to understand all their differences. Often a meeting report is produced as a journal article. It has been the custom not to identify the outputs of individual codes in those meeting reports.

The practice in the NLTE and SLSP code comparison workshops is that each participant must contribute results for some test cases and each test case is supposed to attract several contributions. The same expectation will apply for the present proposed workshop.

Some of the expected constraints on the test cases are given below.

  • We expect that the test cases will all specify uniform plasma conditions and pure beam conditions; i.e., the beam particles have a definite velocity and a uniquely specified initial atomic state. Contributors are asked to simulate beam penetration and photoemission.

  • Sources of emission must be distinguished: beam particles, halo neutrals, plasma impurities. In practice, we focus on emissions from the neutrals (BES and MSE) and downplay the simulations of impurity emissions (CXRS/CHERS).

  • Plasma would be semi-infinite (x > 0), uniform, normally including a uniform magnetic field in the y-direction. Plasma electron density might be fixed at 1014 cm-3; the bulk species is deuterium. (Some cases with a significantly higher plasma density might be included to highlight possible issues with multi-step atomic processes.)

  • Plasma temperatures to consider might be 100 eV, 500 eV, 2 keV, 10 keV. If there is an impurity then we might agree that Zeff=2 in each case, and there is only a single impurity; say Be, C, N, Ne or Ar. (This needs some care if the impurity would not be fully stripped.)

  • The beam is propagating in the positive x-direction along the x-axis; ignore beam width on entry. The beam species would be deuterium and the energy might be 50 keV, 200 keV, 1 MeV. Initial state is pure, D(1s) or D(2s) or D(2p) in any polarization. Ignore beam density effects; assume an independent beam particle model.

  • We may specify cases with a field of 1T and cases with a field of 5T.


Beam Attenuation, not Beam Deposition

Beam attenuation is the removal of fast neutrals due to ionization or charge transfer. Either way, a fast ion is produced. Beam deposition, in fusion parlance, is the production of thermal ions. It involves beam attentuation as a source of fast ions and then the slowing down and thermalization of those fast ions. In practice, in beam deposition theory and codes the fast ion processes receive much more attention than the atomic physics source term for the fast ions.

Integrated Atomic Modelling, not Integrated Tokamak Modelling

The proposed event will involve codes that calculate beam penetration and beam-based photoemissions. Test cases will involve simple beam and plasma conditions, but multiple atomic processes: change of excitation state, ionization and charge transfer. The codes must include enough of the atomic physics that the code results are relevant for applications.

On the other hand, the plasma is as simple as possible; semi-infinite geometry, uniform and stationary plasma conditions, uniform magnetic field. The present proposed exercise is not concerned with the manner in which the atomic processes affecting the beam particles are finally coupled to plasma dynamics in tokamak (or other) geometry.

Code Comparison, not Data Evaluation

The principal objective of this code comparison exercise is to identify the range of differences among various codes, each implementing some set of atomic physics data, for simulations of beam penetration and beam-based photoemissions. As a result of this comparison it may be found that some atomic data really need attention. However, this code comparison workshop is not set up to assess the uncertainties in the atomic data or to recommend best data.

We are not comparing Fusion Science Neutral Beam Codes

The present code comparison workshop needs to be set apart from other comparison and benchmarking activities for neutral beam codes in the world-wide fusion programme. A benchmarking exercise coordinated by T. Oikawa for the International Tokamak Physics Activity (ITPA) Energetic Particle Physics Topical Group is documented in [1] and a more recent activity coordinated by M. Schneider in the context of the European Integrated Modelling Framework is documented in [2]. The activity [2] is now going further in the context of the ITER Integrated Modelling and Analysis Suite (IMAS).

These benchmarking exercises are really devoted to the issue of fast ion slowing down and thermalization and therefore issues of fast ion orbits in tokamak geometry are at the fore. The atomic physics that provides the source of the fast ions is pretty much taken for granted in [1,2]. By contrast, the present proposed code comparison workshop is entirely devoted to atomic physics of the neutral beam particles; operationally speaking, the fast ions produced by beam ionization or charge transfer are immediately lost from the comparison exercise.

[1] T. Oikawa, J. M. Park, A. R. Polevoi, M. Schneider, G. Giruzzi, M. Murakami, K. Tani, A.C.C. Sips, C. Kessel, W. Houlberg, S. Konovalov, K. Hamamatsu, V. Basiuk, A. Pankin, D. McCune, R. Budny, Y-S. Na, I.Voitsekhovich, S. Suzuki: "Benchmarking of neutral beam current drive codes as a basis for the integrated modeling for ITER." In Proc. 22nd Int. Conf. on Fusion Energy. 2008.

[2] M. Schneider, O. Asunta, T. Johnson, D. Kalupin, R. Coelho and EU-IM team: "Benchmarking Neutral Beam Injection Codes within the European Integrated Modelling Framework." Extended abstract for presentation at the EPS DPP meeting, 22–26 June 2015, Lisbon, Portugal.