Comparison of lower-hybrid (LH) frequency spectra between at the high-field side (HFS) and low-field side (LFS) in Alcator C-Mod

Transcription

1 Comparison of lowr-hybrid (LH) frquncy spctra btwn at th high-fild sid (HFS) and low-fild sid (LFS) in Alcator C-Mod S. G. Bak, R. R. Parkr, S. Shiraiwa, G. M. Wallac, P. T. Bonoli, D. Brunnr, I. Faust, A. Hubbard, B. LaBombard, C. Lau, M. Porkolab, Y. Takas*, J. Trry Plasma Scinc and Fusion Cntr, MIT * Univrsity of Tokyo 54 th Annual Mting of th Division of Plasma Physics Providnc, Rhod Island Octobr 29, /11

2 Anomalous loss of lowr hybrid currnt driv (LHCD) fficincy is obsrvd in Alcator C-Mod whn > 1x10 20 m -3 in L-mod plasmas (s -1 ) Lin Intgratd HXR Count Rat 0.6 Ray Tracing w/ SOL Full Wav Exprimnt Discrpancy du to Non-linar ffcts? n - (10 20 m -3 ) Whn > 1x10 20 m -3, modling ovr-prdicts hard X-ray count rat than xprimnts 1 No non-linar ffcts in th cods To idntify any additional loss mchanisms, spctral broadning of LH wavs is xamind with dg Langmuir probs. In this talk, th focus is givn to th xcitation of ion cyclotron paramtric dcay instability (PDI) 2/11 1 G. M. Wallac, PoP, 19, (2012)

3 Ion cyclotron Paramtric dcay instability (PDI) can dgrad LHCD prformanc. Non-linar wav-wav intraction Pump Studid xprimntally and thortically in svral tokamaks 1,2,3 Dcay into Daughtr LH wav + ion cyclotron (quasi) mod Pump dpltion dcrass ffctiv LH powr Spctral broadning : Daughtr LH wavs with highr n can b Landau dampd at th plasma priphry with rducd fficincy Spctral Powr Ion cyclotron (quasi) mod ʃ ʃ Daughtr LH wav (highr n ) f ω ci ~ ω ci 4.6 GHz - ω ci 3/11 Daughtr Daughtr 2 Pump Frquncy 4.6 GHz Drivn Currnt I LH 1 n 2 1 M. Porkolab, PRL, 38, 230 (1977) 2 Y. Takas, Phys. Fluids 28, 983 (1985) 3 R. Csario, Nucl. Fusion, 27, 435 (1987)

4 Spctral rcordrs ar dvlopd to continuously monitor LH frquncy spctra. Dvlopmnt of spctral rcordrs Htrodyn systm Rsolution bandwidth : ~ 100 khz Frquncy bandwidth : ~ 250 MHz Rptition rat : ~ 60 Hz Powr (kw) Frquncy (GHz) LH Powr Pump Tim- and Frquncy- Intrpolatd Spctrogram Tim (Sc) 4/11 (db) RF (~ 4.6 GHz) Simplifid schmatic of powr dtcting stag DC Output Amplifir Low Pass Filtr Powr Dtctor LO 3.5 GHz Variabl LO ~ 1.1 GHz

5 Excitation of ion cyclotron PDI with thrshold lik bhavior abov m -3 Obsrvations in L-mod plasmas during 2012 campaign Probs on th launchr, outr divrtor, and innr wall Excitation of PDI with thrshold lik bhavior abov m -3 Lin-avragd Dnsity (10 20 m -3 ) Frquncy (GHz) n Pump Sidband (db) HFS Prob LFS Prob on th launchr 4.4 HFS Prob Tim (Sc) -60 Outr Divrtor Prob 5/11

6 In USN plasmas, ion cyclotron PDI is xcitd nar th outr plasma dg abov m -3 Abov m -3 Excitation of ion cyclotron mod Frquncy sparation : ~ 33 MHz Instability occurs at th outr dg Powr sidband << Powr pump Consistnt with past obsrvations in othr tokamaks Broadband fatur is mor or lss indpndnt from th prob locations 1,2 LFS Prob n- 0.9x10 20 m -3 n- 1.15x10 20 m -3 Outr Divrtor Prob HFS Prob f 33 MHz USN Plasma Frquncy (GHz) 1 Y. Takas, Phys. Fluids 28, 983 (1985) 2 R. Csario, Nucl. Fusion, 27, 435 (1987) 6/11

7 In LSN plasmas, ion cyclotron PDI is xcitd nar th innr plasma dg abov m -3 Abov m -3 No ion cyclotron PDI occurring at th outr dg within th dtctor snsitivity Innr wall prob dtcts ion cyclotron PDI occurring at th innr dg Evidnc of pump dpltion LFS Prob n- 0.9x10 20 m -3 n- 1.2x10 20 m -3 Outr Divrtor Prob LSN Plasma Prob location can b important in idntifying PDI occurring nar innr dg Not rportd in th litraturs HFS Prob f 65 MHz 7/ Frquncy (GHz)

8 In IWL plasmas, ion cyclotron PDI occurring nar th innr dg shows a highr thrshold dnsity. Abov 1.25 x10 20 m -3 Rlativly wakr ion cyclotron PDI occurring at th outr dg Strong ion cyclotron PDI dvlops at th innr dg Broadband fatur in th dcay spctra can b dpndnt on th prob locations. LFS Prob n - 1.2x10 20 m -3 n - 1.3x10 20 m -3 Outr Divrtor Prob f 33 MHz HFS Prob f 65 MHz IWL Plasma 8/ Frquncy (GHz)

9 Dpnding on magntic configurations, ion cyclotron PDI is xcitd at diffrnt radial locations. away X-point toward X-point Obsrvations in both forward and rvrsd & Innr Wall Limitd Obsrvations in forward & ƒ ω ci (R = R a ) ƒ ω ci (R = R 0 -a) ƒ ω ci (R = R 0 -a) PDI at LFS PDI at HFS PDI at HFS IWL 9/11

10 Minimizing PDI can lad to bttr LHCD prformanc (1/s) HXR Count Rat 2008 Campaign USN w/ wakr PDI LH Frquncy Spctrum - n 1.2x10 20 m -3 USN USN w/ wakr PDI f 33 MHz n- (10 20 m -3 ) Frquncy (GHz) Quantitativ assssmnt is yt difficult. Wakr ion cyclotron PDI occurring at LFS corrlats with th incras of hard X-rays. Obsrvd non-linar ffcts may xplain th discrpancy btwn xprimnts and simulations abov 1.0 x10 20 m -3 10/11

11 Conclusion : PDI can b also rsponsibl of loss of currnt driv fficincy Survy of LH frquncy spctra on Alcator C-Mod throughout 2012 campaign Various PDI activity is obsrvd in L-mod plasmas Thrshold-lik xcitation of PDI activity abov 1.0 x10 20 m -3 PDI can b localizd and th location of probs ar important. Evidnc of pump dpltion and spctral broadning PDI may xplain th discrpancy btwn xprimnts and simulations abov 1.0 x10 20 m -3 Strong singl-pass absorption (Shiraiwa Postr JP Tusday Aftrnoon) will prvnt from ion cyclotron PDI occurring at HFS. Furthr modling work including non-linar ffcts is ncssary. 11/11