# Presentation: 852604

## Observation of D I and He II series transitions in NSTX divertor and prospects for burning plasma divertor diagnostics

***Acknowledgements:***
**R. Bell****2****, R. Kaita****2****, A. L. Roquemore****2**
***1 ******Lawrence Livermore National Laboratory, Livermore, CA***
***2 ******Princeton Plasma Physics Laboratory, Princeton, NJ***
**NSTX Results Review**
**23 July 2007**
**Princeton, NJ**

**Observation of D I and He II series transitions in NSTX divertor and prospects for burning plasma divertor diagnostics**

**Supported by**

## Summary

- Application of Stark broadening measurements of Balmer (UV) and Paschen near-infrared (NIR) D I emission lines for* n**e* diagnostic of tokamak recombining divertor is presented
- Several Paschen lines measured for the first time at “low” densities (tokamak divertor - 1 x 1019 - 1 x 1021 m-3)
- Paschen 4 - 6...14  emission line profiles demonstrate
- Substantial Stark broadening (scales as ~*n**e**2/3*,* l**2* for series)
- Low sensitivity to instrumental and Doppler broadening
- Good sensitivity to tokamak divertor-relevant densities
- Comparison with several theoretical models and independent divertor density measurements is commencing
- For the first time, He II Pfund and Humphrey series lines have been measured in a recombining NSTX divertor
- NIR spectroscopy - attractive diagnostic technique for burning plasma recombining / detached divertor

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Tokamak divertor plasma is well suited for high-n series line measurements

- Electron impact excitation much less efficient for high-*n* levels at low *T**e*
- 3-body recombination rate is high at high *n**e*, low *T**e*
- Plasma is optically thin for UV-vis-NIR lines
- Dominant broadening mechanism: Stark
- Neglected in NSTX: Zeeman spitting, Van der Waals and natural broadening

**Tokamak divertor plasma is well suited for high-*****n***** series line measurements**

**D I:                VUV      UV-vis     NIR     - NIR - IR -**
**He II              XUV      FUV         UV     UV-vis    vis-IR**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Significant volume recombination is observed at outer strike point during detachment

- Balmer series lines 2-6...11
- Sign of strong volume recombination
- Stark broadening yields *n**e*
- Intensities determined by Saha-Boltzman level population distribution
*n**e** *~ 2-3 x 1020 m-3 in inner divertor (bottom panel)
*n**e** *~ 4-6 x 1020 m-3 at outer strike point (top panel)
*T**e** *< 1.5 eV
- Interim calibration used
**Soukhanovskii et. al., Rev. Sci. Instrum. 77, 10F127 (2006)**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Stark broadening of Paschen lines indicates ne ~ 1-5 x 1020 m-3 in NSTX inner divertor

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Paschen P10, P11 lines yield similar densities in inner recombining (detached) divertor region 

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Divertor densities inferred from Balmer and Paschen lines are similar

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## He II Pfund (5-8...19) and Humphreys (6-12...16) series lines have been observed in He plasmas

- He II is a hydrogen-like system
- Pfund series lie in visible & NIR range, FWHM ~ 0.5-2 nm
- Humphreys series are in the NIR range, lines are close to D I Paschen series lines
- First observation of high-*n* He II series lines (?)

**Pf18    Pf17      Pf 16        Pf15**

**N I    Hu14   **

**Pf 7 **

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Theory of Stark broadening of deuterium lines has made much progress in last 10-15 years

- Textbook material, e.g. Griem, Sobelman
- Hydrogen atom: degenerate levels, linear Stark effect
- Ions: quasi-static microfield distribution (Holtzmark field *F**0 **= 2.603 e N**e**2/3* )
- Electrons: impact approximation
- Well suited for tokamak divertor plasma conditions (*T**e* < 100 eV, *n**e* < 1021 m-3)
- Useful tabulations: C. R. Vidal *et al*. ApJS 25, 37 (1973), M. Lemke A&A 122, 285 (1997)
- New methods and modeling developed in recent years
- Model Microfield Method (MMM)
- U. Frisch and A. Brissaud, JQSRT 11, 1753 (1971), C. Stehle and R. Hutcheion A&A Sup. Ser. 140, 93 (1999)
- Extensive tabulation of line profiles available
- Frequency Fluctuation Model (FFM)
- B. Talin et. al Phys. Rev. A 51, 1918 (1995); B. Talin et. al. JQSRT 58, 953 (1997); S. Ferri et. al. 14th ICSLS, V. 10, p 115 (1999)
- Tabulated data fitted to formula *** = 2.5 10**-13** (**** N**e**2/3** + **** N**e**)*

**Theory of Stark broadening of deuterium lines has made much progress in last 10-15 years**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Tabulated Model Microfield Method calculations of Stark profiles used for spectra interpretation

**Tabulated Model Microfield Method calculations of Stark profiles used for spectra interpretation**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Stark widths of Paschen D I lines sensitive to lower density than Balmer lines

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## CRETIN code modeling of Balmer and Paschen spectra is commencing

- Preferred way of analysis - model whole spectrum including bremsstrahlung, line emission, line profiles, self-absorption, ionization and recombination
- Code developed by H. Scott (LLNL)
- Includes non-LTE population kinetics, radiation transport, neutral diffusion, diagnostic simulators
- Line profile code: TOTAL
- Quasi-static approximation for ions, impact approximation for electrons
- Electric dipole momentum reduced matrix elements must be calculated elsewhere
- Good candidate for future analysis

**CRETIN code modeling of Balmer and Paschen spectra is commencing**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## NIR spectroscopy may be a better spectroscopic technique for a burning plasma experiment

**Signal extraction techniques is a big issue for future burning plasma experiments**
**First mirror optical properties (reflectivity, polarization) degrade due to surface erosion and material deposition**
**Various materials and mitigation **
**	****techniques are studied**
- Mo, W, Rh, CU and Stainless Steel
- 	mirror materials
- Single crystal mirrors
- Polycrystalline mirrors
- Heated mirrors
**Total reflectivity is higher in NIR than in UV-VIS**
- Specular reflectance ***R*** is a function of wavelength **** and mean surface roughness ***d***
- Benett’s formula for total reflectivity

**References:**
- A. Litnovsky, et. al., to be published, JNM 2007
- M. J. Ruben, et. al. RSI 77, 063501 (2006)
- D. L. Rudakov et. al. RSI 77, 10F126 (2006)
- V. S. Voitsenya et. al. RSI 76, 083502 (2005)

**D. L. Rudakov et. al.**

**NIR**

**UV-VIS**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Future work

- HTPD 2006 Conference (Rev. Sci. Instrum. 77, 10F127 (2006))
- International Conference in Atomic Processes in Plasmas 2007 (Gaithersburg, MD)
- Paper in preparation for submission in Phys. Rev. E
- APS 2007 poster abstract submitted
- New imaging spectrometer is needed for multi-channel divertor measurements with existing imaging system on NSTX
- Plan to propose with focus on burning plasma diagnostic in 2008 in DoE Diagnostic proposal competition for national labs

**Future work**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Lower divertor imaging system enables high spatial resolution spectroscopic measurements

**Lower divertor imaging system enables high spatial resolution spectroscopic measurements**

**32-fiber**

- Fiber-optic imaging bundle:
- Imaging lens - Nikon 85 mm *f */1.4 lens
- Mounted on upper window port
- V-groove design allows focal-plane individual fiber mounting and adjustments during alignment
- Fiber-optic bundle: eight 1 mm quartz fibers for outer divertor - 1.5 cm spots,      4-5 cm inter-center, twenty four 0.6 mm fibers for inner divertor - 1 cm spots, 2 cm inter-center; total length - 20 m
- Lines of sight are spatially calibrated with He-Ne laser and computerized measurement arm with *mm*-precision
- Spectrometer:
- Acton Research SpectraPro 500i 0.5 m,     *f* / 6.5 Czerny-Turner scheme
- Three input fibers with *f*-matching optics
- Three gratings: 600, 1200, 2400 l/mm
- CCD detector - 1340 x 100 pixel  Princeton Instruments Model Spec-10:100B
- CCD operated in binned mode
- Typical readout times 15-50 ms / chip
- Spectrometer and imaging system are photometrically calibrated with URS-600 LabSphere radiation standard *in-situ*

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Measured instr. function and properties of convol. integral are used to obtain pure Stark profiles

-  Mercury lamp used
-  For 1200 l/mm grating, instr. function is single Gaussian with FWHM = 1.13 A
-  For 600 l/mm grating, instr. function is not Gaussian. Use Gaussian with FWHM 2.30 A
- Convolution with thermal (Gaussian) FWHM for 2 eV gives FWHM 1.7 A for 1200 l/mm grating, and FWHM = 4 A for 600 l/mm grating

**Measured instr. function and properties of convol. integral are used to obtain pure Stark profiles**

-  Spectral analysis procedure:
- Fit Voigt profile to D lines, Gaussian to impurity lines, with quadratic background
***V******tot****** = G******therm******+G******instr******+L******stark******+U******other***
- If ***V = G * L*** then ***f(V) = f(G) x f(L)*** and ***L = f****** -1******( f(V)/f(G) )****, *where “*” is integral convolution,*** f*** is integral Fourier transform
- Recover pure Stark profile, fit Lorentzian profile, obtain FWHM and compare to calculations
- All routines are written in IDL, the MPFIT package is used for line fitting
- Fourier transforms and convolutions are done numerically

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ

## Multi-diagnostic analysis needed for comparisons with independent Langmuir probe measurements

- Need other diagnostic measurements and divertor plasma models to reconstruct complex divertor picture

**Multi-diagnostic analysis needed for comparisons with independent Langmuir probe measurements**

**C. Bush (ORNL)**

- V. A. Soukhanovskii, NSTX Results Review, 24 July 2007, Princeton, NJ