13th iea/rfp workshop october 9-11, 2008 stockholm measurements of characteristic pressure profile...
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Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
113th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Measurements of characteristic pressure profile lengths
and comparison to turbulence scale lengths in RFX-mod edge
P.Scarin, M.Agostini, R.Cavazzana, F. Sattin, G.Serianni, M. Spolaore, N. Vianello
Consorzio RFX, Associazione EURATOM-ENEA sulla fusione, C.so Stati Uniti 4, Padova, Italy
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
213th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
T O P I C S• Edge pressure profile measurements with Thermal Helium Beam
• Toroidal spatial scales of edge turbulence structures (blobs) routinely measured with Gas Puffing Imaging diagnostic
• Scaling of edge electron density and temperature with n/nG
• Link between the e-folding characteristic length Lp of the edge electron pressure profile and the blobs dimensions
•Scaling of average blob sizes with the ion sound radius s
• Toroidal Mach number M⊥=v/Cs has been determined in different plasma conditions
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
313th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
to spectrometer [ne,Te]
plane mirror
GPID
Radial resolution (5mm) in plasma edge (45mm)
Radial profiles of density and temperature with line intensity ratio
I(7281)/I(7065) Te
I(6678)/I(7281) ne
Sampling rate : 2MHz (PMT in progress)
slow with CCD
Time resolution of ne ,Te about 100kHz (in progress)
He cloud
spherical mirror
vacuum window
lens
to photomultipliers with HeI filters
Thermal Helium Beam Diagnostic
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
413th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
IØ
GPI diagnostic with Triassial Magnetic Coils
32 chords ( 16 + 8 + 8 ) HeI (668nm) View area : 70 mm x 30 mm Flux 1018 ÷ 1020 atoms/s Cloud Density no 3·(1017÷1018) m-3
Bandwidth: 2 MHz Sampling rate : 10 MSamples/s
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
513th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Experimental conditions
Edge (45mm) electron profile Pe(r) has
been measured with a low time
resolution in different plasma
conditions in terms of plasma current I,
normalized density n/nG and reversal
parameter F
42.5 mm
32.5 mm
22.5 mm
12.5 mm
I=1.5 MA, n/ng= 0.1-0.2, F = -0.02
I=0.4 MA, n/ng= 0.1-0.2, F = -0.07
I=0.4 MA, n/ng= 0.4, F = -0.07
I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1
I=0.4 MA, n/ng=0.2 , F = -0.2
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
613th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Te,edge versus ne,edge scaling
The measured ratios are compared with a look-up table derived from (CRM) atomic
model [B. Schweer et al.J. N. Mater.266 (1999) 673] from which Te,edge and ne,edge are
interpolated
The data dispersion may be due both to different n/nG discharges and to the different
local plasma-wall interaction Te,edge = 10÷100 eV ne,edge = (0.5÷10) 1019m-3
ne,edge[m-3]
Te,
edge
[eV
]
Te,
edge
[eV
]
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
713th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Radial Profile
the pressure characteristic length can be estimated as Lp = min(-Pe/grad Pe) 15÷40 mm,
it depends from plasma current conditions and changes along the pulse time
88 ms
171 ms
255 ms
338 ms
422 ms
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
813th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Scaling of blob size vτa versus Lpv
τa
I=1.5 MA, n/ng= 0.1-0.2, F = -0.02
I=0.4 MA, n/ng= 0.1-0.2, F = -0.07
I=0.4 MA, n/ng= 0.4, F = -0.07
I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1
I=0.4 MA, n/ng=0.2 , F = -0.2
v : toroidal velocity of edge fluctuations measured with cross-
correlation technique
τa : turbulence autocorrelation time
The largest blob size Lb = vτa scales
with the electron pressure characteristic
radial length Lp = min(-Pe/grad Pe) for
different plasma conditions
This trend indicates grad Pe as a source of instability for the
development of edge turbulence [J. Myra et al. POP 13, 2006, 092509]
and Lb can be thought as the injection scale of free energy and
then it develops and feeds smaller and smaller blobs
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
913th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Power spectra & Intermittency behavior
I=1.5 MA, n/ng= 0.1-0.2, F = -0.02
I=0.4 MA, n/ng= 0.1-0.2, F = -0.07
I=0.4 MA, n/ng= 0.4, F = -0.07
I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1
I=0.4 MA, n/ng=0.2 , F = -0.2
The GPI emission spectra are power-law for ƒ>ƒ*= 100kHz (typically) and the decay index depends from the plasma
parameters
The frozen-turbulence hypothesis has been verified recovering a linear dispersion between wave-number k and ƒ
ƒ -3
ƒ -1.5
ƒ*
Multi-scale analysis (wavelet transform) of signals fluctuation PDF manifests intermittency for τ < 10 μs [PDF flatness F(τ) > 3 ]
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1013th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Width 2σ() of emission structures from conditional average technique in RFX-mod
I=1.5 MA, n/ng= 0.1-0.2, F = -0.02
I=0.4 MA, n/ng= 0.1-0.2, F = -0.07
I=0.4 MA, n/ng= 0.4, F = -0.07
I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1
I=0.4 MA, n/ng=0.2 , F = -0.2
The toroidal width 2σ() increases non-linearly (15÷60mm)
with the time scale (1÷10μs) and depends on plasma parameters
The width normalized to ion sound radius s shows a ratio
2σ/s5÷10 at low current (increasing with time scale τ) and
2σ/s 10÷20 at higher current
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1113th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Turbulence scales 2σ() are comparable with gradient scale length Lp
The normalization of toroidal width 2σ() to the pressure gradient length Lp gives a ratio
2σ()/Lp 0.7÷2 (increasing non-linearly with time scale)
For each time scales the scattering of data is about ± 15% in the different plasma
conditions, so we can consider the ratio 2σ()/Lp independent from RFX-mod operations
The normalization of 2σ() to the injection scale Lb=vτa shows a similar trend for the
ratio 2σ()/Lb 0.3÷1
2σ /v
τa
2σ /L
p
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1213th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Sound radius scaling of blob sizes: Lb=vτa , 2σ()
The largest toroidal blob size
Lb scales about linearly with
ion sound radius, Lb/s10÷20
and depends on plasma
parameters
Similar trend has been
observed for the width 2σ() at
different time scales obtained
with conditional average
technique
The observation of blob size scaling with ion sound radius is consistent with the theory
that attributes this scaling to blob stability [D. D’Ippolito et al.POP 11, 4603 (2004)]
Lb = vτa
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1313th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Edge toroidal Mach number M⊥=v/Cs
The local toroidal Mach number
has been estimated in the edge
from spectroscopic measurement:
v flow from GPI measurements
Cs sound speed: Te from THB
measurements and Ti from Doppler
broadening of BII line
(703.0÷703.2nm)
In the different plasma conditions M ⊥ ranges in 0.15 ÷ 0.35 and shows a trend
that increases with the characteristic pressure length Lp
This means a toroidal convection transport competitive with the parallel one
[G. Antar et al., POP 10 (2003) 419] that increases with Lp
v/
Cs
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1413th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Toroidal wave-number power spectra
I=1.5 MA, n/ng= 0.1-0.2, F = -0.02
I=0.4 MA, n/ng= 0.1-0.2, F = -0.07
I=0.4 MA, n/ng= 0.4, F = -0.07
I=0.8 MA, n/ng= 0.2, F = -0.05 ÷ -0.1
I=0.4 MA, n/ng=0.2 , F = -0.2
k -1.5
k -3
The wave-number power spectra has
obtained as integration of S(k,ƒ) spectra
evaluated from 2 GPI line of sight, in
different plasma conditions
Typically the spectra are power-law for
k > k* = 40m-1 and the decay index
depends from plasma parameters
The scaling of edge turbulence for the
toroidal size in the different plasma
conditions gives k*s 0.05÷0.3 and it
is consistent with the model that consider
the blobs to extract free energy from
density gradient [P.W.Terry et al. POF 28 (1985) 1419]
k*
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1513th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
S u m m a r y
The blob sizes scale with the characteristic radial pressure profile length, this
indicates grad Pe as a source for the development of edge turbulence (see J. Myra)
The blob sizes scale also with the ion sound radius s, this is consistent with the
theory of blob stability (see D. D’Ippolito)
The toroidal Mach number M⊥ measurements in the edge shows that there is a
toroidal convection transport competitive with the parallel transport (see G. Antar)
The toroidal wave-numbers scale as k*s 0.05÷0.3 consistent with the model that
consider the blobs to extract free energy from density gradient (see P.W.Terry )
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1613th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
CCD image of He cloud
Tangential view of the porthole where is insert the GPI & THB diagnostic with the
image of back-illuminated fiber optics
GPI
THB
The He cloud is monitored with interferential filter-CCD camera
# 24171
1cm
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1713th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Spectrum measurements
First measurements at low time resolution with standard CCD
In near future : high time resolution with Detector Multianodes R5900U-20-L16 coupled to the focal plane of ƒ# = 4 spectrometer
667,8 nm
706,5 nm
728,1 nm
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1813th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Wave-number Spectrum S(k,ƒ) in RFP plasma
The spectral content due to fluctuations:
|k|<100m-1 and ƒ<500 kHz for RFX-mod
|k|<50m-1 and ƒ<250 kHz for TPE-RX
At spectral width k(ƒ) corresponds a characteristic length for turbulence structures
Lch k-1 20÷40 mm RFX-mod
50÷100 mm for TPE-RX
RFX
ƒ [kHz]
Coh
eren
ce(
ƒ)
RFX
ƒ [k
Hz]
k [m-1]
σk
TPE
ƒ [k
Hz]
k [m-1]
σk
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
1913th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Width 2σ() of emission structures from conditional average technique in RFPs
Non linear increase of δ⊥()
15mm < 2σ < 45mm VS # (Br ~0.5mT)
40mm < 2σ < 90 mm NVS # (Br ~3mT)
2σ() depends more on radial magnetic field Br and less on n/nG
2σ(
) [m
m]
Spatial features of bursts is estimated in RFPs with the GPI central fan
CWT has applied at reference signal to allow the detection of events on different with LIM technique
From measurements of the burst intensity it is possible to put together ensemble data for all LoS at each time scale and so to reconstruct the toroidal pattern
Best fit exp[-(x/ σ)2] has been superimposed to average pattern of the structure
I /
I [a
.u.]
2σ
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2013th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Comparison between THB & Uprobe
• A comparison between the measurements of the different diagnostics has been done
• The different evaluation can be enquired in the different local plasma-wall interaction
THB = 322°
Uprobe = 217°
INT = 22°Int_7B
T Uprobe: 85-110 ms
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2113th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Considerations about the local shift of the modes (F-0.035)Uprobe
THB
INT
• The different diagnostics in different toroidal positions see different plasma-wall interactions
• Uprobe see an effective SOL (in this time window)
• THB and INT see regions fully embedded in the main plasma
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2213th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Different levels of Br1 characterize the different toroidal positions
Uprobe
THB
INT
Different levels of Br1 are generally hints of different
levels of plasma-wall interactions
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2313th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
# 24171Standardinteraction
# 24439
Strong interaction
Different plasma-wall interactions from CCD camera of THB position
The level of interaction depends from the local distance of the last closed magnetic surface (defined by linear overlapping of m=1 and m=0) from the wall, that quantifies the local deformation
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2413th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Ln Lp Lb= vτa v
1 (178)mm (146)mm (363)mm (-148)km/s
2 (4912)mm (3012)mm (8337)mm (-207)km/s
3 (20 7)mm (27 7)mm (6027)mm (-181)km/s
4 (37 11)mm (28 13)mm (544)mm (-207)km/s
5 (35 14)mm (39 9)mm (8424)mm (-2811)km/s
s Cs
1 (1.40.06)mm (864)km/s
2 (6.90.8)mm (11319)km/s
3 (4.40.4)mm (665)km/s
4 (2.50.6)mm (10612)km/s
5 (7.02.1)mm (8811)km/s
2s evi
ev T
m
12i
s ev
mT
B e
Cs
s
Useful data
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2513th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Reconstruction of the 2D emission structure pattern has
done with back-projection technique and inversion algorithm from the line
integrals.
Emission structures (blobs), that move along the ExB flow, emerge from the background
turbulence.
High toroidal resolution
Lower radial resolution
The high time resolution allows to obtain a 2D image
every 0.1 s
Emission structures movie in RFX-mod
[ G. Serianni et al., Plasma Phys. Control Fusion 49 (2007) 2075 ]
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2613th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Edge Structures as Current Filaments in RFX-mod
The bursts detected by the GPI have a magnetic structure compatible
with a current filament elongated along the poloidal direction that
propagates toroidally and radially
(a) conditional average structure from GPI signal
(b) correspondent structures of time derivative of the local magnetic
field fluctuations
continuous lines: experimental measurements; dashed: result of the
current filament model.
[ M. Agostini et al., EFTSOMP 2008 Crete ]
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2713th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Emissivity structure associated to magnetic perturbation at all the range of current explored
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2813th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
…..about the shape of Power Spectrum S(ƒ) (1)
S(ƒ) tails can
switch from exp to
power-law on the
same plasma pulse
Power-law exp
…it depends from the
distribution of burst times ?
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
2913th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
…..about the shape of Power Spectrum S(ƒ) (2)
…with the time scale analysis ones obtain different
distributions, with different width:
• S(ƒ) power-law wide distribution
• S(ƒ) exp narrow distribution
The S(ƒ) of edge turbulence may be interpreted as arising from superposition of signals
with different coherent structures and the time width distribution could depend from the
displacement of magnetic surface (radial component)
τ [μs]
Measurements of characteristic pressure profile lengths and comparison to turbulence scale lengths in RFX-mod edge
3013th IEA/RFP WorkshopOctober 9-11, 2008 Stockholm
Edge behavior of QSH plasma
GPI diagnostic is
used to monitor the
edge plasma
There is a radial
movement of HeI
emission cloud
inward-outward as the
local shape of
dominant toroidal
mode n=7
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