1. SAWTOOTH AND M=1 MODE BEHAVIOUR IN FTU PELLET ENHANCED DISCHARGES
E. Giovannozzi, P. Buratti, D. Frigione, L. Panaccione, O. Tudisco, P. Smeulders and FTU team
Associazione EURATOM-ENEA sulla Fusione, CRE Frascati, C.P Frascati, Italy

2. FTU Pellet injector Top results: Bt_max = 8.2 T R0 = 0.935 m A = 0.3 m
Ip_max = 1.6 MA
Pellet injector
Number of pellets: 8
Pellet mass: (1 1020) md (md deuteron mass)
Pellet speed: ~1.2 km/s
The high pellet speed allows a central fuelling of the plasma.
Top results:
line averaged density up to 4.21020 m-3
neutron yield 1.51013 s-1 at Bt = 8 T and Ip = 1.2 MA

3. Pellet injected plasmas show four different regimes of MHD activity
1. Both sawteeth and m=1 modes are fully stabilized. Hollow temperature profiles and double tearing modes tend to appear in this regime.
2. Sawteeth are replaced by continuous m=1 oscillations, possibly preceded by a quiescent phase.
3. Sawteeth survive pellet injection but their period becomes very long (>20 ms, i.e. about four times the typical pre-pellet value).
4. Sawteeth are essentially unperturbed by pellet injection

4. Examples of regimes 1) and 2) from pulse #12744 (B=7.1T, Ip=0.79MA).
horizontal soft-x emission with impact parameter -2 cm
Temperature outside the sawtooth inversion radius
The first pellet originates type 2 regime, i.e. absent sawteeth and later appearance of m=1 oscillations that are evident in x-ray emission . Type 1 regime is found after the second pellet, with no m=1 oscillations

5. Different sawtooth evolution
in regime 3.
Upper trace: central temperature in shot #18566 (B=8 T, Ip=1.08 MA). The sawtooth period gradually increases after each pellet.
Lower trace: pulse #12821 (B=5.6T, Ip=0.67MA). Sawteeth disappear for 87 ms and then reappear with long period (22.9 ms).

6. After pellet injection we may have a prompt change in sawtooth behaviour and a delayed one.
The delayed change of the sawtooth period is associated with an increase of density gradient that takes place on transport time-scale after pellet injection

7. The sawtooth period just after pellet injection is strongly correlated with the pre-pellet temperature (for plasma current higher than 0.65 MA).
This is a clear evidence that the change in the sawtooth behaviour is correlated with the depth of pellet penetration.
Data from 1997 and deuterium experimental campaigns
BT from 5.3 T to 8 T
Ip from 0.48 MA to 1.57 MA
ne_pre from 0.41020 m-3 to 3.41020 m-3
ne_post from 0.9 1020 m-3 to 4.3 1020 m-3
Te_pre from 1.1 keV to 3.2 keV;
Te_post from 0.4 keV to 2 keV

8. Snakes
At high plasma current soft-X ray oscillation have nearly constant amplitude in the central channels, whereas channels near sawtooth inversion radius show a strong modulation

9. An m=1 mode recognized as a snake, is present in about one third of the discharges. Snakes are characterized by peaks in Soft-X ray emission and temperature dips, as seen on ECE signals. The oscillations have an odd parity. They are well described by a cold bright spot (in Soft-X ray). They may survive sawtooth crashes, and they are located inside the sawtooth inversion radius.

10. Dynamics of long lived snakes
When sawteeth disappear the snake region widens and the central m=1 n=1 mode couples with an m=2 n=1 mode (figure 3). If the m=2 mode is strong enough it may lock to the wall and possibly lead the discharge to disruption.
Sometimes the sawteeth reappear and, in this case, there is no locking to the wall of the m=1 mode.

11. The snake in this discharge becomes visible 75 ms after pellet injection. Sawteeth are completely stabilized. The snake disappears before the small m=2 mode lock to the wall.

12. After pellet injection the m=1 mode rotation speeds up gradually
After pellet injection the m=1 mode rotation speeds up gradually. A similar behaviour is found in the electron diamagnetic frequency
The derivatives are approximated by:

13. First of all, we fitted the SX signals in order to obtain a more smooth profile. We chose a time where the SX signals were stationary. Then we calibrated the SX signals along all the discharge.
A Soft-X tomographic reconstruction is attempted using only the horizontal lines of view. The tomographic reconstruction is based on different line integrated projections taken at different angles of view. The natural rotation of the mode is used to select the different angle projections. This reconstruction assumes that the rotation frequency is constant along a period, and that the rotation is rigid. f1 f2
The horizontal chords, along which the soft-x emission is integrated, are not parallel each other. The spread in angle is about 40°, from -25° to 15°.

14. To solve this problem, the data are corrected for their viewing angle
To solve this problem, the data are corrected for their viewing angle. Each signal is shifted in time by the equivalent of its phase.
Then the signals are fitted along the spatial direction as the used routine* needs a uniform sampling to make a tomographic inversion.
(*) Matlab iradon

15. The snake get closer to the magnetic axis during a sawtooth crash while its rotation frequency slows down.
The original location is readily restored during the recovery phase

16. Tomographic reconstruction just before and after a sawtooth crash.

17. Snake and sawtooth crash
A central part of the discharge is ejected during a sawtooth crash.
The snake gets closer to the magnetic axis after the sawtooth crash.

18. The continuous change in snake behaviour, from a clear cold snake, associated with sawteeth, to a wider kink deformation of the plasma core, and then back is an indication of the continuous presence of an island, the kink like deformation being associated with a wider island.

19. The snake survive a sawtooth crash

21. A Soft-X tomographic reconstruction is attempted using only the horizontal lines of view. The tomographic reconstruction is based on different line integrated projections taken at different angles of view. The natural rotation of the mode is used to select the different angle projections. A phase angle correction is added in order to take into account the different inclination of the viewing lines. This reconstruction assumes that the rotation frequency is constant along a period, and that the rotation is rigid. f1 f2
The horizontal chords, along which the soft-x emission is integrated, are not parallel each other. The spread in angle is about 40°, from -25° to 15°.
To solve this problem, the data are corrected for their viewing angle. Each signal is shifted in time by the equivalent of its phase.