On resolving the optical spectra of the edge plasma radiation against a strong background of the divertor stray light

Transcription

1 Journal of Physcs: Conference Seres PAPER OPEN ACCESS On resolvng the optcal spectra of the edge plasma radaton aganst a strong background of the dvertor stray lght To cte ths artcle: L Ognev and V S Lstsa 06 J. Phys.: Conf. Ser Related content - Hgh-Z plasma facng components n fuson devces: boundary condtons and operatonal experences R Neu - Atomc and molecular database for fuson plasma edge studes R.K. Janev, M.F.A. Harrson and H.W. Drawn Vew the artcle onlne for updates and enhancements. Ths content was downloaded from P address on 3/03/08 at 00:56

2 X nternatonal Conference on Modern Technques of Plasma Dagnostcs and ther Applcaton OP Publshng Journal of Physcs: Conference Seres 666 (06) 0005 do:0.088/ /666//0005 On resolvng the optcal spectra of the edge plasma radaton aganst a strong background of the dvertor stray lght L Ognev,3 and V S Lstsa, Natonal Research Centre "Kurchatov nsttute", Moscow 38 Russa Natonal Research Nuclear Unversty MEPh, Moscow, Russa E-mal: lognev@gmal.com Abstract. A method of detectng a weak radaton of the Balmer lnes from the hydrogen sotopes n the edge plasma of a tokamak (SOL) on the strong optcal background formed by reflecton of the lght emtted from the dvertor plasma s proposed. The method s based on a parallel regstraton of the radaton from the optcal dump and from the wall near the dump. Calbraton of the attenuaton of the stray lght radaton by the optcal dump s beng performed n the course of the measurements, usng the ntensty of the hgh-frequency pulsatons that are not correlated wth the edge plasma emsson. Statstcal estmates show that a 0-fold attenuaton of the lght by the dump allows regstraton of the weak edge plasma radaton makng down to 0.5% of the background ntensty.. ntroducton The ssue of regsterng the Balmer lne radaton from the hydrogen sotopes n the edge tokamak plasma (SOL) has become partcularly mportant after frst assessments of the background formed by reflecton of the powerful lght radaton from the dvertor plasma. The rato of the dvertor to the SOL radaton ntensty n the H-alpha lne n modern tokamaks (JET) s about an order of magntude. Gven the parameters of TER, ths rato can reach a value of 0 [].. Methods of detectng a weak radaton on a strong background Several methods have been proposed to solve ths problem [, ]. Utlsaton of an optcal lght dump (an absorbng cavty n the wall) was proposed n []. The method [] s based on a soluton of the nverse problem of recoverng the SOL sgnal usng the data of measurements along dfferent chords, ncludng those vewng the dvertor. Parallel detecton of the radaton from the dump and wall ("double chords" scheme) mproves the stablty of the method... The dump calbraton method usng hgh-ntensty fluctuatons of the background n ths paper we analyze the possblty of detectng a weak SOL sgnal on the background of a strong dvertor radaton n the scheme wth the lght dump [] ("double chords"), takng advantage of the statstcal ndependence of the fluctuatons of the sgnals from both sources. The typcal frequences of 3 To whom any correspondence should be addressed Content from ths work may be used under the terms of the Creatve Commons Attrbuton 3.0 lcence. Any further dstrbuton of ths work must mantan attrbuton to the author(s) and the ttle of the work, journal ctaton and DO. Publshed under lcence by OP Publshng Ltd

3 X nternatonal Conference on Modern Technques of Plasma Dagnostcs and ther Applcaton OP Publshng Journal of Physcs: Conference Seres 666 (06) 0005 do:0.088/ /666//0005 such fluctuatons n modern tokamaks, accordng to the observatons of the spectra of optcal radaton from helum [3] and hydrogen [4], reach the values up to 00 khz. n the method proposed, the hgh frequency part of the fluctuaton spectrum s used to calbrate the optcal dump. Ths method s based on comparng the ntensty of the radaton beams reflected from the attenuatng dump and from the frst wall wthout attenuaton (fgure ). Both beams nclude, as a mnor fracton, the radaton from the neutral atoms n the edge plasma (SOL), x and x, respectvely, whch s regstered wthout attenuaton and s the useful sgnal to be resolved. The background radaton reflected from the vcnty of the dump, y, s not attenuated unlke that reflected from the dump tself, y (fgure ). Therefore, x x x y, y y, x y () t, x y () t, where s the lght attenuaton factor n the dump. Now the ntensty of the weak sgnal can be found as: x ( ) ( ) () Dagnostc requrements for montorng the H-alpha sgnal foresee the tme resoluton of 0. s. Therefore, the tme-average values should be taken for the ntenstes and n equton. The ntensty of the hgh frequency part of the fluctuaton spectrum n the text below wll be noted as. So, n order to detect the weak sgnal on the strong background one needs to know the reflectvty of the lght beam dump, whch may vary n tme over the measurement perod of the sgnal T = 0. s. Unlke the "double chords" [], ths method does not requre detaled nformaton about the shape of the lne. Fgure. Scheme of the formaton of the twochannel sgnal and by the reflecton of the dvertor stray lght from the wall (chord ) and from the optcal dump (chord ) wth the addton of the radaton contrbuton of the edge plasma. M s the frst dagnostc mrror, D the detector wth separate regstraton of the channels. The radaton zones are shadowed. The reflectvty of the lght dump can be calculated from the tme-averaged power of the fluctuatons of the hgh ntensty dvertor radaton n the lght beams reflected from the dump and from the chamber wall as ~ ~ ~ y ~ y () snce, due to the statstcal ndependence, ~ ~ y ~ x аnd ~ ~ x. y.. Statstcal accuracy of the dump effectveness and the bandwdth of the background fluctuatons. Hgh ntensty fluctuatons of the H-alpha lne are collected n a frequency band [ω, ω 3 ] and subjected to

4 X nternatonal Conference on Modern Technques of Plasma Dagnostcs and ther Applcaton OP Publshng Journal of Physcs: Conference Seres 666 (06) 0005 do:0.088/ /666//0005 quadratc detectng followed by solaton of the low-frequency sgnal components ~ n the dagnostc range [0, ω ]. We wll use the relaton between the dsperson of the current through the quadratc detector squared and the frequency ranges used for regstraton of the hgh- and low-frequency sgnals n the model of the whte nose wth the Gaussan statstcs [5]: 4( ) ( ) 4 ( )d ( ) ( ) S and S ( ) ) d S d ( )d 0 ) S f ( ) 0 0 ( ) 0 3 where f () are the spectral denstes of the fluctuatons of the processed sgnal and ts constant component. Snce / (/) / from (3) one obtans / (/) ) 4 /( ) 3 (3) ~ y y x(t) ~ y y Fgure. Schematcs of the measurement of the optcal radaton sgnal from the neutral hydrogen atoms (x (t)) on the background of the dvertor stray lght (y (t)). The total radaton ntensty (t) and the total ntensty of the radaton from the optcal dump (t) s what s actually measured. The shaded areas correspond to the rapdly oscllatng component of the radaton comng from the dvertor. The bandwdth of the hgh-frequency flter necessary to satsfy the accuracy / 0. can be found from the relaton 4 / Hence 3 (4 / ), and for T 0. s, the frequency of the hgh-frequency pulsatons used for the dump calbraton f 300Hz s adequate for the dump 3 calbraton. Ths corresponds to the mnmum reference tme nterval for the nose t 0.30 s. The requred photon flux through the hgh-frequency flter n the nterval of t / s 3 5 N / ( t/ ) 00 / (0.30 s/ ) 7 0 ph/s, and ths can ndeed be regstered n experment... Senstvty of the method. The uncertanty of the xt () calculaton s attrbuted to varatons n the reflectvty of the dump () t () t. Let us express the varaton through the partal dervatve of x a(, ) wth respect to α: x( x/ ). After substtuton () t () t x x, takng nto account the smallness of x we obtan ( x) ( ) ( ) (4) 3

5 X nternatonal Conference on Modern Technques of Plasma Dagnostcs and ther Applcaton OP Publshng Journal of Physcs: Conference Seres 666 (06) 0005 do:0.088/ /666//0005 Therefore, mnmzaton of, rather than the smallness of tself, s mportant for mprovng the senstvty. Assumng / 0., f 0.05 [], the mnmum detectable sgnal-to-nose rato x mn / can be estmated as less than Acknowledgements The authors wsh to thank A. B. Kukushkn and V. S. Neverov for frutful dscussons. Ths work was supported n part by the grant NSH of the Presdent of the Russan Federaton for the state support of leadng scentfc schools of the Russan Federaton. References [] Andreenko E N, Alekseyev A G and Gorshkov A V 04 AP Conference Proceedngs 6 7 [] Kukushkn A B et al 0 Book of Abstracts, 4th AEA Fuson Energy Conference, San Dego, USA, 8-3 October 0 TR/P5-44 [3] Rosmej F B, Ohno N, Nakamura S and Kajta S 008 Contrb. Plasma Phys [4] Zelnsk J J, van der Meden H J, Morgan T W, Hoen M H J, Schram D C and De Temmerman G 04 Appl. Phys. Lett [5] Akhmanov S A, Dyakov Yu E and Chrkn A S 98 ntroducton to statstcal radophyscs and optcs (Moscow: Nauka) 4