Preliminary study for Soft X-ray Spectroscopy in VEST Jungmin Jo, Jeong-Jeung Dang, Young-Gi Kim, and Y.S. Hwang Center for Advance Research in Fusion Reactor Engineering Department of Nuclear Engineering Seoul National University

Impurity characteristic radiation Future Experimental plan Summary Contents Introduction Background theory Experimental setup Result Continuum radiation Impurity characteristic radiation Future Experimental plan Summary

Passive Spectroscopy Introduction Near Infrared Visible [1] Spectral region Wavelength (nm) Energy (eV) Near infrared 700 to 1200 1 to 2 Visible 400 to 700 2 to 3 Ultraviolet 200 to 400 3 to 6 Vacuum ultraviolet 30 to 200 6 to 40 Extreme ultraviolet 10 to 30 40 to 120 Soft X-ray 0.1 to 10 120 to 12,000 Near Infrared Visible Vacuum UltraViolet Extreme UltraViolet Soft X-ray Target Measuring value Purpose Note Continuum radiation (bremsstrahlung, recombination) Density, temperature, impurity convolution value (with tomography technique) Plasma imaging MHD phenomena EC wave or EBW heating effect Radiation from electrons heated by EC wave Line radiation Impurity Certain impurity lines Impurity content inform. Impurity transport Inform. of Te Appropriate for diagnose core region plasma

Background Theory – mechanism of Soft X-ray Continuum radiation Coulomb interaction between free electrons and ions Bremsstrahlung radiation (free – free transition) Spectral power density [2] + - Recombination radiation (free – bound transition) Spectral power density [2] + - Line radiation characteristic line radiation from ionized impurity + -

Signal processing circuit Experimental Setup VEST Plasma Detector position 128 mm R = 1900 mm Detector AXUV 16ELG Filter foil holder & Foil filter In-vacuum component Limit the line of sight Collimator (SUS pipe) Signal processing circuit Vacuum feedthrough

Result VUV, EUV and SXR range radiation from VEST Signal level examination Aluminum 1.5 μm Aluminum 1.5 μm thickness foil can be used as VUV, EUV and SXR region filter. Shot #9620 Lots of radiation signal is detected. Because the spectroscopic applications are depends on radiation source so firstly we have to identify the radiation mechanism. Possible sources for this radiation Continuum radiation Impurity characteristic radiation. High energy radiations caused by non-thermal energetic electrons

Result Identifying radiation source_Continuum radiation Theoretical calculation of expected continuum radiation signal (At the current peak time) Plasma volume at the line of sight 0.04 m^3 Plasma density, temperature Ne= 5 E18 Te=100eV Generated bremsstrahlung radiation power 0.15 W Solid angle 5.47 E-8 Filter Carbon, 4.5 um Generated Photocurrent 9.5395e-12 (A) Dark current ~0.1 nA Circuit gain : 𝟏𝟎 𝟔 V / A Expected Voltage signal 9.5395e-6 (V) In our case Continuum radiation effect will be negligible

Result Identifying radiation source_Continuum radiation To check the continuum radiation level experimentally, choose impurity line free region Expected major impurity(oxygen, carbon) characteristic line radiation (vertical lines) and Transmission curve of appropriate filters Experimental result (shot #9464) By using carbon 4.5 μm thickness filter we can filter out most of expected major impurity characteristic radiation Negligible continuum radiation in VEST Negligible high energy radiation VUV, EUV and SXR range radiation in VEST is mostly caused by impurity characteristic lines

Result Identifying radiation source_Impurity characteristic radiation C III C VI C V O V O VI O VII O VIII Fe XV Fe XVI Fe XXIII Fe XXIV W (quasi-continuum) 97.7 154.82 4.03 3.37 15.61 15.01 12.03 10.24 28.42 33.54 13.28 19.2 4.5~6.5 38.62 155.08 22.72 2.84 19.28 17.3 12.85 7.59 36.08 25.51 41.96 24.87 2.7 21.5 18.4 2.16 1.9 38.41 13.49 62.97 103.19 31.24 18.22 76.03 103.76 24.49 Expected minor impurities in VEST : Iron, Tungsten(limiter material) Goal : Identifying impurity species and lines Result Identifying radiation source_Impurity characteristic radiation By using list of intense spectral lines we can reduce the expected impurity lines. (This data mostly based on spectra which were measured at TEXTOR and JET and to some extent on data from literature.) Expected major impurities in VEST : Oxygen, Carbon [3] Ion species C III C VI C V O V O VI O VII O VIII wavelength 97.7 154.82 4.03 3.37 15.61 15.01 12.03 10.24 38.62 155.08 22.72 2.84 19.28 17.3 12.85 7.59 41.96 24.87 2.7 21.5 18.4 2.16 1.9 38.41 13.49 62.97 103.19 31.24 18.22 76.03 103.76 24.49 Oxygen Carbon Firstly, assume this list also intense lines in VEST and check the existence

Result Identifying radiation source_Impurity characteristic radiation Spectrum scanning and line radiation existence check Carbon 3 μm Transmission Shot #9613 Line Transmission (%) Al 3 μm C 3 μm 652.62 (O VIII) 4.70 1 If the aluminum filter signal is mainly from the highest energy line radiation (upper yellow circle), the carbon filter signal level should be one fourth of aluminum signal (~ 5 mV) due to the transmission rate difference. Aluminum 3 μm Transmission However there is no signal in carbon filter, so the highest energy line is negligible and aluminum filter signal is more likely from low energy region 652.61726 line negligible

Result Identifying radiation source_Impurity characteristic radiation Spectrum scanning and line radiation existence check Carbon 1.5 μm Transmission Large signal difference between two filters, about hundreds times Most of EUV~SXR radiation is from the low energy region Aluminum 1.5 μm Transmission To check low and high energy region each, other material filters are necessary.

Necessary filters for identification of the other impurity lines Future Experimental plan Identifying radiation source_Impurity characteristic radiation Necessary filters for identification of the other impurity lines Ni (250 nm) + Cr (500 nm) + Parylene (200 nm) 1 Al (400 nm) + V (600 nm) 2 Ti (400 nm) + CaF2 (600 nm) 3 1 2 Identify O VII line effect 2 Identify C VI line effect 3 Identify C V line effect Necessary for Verification Method Because of possibility of existence of other lines verification method are needed Multi-Layer Mirror monochromator Spectrometer (e.g. grazing incidence spectrometer)

Summary Conclusion Summary Preliminary study for SXR spectroscopy was progressed For the identification of impurity lines necessary filter foil is selected Because of possibility of existence of other lines verification method are needed and MLM method or spectrometer can be used. Summary Summary Preliminary study for SXR spectroscopy was progressed Radiation source identifying experiments are performed using filtered diode method For the identification of impurity lines more precisely necessary filter foil is selected Because of possibility of existence of other impurity lines verification method are needed. MLM monochromator or spectrometer can be used.

Reference [1] Fusion Science and Technology, Vol. 53, 2008, Special issue on plasma diagnostics for magnetic fusion research [2] I. H. Hutchinson, Cambridge univ. press, Principles of plasma diagnostics [3] W. Biel, FZJ, Design of the ITER VUV spectrometer system, Draft report 15 October 2007