Beam-plasma atomic data needs for fusion devices

Contemporary areas of application on fusion machines:

● Beam stopping (H/D/T heating beams)

– Beam shinethrough in small machines and/or at low density. Power from heating beams transmitted through the plasma can damage the inner wall. Cross section data could make the protection calculations more or less conservative

– Beam density in the centre of the plasma, especially in large machines. Data would affect calculations for ITER diagnostic performance

● Charge exchange (impurity and hydrogen) population cross sections

● Stark effect for magnetic field magnitiude and direction measurement

– Non-statistical population of sub-states affecting line ratios (for magnetic field direction measurements that are based on the intensity ratios)

– Magnetic field magnitude

● Lithium beam stopping: density profile dependent on cross-section for stopping (but eliminated from calculation by boundary conditions?)

● Zeeman effect in Lithium: population of sub-levels (as Stark) when used to measure magnetic field direction.

● Thermal He 'beam' excitation and charge-exchange

● Helium (high energy) beam excitation.

● Ion source physics, especially negative ion formation

● Neutral beam reionisation by neutral collisions

● (Positive and negative) ion neutralisation in neutral gas

Beam stopping cross-section (E Delabie)

• Neutral beam stopping• above 40 keV/amu dominated by proton impact ionisation from ground state• (Below 40 keV/amu CX dominated)

• Recommended data from Janev & Smith, 1993 (and ADAS) based on experimental data (Shah & Gilbody, 1982) which at the time agreed with theoretical predictions

• Theory later found to be unconverged (Toshima, 1998). Several theoretical studies since then consistently predict 10% higher cross sections

Beam stopping cross-section (E Delabie)

● Impact of a 10% change in beam-stopping cross sections

● For JET-like conditions, f=0.22, reduces core density by 15%—mild effect (although reduces shinethrough by 30%)

● For ITER conditions, f=0.01, reduces core density by 46%—significant impact on the performance of the core charge-exchange system

– Uncertainties in modelling (not just atomic data) acknowledged: need to measure local beam emission for quantitative analysis of impurity spectra

● 30% discrepancy in total emission (but potential causes other than atomic physics)

● Enhanced Lorentz ionisation from upper n-levels thought to be adequately trreated

f=e−σx n

∂ ff

=∂σσ

ln f

• Seems like it needs experiments to check atomic data.

• Experiment is not a 'clean' measurement of a single cross-section, (CX as well as impact excitation involved) so needs to have a self-consistent modelling of the experimental results.

• Can an experiment achieve the required accuracy (propagation of errors in electron density etc) ?

FIDASIM (Heidbrink, Grierson)

● Code to fully model the beam emission spectrum of hydrogen beam into a hydrogen plasma. Attempts to model all processes, including

– Thermal charge exchange from the edge (donor energiesbelow 1 keV)

– Effect of 'halo': charge transfer between C5+ and D+, want to add this process to the modelling

FIDASIM (Heidbrink, Grierson)

● Code to fully model the beam emission spectrum of hydrogen beam into a hydrogen plasma. Attempts to model all processes, including

– Thermal charge exchange from the edge (donor energiesbelow 1 keV)

– Effect of 'halo': charge transfer between C5+ and D+, want to add this process to the modelling

● Data on nl resolved cross sections for D0+D+→D++D0 for energies below 1 keV (62eV—2.5keV) seems to be available already in ADAS for n=1, 2, 3, 4/home/adas/adas/adf24/scx#h0

● no data on C5++D+→C6++D0

Charge-exchange from high n-states of the beam (R E Bell)

● Intensity ratio of C VI n=8-7 to C VI n=14-10 are 28:1 (TFTR) and 30:1 (DIII-D).

● Using Einstein A-coeffefficients, deduce the population ratio of the respective upper n-levels (n=14 and n=8): about 1.5x

● This ratio is larger than can be explained by a collisional-radiative model based on donor n=1 and n=2 populations only

– Implies that n>2 donorpopulations need to beconsidered, but thereis no data for these.

Charge-exchange from high n-states of the beam (R E Bell)

● Some data exists in ADAS for n=3,4 donor to C receiver, but derivation uncertain.

● Was studied briefly in the early days of JET but dismissed as contributions found insignificant for the spectral features of interest.

Suggestions from M O'Mullane:

● There is very little in the literature for H(n>3) donor and nothing which resolves the spread in the n-shells of the capturing carbon

● Without more fundamental data one could:

– Extrapolate total cross section data to H(n>2) donorbased on [Janev, Phys Lett A160, p67, 1991]

– Split the total cross-section over n-states, using 'universal scaling' developed for W

– Beam populations in the n>2 levels adequately covered in existing models/data.

● Fundamental calculations could be done instead to provide this data

Thermal helium 'beam' data (M Brix)

● Important processes for thermal helium beam diagnostics ~.1—10 eV

– proton impact excitation

– Charge exchange with impurities

– Collisional redistribution

● Data is pre-1990's and no-longer 'state of the art'

Lithium beam

● Electron impact excitation dominant at low energies—data believed to be good (need to check with the Garching group)

● Thermal sodium beams being considered for machines that are lithium wall conditioning—how reliable is this data?

Neutral beam ion sources (E Surrey)

• Electron collision processes, Te from 1-100 eV, depending on whether filament or RF driven (cf ITER)

• Accuracy of atomic data thought to be 20-100% but this is probably adequate (since many microscopic processes involved tends to “average out” the errors). Overall comparison with extracted beam properties is okay. Probably not worth investing a lot of effort to improve.

• Three ionisation processes: H→H+

H2→H2+

H3→H3+

• Neutralisation occurs on surfaces: concept of “cross section” may not be appropriate

• Formation of H- occurs at surfaces, processes not well understood. Caesium coating improves yield. (ITER importance)

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