8.4 Slow confinement transitions
8.4.3 Comparison between the radial profile of fluctuations
The measurements covered in this subsection have also been published in [133], [14] and [176].
Up to this point we have exclusively presented measurements of density fluctuations line integrated along the measurement volumes. However, as we have described in chapter 3, some localisation is possible with one of two different techniques. In the following we describe results of both methods and thereafter compare the outcome. The density fluctuation profile is in both instances analysed for k⊥ = 15 cm−1, where the fluctuation amplitude increases for degraded confinement.
Dual volume localisation
Figure 8.50 shows crosspower amplitude spectra originating at the bottom (left-hand plot) and top (right-hand plot) of the measurement volumes. In both cases, spectra are shown during good (dotted lines) and bad (solid lines) confinement in static discharges. Our initial observation is that negative and positive frequencies form two separable spectra. The low frequency feature exists for negative frequencies at the bottom and positive frequencies at the top, meaning that it rotates in the ion d.d. direction.
Conversely, a higher frequency feature travelling in the electron d.d.
direction is present. The low frequency ion feature has a larger amplitude than the high frequency electron mode. A strong up-down asymmetry is found for the electron feature by comparing the top and bottom
measurements: The frequencies at the bottom are considerably higher (roughly a factor of two) than at the top, an effect probably due to the difference in flux compression between the bottom and top of the plasma.
There are two major differences between the spectra from good to bad confinement:
1. The amplitude of the ion feature increases, but the frequency does not change.
2. The frequency of the electron feature decreases, and the amplitude increases.
Further measurements of the crosspower amplitude during confinement transitions induced by slow current ramps are presented in subsection 8.4.4 describing the details of the confinement transition.
Bottom of plasma Top of plasma
Frequency [MHz] Frequency [MHz]
-3 3 -3 3
102
10-1
Crosspower amplitude [a.u.]
Figure 8.50: Crosspower amplitude spectra for k⊥ = 15 cm−1. Spectrum at good (dotted lines) and bad (solid lines) confinement in static discharges.
Left: Signal due to fluctuations at the bottom of the plasma, right: Signal due to fluctuations at the top of the plasma.
Single volume localisation
The principles of single volume indirect localisation have been detailed in chapter 3. We made an α-scan (at k⊥ = 15 cm−1) of initially good confined plasmas with a fast current ramp at the end. Six shots were made, where α was set to 12◦ (bottom), 8◦, 6◦, 4◦, 0◦ and -4◦ (top).
Figure 8.51 shows the autopower for the six angles at good (thin lines) and bad (thick lines) confinement. If we begin by commenting on the good confinement spectra, it is clear that for the outermost measurement volumes (upper left and lower right plots), two features are visible: The dominating one in amplitude is at low frequencies (a few hundred kHz) and slopes outward; the other feature at high frequencies has a broader shape and the frequency sign is opposite to that of the low frequency feature. We conclude that the high amplitude, low frequency fluctuations travel in the ion d.d. direction, while the low amplitude, high frequency feature travels in the electron d.d. direction. As one moves further into the plasma both features diminish and vanish almost completely in the core of the plasma.
The limited spatial resolution makes it impossible to determine whether the radial positions of the counterpropagating features coincide or if they are separated.
Turning to the bad confinement spectra, we immediately observe that both the ion and electron feature increases in amplitude. Additionally, the electron feature decreases in frequency, i.e. it spins down. This is most clearly seen at the bottom of the plasma, compare to the dual volume
CHAPTER 8. INVESTIGATED PHENOMENA 174 localised measurements in figure 8.50.
Autopower [a.u.]
5 10
110
-15 10
15 10
110
-110
-1Frequency [MHz] Frequency [MHz]
-3 3 -3 3
Bottom of plasma
Top of plasma
a= 12o a = 8o
a= 6o a= 4o
a= 0o a = -4o
Figure 8.51: Autopower spectra for the α-scan in fast current ramp good confinement discharges. From top left to bottom right: α = 12◦ (bottom), α = 8◦, α = 6◦, α = 4◦, α = 0◦ and α = −4◦ (top). Thin lines are during good confinement, thick lines during bad confinement.
We have just discussed two snapshots of the fluctuations at good and bad confinement. In figure 8.52 we display the transition in a more dynamical fashion: We show contour plots of the low frequency density fluctuations versus spatial position and time. The traces above the contour plots show traces of plasma current, particle fuelling and plasma energy. We again conclude that during the steady-state good confinement phase negative frequency fluctuations dominate at the bottom of the plasmas while positive frequencies dominate at the top, indicating movement in the ion d.d. direction. As the plasma enters the transient bad confinement phase, massive changes are observed in the profile: From being confined mostly to the edge plasma region, the fluctuations extend to the very core.
In figure 8.53 we show a contour plot like those shown in figure 8.52, but now displaying the autopower integrated over all frequencies. The same general behaviour is observed as for the low frequencies; note that the fluctuations at the bottom seem to reach deeper into the plasma than the
[-120,-100] kHz [100,120] kHz
Time [s] Time [s]
0.3 0.7 0.3 0.7
r 1
-1 2 2 15 I [kA]p
e/s Wdia[kJ]
Figure 8.52: Traces of plasma quantities (net currentIp, gas fuelling in units of 1021 and stored energy) and contour plots of density fluctuations in the [-120,-100] kHz (left) and [100,120] kHz (right) frequency bands. The contour plots are shown versus time and plasma position in normalised coordinates:
-1 corresponds to the bottom position of the LCFS, 0 to the center and 1 to the top of the LCFS.
top fluctuations. This is due to systematic errors in the calibration of the angle α, see the analysis below.
Time [s]
0.3 0.7
r 1
-1 2 2 15 I [kA]p
e/s Wdia [kJ]
Figure 8.53: Traces (see figure 8.52) and contour plot of the density fluctua-tions integrated over all frequencies.
∗ ∗ ∗
The final subject treated here is how the radial density fluctuation profile changes at the confinement transition. In chapter 3 we found that the
CHAPTER 8. INVESTIGATED PHENOMENA 176 measured autopower due to scattering in a single measurement volume is proportional to the integral along the measurement volume of the local spectral density multiplied by an instrumental selectivity function, see equation 3.52. Experimentally, both the autopower and the terms in the selectivity factor are known; the local spectral density is assumed to be a function of 3 free parameters (b, cand p), see equations 3.54 and 3.56:
f = δn(r)
n(r) =b+c|ρ|p (8.28)
Making an α-scan as described above, these 3 parameters can be determined by fitting to the measured autopower.
The measurement results are shown in figure 8.54. The curve connected by diamonds in the upper plot is frequency integrated scattered power during good confinement vs. spatial position in normalised coordinates (-1 bottom LCFS, 0 center, 1 top LCFS of plasma). The triangle curve shows the profile during bad confinement. Finally, the asterisk curve in the lower plot shows the bad/good profile ratio. The measurements were averaged over 50 ms, the good confinement data from 300 to 350 ms and the bad confinement ones from 500 to 550 ms. We can make the following statements:
1. The turbulence level is generally low in the central plasma as compared to the edge.
2. The turbulence level increases at all radial positions in going from good to bad confinement.
3. The increase in turbulence is largest in the central plasma.
4. The ratio between bad/good profiles is shifted somewhat with respect to ρ = 0. This indicates that our originalα calibration is somewhat off with respect to the ’real’ calibration.
After modelling the δn/n-profile, we need the density profile to calculate δn2, which is obtained from a fit to the profile measured by Ruby laser Thomson scattering. This fit is shown in figure 8.55 and is a density profile for the good confinement phase. Since the line density was kept constant with gas puffing, this profile is used for both the good and bad confinement profile fit.
The first step of the fit procedure was to re-calibrate α; this was done by making α into a fourth fit parameter and performing the fit. In fitting to all 6 points for both the good and bad confinement data it was found that α increased for both cases, but not by the same amount. Excluding the spatial
Auto-power [a.u.]
3.5
0 8
0 Ratio
-1 r 1
Figure 8.54: Measured turbulence profiles (top) and ratio between them (bottom). The symbols have the following meaning: Diamonds (connected by solid lines) are good confinement, triangles (connected by dotted lines) bad confinement and asterisks the bad/good ratio.
8 10 19
n [m ]e -3
0 -1 r 1
Figure 8.55: Density profile used in the fit procedure. The profile is a fit to Ruby laser Thomson scattering measurements.
point ’pushed out’ of the plasma in the direction indicated by the initial fits - and now only fitting to 5 points - the fitted α shifted by the same size in both cases, namely 1.65◦ ± 0.03◦ (of 16◦ in total, a change of 10 %). The resulting positional change can be observed by comparing figures 8.54 and 8.56. In the fits described below, α was set to the re-calibrated value.
Unfortunately, this means that the bottom point of the profile can no longer be used in the fit since it is outside the plasma. The very limited number of measurement points of course questions the validity of the
following procedure, since we use 5 data points to arrive at 3 fit parameters.
However, this was the only series of similar discharges where we have data using the setup presented, and performing a least squares fit is still the best way forward in the analysis of these discharges.
The result of the fits is shown in figure 8.56. The measured profile is
CHAPTER 8. INVESTIGATED PHENOMENA 178 displayed using the same symbols as in figure 8.54; squares are now the fit to good confinement and crosses fit to bad confinement. The errorbars on the measured data are set to 10 %, see chapter 7. The fitted parameters were: (b, c, p)good = (0.0067, 0.53, 8.0) and (b, c, p)bad = (0.019, 0.57, 6.2), where the subscripts refer to the confinement quality. Since we measure in arbitrary units, it is only the relative values (c/b)good = 79 and (c/b)bad = 30 that are important.
Figure 8.56: Measured and fitted profiles. Left: Good confinement (diamonds measurements, squares fit), right: Bad confinement (triangles measurements, crosses fit). The point beyond the bottom of the plasma is not included in the fits. The errorbars on the measured profiles are calculated assuming a 10
% error.
The relative (δn/n) and absolute (δn2) fluctuation profiles are shown in figure 8.57. Note that the relative profiles are shown on a logarithmic plot to elucidate the core behaviour. The errorbars on the relative profiles are found using the covariance matrix obtained when applying the
Levenberg-Marquardt method for the fit [118]. The error ∆f on the profile function f is given by
(∆f)2 = where the σ2’s are elements of the covariance matrix [8]. Note that it is important to retain the off-diagonal terms since they can act to reduce the error on the fit [18]. We conclude that the relative fluctuation level
increases significantly in the core region of the plasma during degraded confinement. This is also the case for the absolute fluctuations, where the bad confinement profile furthermore develops a ’hump’ somewhat inside the LCFS. The errorbars on the relative profiles show that the increased level of core turbulence during bad confinement is a real effect. Outside half-radius, the profiles are identical within errorbars. The errorbars on the absolute profiles are qualitatively identical to those on the relative profiles and are not shown.
10
110
-30.25
0.0 -1 1
dn/n [a.u.]
r dn [a.u.]
2Figure 8.57: Fitted relative (top) and absolute (bottom) fluctuation profiles.
Solid lines are good confinement, dotted lines are bad confinement profiles.
Note that the data in the top plot are on a logarithmic scale. The errorbars on the relative profiles are calculated using a 10 % error on the measured data. We have left out the errorbars on the absolute profiles for clarity.
The procedure used above is identical to the one used by the ALTAIR team at the Tore Supra tokamak to study differences between L-mode and
reversed shear (RS) discharges [4] [76].
We have seen that the same model can be used in Tore Supra and W7-AS to fit the measured data; however, this is not surprising in our case, since our number of data points is very small compared to the number of fit parameters. Nevertheless, a direct comparison of L-mode parameters stated in [4] and derived above yields:
• (c/b)W7−ASL−mode = 79, pW7−ASL−mode = 8.0
• (c/b)Tore SupraL−mode = 14,pTore SupraL−mode = 8
CHAPTER 8. INVESTIGATED PHENOMENA 180 From the above parameters it is quite difficult to make quantitative
comparative remarks concerning the fluctuation profiles. Unfortunately, due to the installation of divertor modules in W7-AS (which severely limits the optical access), it is no longer possible to extend our measurement database.
A second point of some interest is that both the change from L-mode to RS confinement in Tore Supra and the sub-L to L-mode transition in W7-AS is connected to a strong decrease of density fluctuations in the core plasma. A direct comparison of figure 6 in [4] and our figure 8.57 (top) shows that the transition from bad to good confinement is mainly associated with a
reduction of core turbulence.
Further evidence of the connection between reduced core turbulence (measured using beam emission spectroscopy) and improved plasma performance has been found in the DIII-D tokamak for both internal transport barrier [61] and radiatively improved [105] discharges.
Summary of results from the two localisation techniques The results obtained using the two different methods of localisation fit together nicely. In both cases two separate features are observed: A low frequency (±500 kHz) feature propagating in the ion d.d. direction and a high frequency (±2 MHz) feature propagating in the electron d.d.
direction. Both components decrease in amplitude towards the plasma core.
In the transition from good to bad confinement the electron feature spins down, i.e. its frequency drops, and the amplitude of both features increases.
In the bad confinement state, fluctuations extend to the core and the increase in the derived turbulence profile is significant in the core.