Overview of FTU results

Abstract

New FTU ohmic discharges with a liquid lithium limiter at I P = 0.7–0.75 MA, B T = 7 T and n e0 ⩾ 5 × 1020 m−3 confirm the spontaneous transition to an enhanced confinement regime, 1.3–1.4 times ITER-97-L, when the density peaking factor is above a threshold value of 1.7–1.8. The improved confinement derives from a reduction of electron thermal conductivity (χe) as density increases, while ion thermal conductivity (χi) remains close to neoclassical values. Linear microstability reveals the importance of lithium in triggering a turbulent inward flux for electrons and deuterium by changing the growth rates and phase of the ion-driven turbulence, while lithium flux is always directed outwards. A particle diffusion coefficient, D ∼ 0.07 m2 s−1, and an inward pinch velocity, V ∼ 0.27 m s−1, in qualitative agreement with Bohm–gyro-Bohm predictions are inferred in pellet fuelled lithized discharges. Radio frequency heated plasmas benefit from cleaner plasmas with edge optimized conditions. Lower hybrid waves penetration and current drive effects are clearly demonstrated at and above ITER densities thanks to a good control of edge parameters obtained by plasma operations with the external poloidal limiter, lithized walls and pellet fuelling. The electron cyclotron (EC) heating system is extensively exploited in FTU for contributing to ITER-relevant issues such as MHD control: sawtooth crash is actively controlled and density limit disruptions are avoided by central and off-axis deposition of 0.3 MW of EC power at 140 GHz. Fourier analysis shows that the density drop and the temperature rise, stimulated by modulated EC power in low collisionality plasmas are synchronous, implying that the heating method is the common cause of both the electron heating and the density drop. Perpendicularly injected electron cyclotron resonance heating is demonstrated to be more efficient than the obliquely injected one, reducing the minimum electric field required at breakdown by a factor of 3. Theoretical activity further develops the model to interpret high-frequency fishbones on FTU and other experiments as well as to characterize beta-induced Alfvén eigenmodes induced by magnetic islands in ohmic discharges. The theoretical framework of the general fishbone-like dispersion relation is used for implementing an extended version of the HMGC hybrid MHD gyrokinetic code. The upgraded version of HMGC will be able to handle fully compressible non-linear gyrokinetic equations and 3D MHD.

First results of the use of a continuously flowing lithium limiter in high performance discharges in the EAST device

“…In addition, enhanced H-mode pedestals were obtained with Li injection in DIII-D [16]. Also, low recycling, reduced impurity level, and improved confinement were achieved in CDX-U [17], FTU [18], NSTX [19] and T-11 M [20] with Li limiters of different designs. CDX-U in particular showed a four-fold confinement enhancement with liquid Li PFCs, while LTX (Lithium Tokamak eXperiment) has shown up to a ten-fold increase, including energy confinement as high as 3-4 times H-mode scaling [20].…”

“…CDX-U in particular showed a four-fold confinement enhancement with liquid Li PFCs, while LTX (Lithium Tokamak eXperiment) has shown up to a ten-fold increase, including energy confinement as high as 3-4 times H-mode scaling [20]. Finally, a Li limiter with capillaryporous system (CPS) structure successfully suppressed liquid droplet ejection by disruptions, in FTU [18], NSTX [19] and T-11 M [20]. Additionally, high performance discharges obtained in LTX with liquid lithium walls suggests the importance of 'fresh' surfaces when liquid lithium is used [21].…”

Plasma performance improvement with favourable B t relative to unfavourable B t in RF-heated H-mode plasmas in EAST

“…Considering the significance of energy confinement for tokamak operation, many methods have been developed to improve the tokamak plasma confinement such as lithium conditioning [3][4][5][6], internal transport barrier [7], using favourable toroidal field B t with ion ∇B drift towards the primary X-point [8] and so on.…”

Density fluctuations as an intrinsic mechanism of pressure profile formation

“…Experimental plasma parameter profiles were compared with profiles obtained from simulation by the canonical profiles transport model (CPTM) [10]. The problem of plasma density evolution modelling is discussed in many articles by Hoang [11], Angioni [12], Zabolotsky [13], Tuccillo [14], and others, but the reliable results were not obtained.…”