Reduction of fluctuations and edge current during PPCD in MST B. E. Chapman and the MST group
Outline -- The key to sustained fluctuation reduction during PPCD -- Magnetic fluctuations (including single helicity spectra) -- Density and potential fluctuations -- Edge current and heat flux
bt,rms (G) Key to sustained reduction of fluctuations (magnetic and electrostatic) during PPCD is suppression of edge-originating bursts Eθ (V/m) 1-1 - 6 4 m = (b/b 1%), 3-feb-1999, Sh(96) 3-feb-1999, Sh(113) Pohmic (MW) 5 15 5-5 SXR (arb) 1 5 1 1 14 16 18 Time (ms) 1 1 14 16 18 Time (ms)
The bursts -- Same phenomenology in PPCD and EC discharges -- Also observed in IHTM (TPE- 1RM)? -- Cause unknown, but observe increased p and J during PPCD -- Bursts clearly associated with edge-resonant m = fluctuations -- But may or may not originate with the m = 's: in PPCD, m = resonant surface may be well inside the largegradient region
Burst-free PPCD at low current often exhibits a flat and reduced m = 1 (n = 6-15) spectrum 1 3-feb-1999, Sh(19) before PPCD @ 9.5 ms during PPCD @ 18 ms Before PPCD (standard confinement) Mode amplitude (b t, Gauss) 1 8 6 4 Quasi single helicity During PPCD (improved confinement) 1 3 4 5 6 7 8 9 1 11 1 13 14 15 n (toroidal mode number)
Quasi single helicity spectra can appear during PPCD, most often at higher Ip Eθ (V/m) 1-1 - PPCD 9-feb-1999, Sh(3) Mode amplitudes bt (G) 3 1 1-pt smooth n=6 n=7 n=8 SXR ratio (arb) 16 1 8 4 Signature of (1,6) rotation Pohmic (MW) 1-1 1 1 14 16 Time (ms)
Density fluctuation reduction with PPCD is broadband and global PPCD Density fluctuation power spectra vs. time in a 4 ka PPCD discharge (with burst suppression) Data courtesy of D. Brower
Electrostatic fluctuations and particle transport reduced in extreme edge 1 3 @ 48 cm standard 1 3 @ 5 cm n (arb) 1 1 1 PPCD n (arb) 1 1 1 1 3 1 3 Vp (arb) 1 1 1 Vp (arb) 1 1 1 Electrostatic-fluctuation-induced particle flux (particles/m /s)x1 1 4 6 8 1 1 4 6 8 1 f (khz) f (khz) 1 9 8 standard 7 6 5 4 3 PPCD 1 48 49 5 Minor radius (cm) Data courtesy of C.-S. Chiang
Reduction of edge parallel current and heat flux during PPCD
Edge parallel current is reduced during improved confinement, burst-free PPCD E//(a) (V/m) J//.8. -.4 (A/cm ) bt,rms (G) 4 Auxiliary parallel current drive applied 3 cm from plasma boundary burst-free improved confinement period Pohmic (MW) sxr (arb) 8 4 1 1 14 16 18 Time (ms)
During burst-free PPCD, edge current is reduced and edge current profile steepens J// (A/cm ) 15 1 5 PPCD standard J///B (A/cm /G) x 1 3-5 5 15 1 5-5 4 44 46 48 5 Minor radius (cm)
During burst-free PPCD, edge conductivity and the dynamo emf appear to drop Te 3/ (ev) 3/ 14 1 1 8 6 4 standard PPCD 4 44 46 48 5 Minor radius (cm) -- Edge datapoints from Langmuir probe spline fit to Thomson scattering data further inside -- (Zeff) unknown -- MHD dynamo electric field measured at 48 cm with B-dot and spectroscopic probes: Edynamo (standard) =.77 V/m Edynamo (PPCD) =.17 V/m -- Parallel Ohm s law: J = σ(eapplied + Edynamo) -- Edynamo and/or σ decrease more than Eapplied increases -- So changes in fluctuations and transport induced by PPCD play larger role in determining edge current than does PPCD itself
Q // (kw/cm ) Consistent with the drop in edge current, the edge parallel heat flux drops as well Eθ (V/m) 1-1 - PPCD 6-feb-1999, Sh(97) 1 cm from plasma boundary Pohmic (MW) SXR (arb) 8 6 4 4-8 1 1 14 16 18 Time (ms)
How are the fluctuations reduced? -- Current profile modification one obvious explanation for PPCD -- But magnetic and electrostatic fluctuations are reduced globally (got more than we hoped for) -- EC, IHTM, and α-mode discharges have characteristics similar to PPCD (particularly in MST) -- So explanations not based solely on current drive may be needed as well (E B flow shear, etc.)
Which fluctuation reductions account for reduced energy transport? -- Expect core reduction due to reduced m = 1 tearing fluctuations -- In the edge (r/a > r rev /a.85) of standard discharges: (1) little contribution to energy transport from magnetic fluctuations () electrostatic contribution also small, but have no data for r/a <.9 -- With r rev /a.65-.75 (PPCD, EC), is "edge" region much broader?
Energy flux from electrostatic fluctuations small but increasing exponentially (Rempel et al., 199) 1 Poh - Prad Electrostatic-fluctuationinduced energy flux (kw/m ) 1 1.1 r /a =.85 rev.1.84.86.88.9.9.94.96 r/a
Summary -- Suppression of edge bursts key to sustained global fluctuation reduction -- Magnetic fluctuations (m=1 and m=) reduced in core and edge with single helicity spectra occurring in some PPCD and standard discharges -- Density and potential fluctuations are reduced -- Edge parallel current, parallel heat flux, conductivity, and dynamo electric field reduced -- Role of edge fluctuation reduction in decreased energy transport not yet determined