TJ-II: Zeff measurement using visible bremsstrahlung (VB) with NBI heating (II)

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Experimental campaign

Spring 2022

Proposal title

Zeff measurement using visible bremsstrahlung (VB) with NBI heating (II)

Name and affiliation of proponent

López-Miranda, B., Baciero, A., Ochando, M. A., Medina, F., McCarthy K. J., Pastor, I., Liniers M. and the TJ-II team, Laboratorio Nacional de Fusion, CIEMAT (Spain)

Details of contact person at LNF

Description of the activity


The study of the impurity behaviour in fusion plasmas is crucial for its performance and the understanding of its transport. Effective ion charge, Zeff, is one of the basic plasma parameters that estimate impurity content in high-temperature plasmas and its profile is essential for studying impurity transport in the plasma core.

Several methods have been implemented to evaluate the Zeff in different fusion devices, such as visible bremsstrahlung (VB) [1, 2], plasma resistivity [3], soft X-ray emission [4], or neutron flux [5]. Whereas in tokamak plasmas several methods are available to estimate the plasma Zeff, in stellarator devices, without significant current, only bremsstrahlung measurements in visible and soft X-ray ranges can be used for this purpose. Therefore, in the LHD and Wendelstein-7X stellarator devices, the Zeff has been determined from the spectrum of continuous radiation in the visible range [6, 7]. But, because the emissivity of the bremsstrahlung radiation is weak in the visible range, using the VB to estimate the visible Zeff is challenging. Previous attempts for measuring the Zeff profiles in TJ-II plasmas using VB profiles [8, 9, 10] provided unrealistic Zeff values much higher than those estimated by soft X-rays measurements. Accurate deduction of Zeff in ECRH plasma scenarios was possible with an absolutely calibrated measurement of the emissivity and electron density and temperature in a visible range where e-i bremsstrahlung was the dominant source of light [11].


In this work, we propose to study the emission from bremsstrahlung in relatively high-temperature fusion research plasmas created with ECRH and NBI and to measure the Zeff profile by scanning the VB. A general view of the impurity content will be obtained recording spectra between 200 to 900 nm with a PMA spectrometer and the spectral scanning system will be employed to record the VB signals. We will employ the previous system described in [11] upgraded with a plan-achromatic doublet; also, reducing the scanning mirror velocity will maximize the signal to avoid optical aberrations. The effect of the NBI overlapped with ECRH will be studied and the effect of NBI 1 and NBI 2 will be compared. In order to minimize the impurities in TJ-II plasmas, we need a fresh boronizated/lithiumizated wall.

International or National funding project or entity

This work was funded by the projects from the Spanish Ministerio de Ciencia e Innovación RTI2018-100835-B-I00 (MCIU/AEI/FEDER, UE) and PID2020-116599RB-I00.

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation: 3 days
  • Essential diagnostic systems: spectral scanning system, Bolometric arrays, X-Ray detectors, VUV spectrometer, Thomson Scattering, Doppler reflectometer.
  • Type of plasmas (heating configuration): standard configuration (100_44_64), ECRH, and NBI plasmas, this scenario requires a constant high line averaged density (0.9×1019 m-3), similar to 48223.
  • Specific requirements on wall conditioning if any: fresh-lithiated wall.
  • External users: need a local computer account for data access: no
  • Any external equipment to be integrated? Provide description and integration needs:

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)


[1] Chen, Y., Wu, Z., Gao, W., Ti, A., Zhang, L. et al., (2015). Application of visible bremsstrahlung to Zeff measurement on the Experimental Advanced Superconducting Tokamak. Rev. Sci. Instrum. 86, [023509].

[2] Morita, S. y Baldzuhn, J., (1994) Max-Planck-Institut für Plasmaphysik, Garching, Germany, IPP Report No. III/199.

[3] Anderson, J. K., (2001). Measurement of the electrical resistivity profile in the Madison Symmetric Torus (Ph.D. Thesis). University of Wisconsin-Madison, (USA).

[4] Galante, M. E., Reusch, L. M., Den Hartog, D. J., Franz P., Johnson, J. R. et al., (2015). Determination of Zeff by Integrating Measurements from X-ray Tomography and Charge Exchange Recombination Spectroscopy, Nucl. Fusion 55, [123016].

[5] Eriksson, J., Hellesen, C., Conroy, S., Ericsson, G., Hjalmarsson, A. et al., (2013). Deuterium Beam Ion Diffusion in JET H-mode Plasmas Studied with Transport Modelling and Neutron Diagnostics, 13th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, Beijing, China. From:

[6] Zhou, H. Y. Moriota, S., Goto, M. and Dong, C. F., (2010). Zeff profile diagnostics using visible bremsstrahlung continuum for nonaxisymmetric plasmas with finite  in large helical Device. J. Appl. Phys. 107, [053306]

[7] Krychowiak, M., Dodt, D., Dreier, H., König, R. y Wolf, R., (2008). Development of a virtual Zeff diagnostic for the W7-X stellarator. Rev. Sci. Instrum. 79, [10F512].

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

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

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

[11] López-Miranda, B., (2021). Study of the impurities and continuous radiation in the visible spectral range by means of spectroscopic techniques and lasers in the TJ-II stellarator. (Ph.D. Thesis). Universidad Complutense de Madrid (Spain).

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