Celebrating the achievements of Alan Gabriel

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Celebrating the achievements of Alan Gabriel

• Laboratory spectroscopy
• Exploring the process of dielectronic
  recombination
• S. Volonte

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Laboratory spectroscopy

• 1969 -1973
• Under Alans supervision development of an
  electromagnetic T-tube
• Study of high density effect in He plasmas
• Plasma Polarization Shift (PPS) of lines from
  ions due to partial screening from neighbouring
  perturbing electrons
• Studies by Griem et al
• Controversial theoretical and experimental
  results

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Laboratory spectroscopy

• First shift measurements of He II resonance
  series up to n4
• Blue shifts measured and predicted n4 scaling
  confirmed
• Shift measurements in overall quantitative
  agreement only with quantum estimate (Griem 1970)
  based on hydrogenic continuum wavefunctions for
  perturbing electrons
• Study showed that if perturbing electrons are
  assumed as bound
• in high orbitals the effect corresponds to
  emission of high dielectronic satellites and is
  negligible in high density plasmas
• Conclusion only free perturbers produce
  measurable PPS (Volonte PhD 1972 , Gabriel and
  Volonte 1973a,b)

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Exploring dielectronic recombination

• In same period Alans main laboratory
  activity-production of highly ionized spectra
  with theta-pinch machine for simulation of
  spectra of solar sources
• In parallel, research work on satellite
  lines-faint features on red side of resonance
  lines of H-like and He-like ions (Edlen and Tyren
  1939)
• With co-workers (a.o C. Jordan) he showed that
  production process was dielectronic recombination
  of free electron bound to parent ion in excited
  state to form an auto-ionzing doubly excited
  state of recombined ion from which satellites are
  emitted
• In developing the theory of dielectronic
  satellites he derived the intensity ratio of the
  satellite to the resonance line as given by
• Is/Ir Ar/Te exp((E0-Es)/kTe) with
  ArZ4
• The ratio varies only with temperature and
  the n scaling shows that satellites
• are much stronger in heavy ions (factor 100
  from O to Fe)

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Exploring dielectronic recombination

• The theory was further developed for inner-shell
  satellites produced by collisional excitation of
  inner-shell transition in the recombined ion.
• For inner-shell satellites the intensity ratio to
  resonance line is given by
• Is/Ir (NHe/NLi)(C/C)F(A)
• Here the intensity ratio varies with the
  relative abundancies (Te dependant) which
  depend on the ionisation equilibrium of the
  source (i.e. wether ionising, recombining or
  steady state)
• These satellite intensity ratios give independent
  access to the electron temperature and the
  ionization state of the source in coronal
  plasmas.

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Exploring dielectronic recombination

• This early work was devoted to the red shifted
  prominent satellites produced from doubly-excited
  states with n2 for the most solar abundant
  He-like ions (O to Fe)
• Confirmation of these calculations was obtained
  from the first ever observations of solar flare
  iron spectra (Fe XXV near 1.85 A) observed by the
  Intercosmos-4 satellite (1972).
• Best fit between the observed and computed
  spectra provided the electron temperature and the
  ionization state of the flaring plasma thus
  demonstrating the diagnostic capabilities of the
  satellite line theory

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Exploring dielectronic recombination

• 1974
  -1986
• The next step undertaken by Alan was to improve
  the diagnostic capability of the method by
  extending the work to include satellites emitted
  from auto-ionizing states with ngt2 for H-like and
  He-like ions
• This entailed calculation of a very large number
  of atomic parameters
• This work was realised through an extended
  collaboration with the atomic physics groups at
  the Nice observatory (F. Bely-Dubau, P. Faucher,
  L. Steeman-Clark) and Meudon (J. Dubau, M.
  loulergue, M. Cornille) and myself at Mons

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Exploring dielectronic recombination

• A series of workshops were held regularly mainly
  at Nice (see photos) but also at Meudon and
  Culham and occasionally at Mons to discuss
  progress of the work
• The early results of the collaboration concerned
  the high n satellite lines of Fe XXV (Bely-Dubau,
  Gabriel and Volonte 1979)
• The results showed the importance of including
  these lines to improve agreement between observed
  and calculated spectra thus enhancing the
  accuracy of the diagnostics.

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Exploring dielectronic recombination

• A series of workshops were held regularly mainly
  at Nice (see photos) but also at Meudon and
  Culham and occasionally at Mons to discuss
  progress of the work
• The early results of the collaboration concerned
  the high n satellite lines of Fe XXV (Bely-Dubau,
  Gabriel and Volonte 1979)
• The results showed the importance of including
  these lines to improve agreement between observed
  and calculated spectra thus enhancing the
  accuracy of the diagnostics.

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Nice workshop
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Nice workshop
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Exploring dielectronic recombination

• The collaboration worked actively on the atomic
  parameters needed in preparation of NASAs Solar
  Maximum Mission (SMM 1980)
• Satellite line data up to n5 were obtained for
  the most solar abundant H-like and He-like
• Examples of solar flare spectra from the BCS
  spectrometer on board SMM are shown below. The
  calculated spectra have adjusted to provide the
  best fit with the observations. Note the Te
  values and the ionization diagnostics obtained
  from the satellite line intensities.

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Exploring dielectronic recombination

• The work on satellite spectra was eventually
  tested on spectra obtained in fusion machines
  (a.o. Princeton and Frascati Tokamak)
• Calculated and observed spectra were in good
  agreement
• The temperatures derived from the satellite
  intensities showed excellent agreement (few )
  with independent measurements from Thomson
  scattering provided high satellites were
  accounted for
• This was a further confirmation of the powerful
  diagnostic capabilities of the satellite lines

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Exploring dielectronic recombination
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Exploring dielectronic recombination
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Conclusion

• Through his leadership, scientific insight and
  as an instigator of international collaboration
  Alans work in laboratory spectroscopy, theory
  and interpretation of spectra has provided not
  only important data but also new diagnostics
  methods (e.g. satellite lines) which had not been
  available earlier. These research topics are
  among the many other important contributions that
  Alan has provided throughout his scientific
  career.