TJ-II:Comparison of the ionic effective charge of TJ-II plasmas after recent vacuum wall boronization/lithiumization with Laser Blow-off contaminated plasmas using visible bremsstrahlung emissions

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Experimental campaign

2019 Spring

Proposal title

Comparison of the ionic effective charge of TJ-II plasmas after recent vacuum wall boronization/lithiumization with Laser Blow-off contaminated plasmas using visible bremsstrahlung emissions

Name and affiliation of proponent

B. López-Miranda1, A. Baciero1, K.J. McCarthy1, M. Ochando1, F. Medina1, I. Pastor1 and TS and TJ-II Team. (1 Laboratorio Nacional de Fusión, CIEMAT, Madrid, Spain.)


Details of contact person at LNF

B. López-Miranda: belen.lopez@ciemat.es

Description of the activity

Motivation.

Study of the impurity behavior in fusion plasmas is crucial for the performance and understanding its transport is needed to achieve fusion research goals. Effective ion charge, Zeff, is one of basic plasma parameters for estimating impurity content in high-temperature plasmas and its profile measurement is essential for studying impurity transport in the plasma core. Whereas in tokamak plasmas several methods are available to estimate the plasma Zeff [1], in stellarator devices, with not significant current, only bremsstrahlung measurements in visible and soft X-ray ranges can be used for this purpose. Previous attempts for measuring the Zeff profiles in TJ-II plasmas by means of visible bremsstrahlung (VB) profiles [2, 3, 4] provided unrealistic Zeff values much higher than those estimated by soft X-rays measurements. Accurate deduction of Zeff is possible with an absolutely calibrated measurement of the emissivity and electron density and temperature in a wavelength region where e–i bremsstrahlung is the dominant source of light. Identification of wavelength regions free of atomic lines and other radiation is a challenge because there are different contributions to the continuum spectrum.


Proposal.

In this work, we propose to study the emission from bremsstrahlung in relatively high-temperature fusion research plasmas and measure the Zeff profile by scanning the visible bremsstrahlung in broad spectral ranges, upgrading the previous system described in [4] by means of a plan achromatic doublet and reducing the scanning mirror velocity in order to maximize the signal and avoid the optical aberrations. We try to compare the experimental results obtained with freshly boronizated/lithiumizated plasmas with the Zeff profiles measured with the introduction of impurities. In order to perform the second part of the experiment, we will inject impurities in the vacuum vessel by means of laser blow-off.


International or National funding project or entity

This work was funded by the Spanish “MINECO” under Grant No. ENE2014-56517-R. B.L.M. would like to acknowledge her scholarship, Grant No. BES-2015-075704.

Description of required resources

Required resources:

• Number of plasma discharges or days of operation: We would need at least 3 operational days: one (compatible with other experiments) to adjust all the variables of the rotational scanning mirror system, another day to perform the experiment with freshly boronization/lithiumization. We will devote the third day to perform the second part of the experiment which requires the injection of impurities by means of laser blow-off ablation.

• Essential diagnostic systems: Main diagnostics will be radiation diagnostics (bolometers, X-rays, VUV spectrometer, etc.) and ECR radiometer data as well as other diagnostics with good time resolution. We need the electron density and temperature measurements from Thomson scattering. Laser blow-off system and a broad range PMA12 spectrometer.

• Type of plasmas (heating configuration): We need standard configuration plasmas (100_44_64)

• Specific requirements on wall conditioning if any: we need freshly lithiumizated and boronizated plasmas. We prefer to perform the experiment after 2 wall conditioning treatments at least.

• We need discharges with solely ECRH and good control with constant density along discharges. Lower densities appear to give more stable target plasmas; however, we need plasma electron densities that are sufficiently high to ensure good Thomson scattering profiles.

• External users: need a local computer account for data access: No.


Preferred dates and degree of flexibility

Preferred dates: (XX-03/4-2019) We would like to perform the experiment during March and April 2019.

References

1. Zheng YZ et al., 2007 CHINESE PHYSICS 16 (4): 1084-1088

2. Baciero A, Zurro B et al., Proc. 30th EPS Conf., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-2.80.

3. Baciero A, Zurro B et al, Proc. 34th EPS Conf., Warsaw, July, 2007 ECA Vol.31F, P-5.089.

4. B. López Miranda, A. Baciero, B. Zurro, et al., Proc. 43rd EPS Conf., Leuven, Belgium, July, 2016 Vol. 40A ISBN: 2-914771-99-1.



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