P.C. de Vries JET-EFDA Culham Science Centre Abingdon OX14 3DB UK EX8/3 22nd IAEA Fusion Energy Conference Geneva
P.C. de Vries1, E. Joffrin2,3, M. Brix1, C.D. Challis1, K. Crombé4, B. Esposito5, N.C. Hawkes1, C. Giroud1, J. Hobirk6, J. Lönnroth7, P. Mantica8, T. Tala9 and JET-EFDA Contributors to the Work Programme* JET-EFDA Culham Science Centre, Abingdon, OX14 3DB, UK. 1) EURATOM/UKAEA Association, Culham Science Centre, Abingdon, OX14 3DB, UK. 2) JET-EFDA-CSU, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK. 3) Association Euratom-CEA, Cadarache, F-13108, France. 4) Department of Applied Physics, Ghent University, Rozier 44, 9000 Gent Belgium 5) Associazione Euratom-ENEA sulla Fusion, Frascati, Italy 6) Max-Planck-Institut für Plasmaphysik, Euraton Association, 85748, Garching, Germany. 7) Association Euratom-Tekes, Helsinki University of Technology, PO Box 4100, Finland. 8) Istituto di Fisica del Plasma, EURATOM/ENEA-CNR Association, Milano, Italy. EX8/3 Internal transport barrier dynamics with plasma in JET P.C. de Vries 9)rotation Association Euratom-Tekes, VTT, PO Box 1000, 02044 VTT, Finland.
Internal transport barriers (ITBs) have been observed in many devices and are considered as a candidate for enhancing the confinement in the Advanced Tokamak scenario. Various physical mechanisms are thought to enable the formation of transport barriers in plasmas by causing a local suppression of turbulence. Two factors play a crucial role: magnetic and rotational shear
At JET the dynamics of transport barriers has been explored by trying to decouple the effects of heating and torque. This has been achieved in experiments in which the plasma rotation profile was modified by tuning the toroidal field ripple. And furthermore by increasing the fraction of ICRH power in order to form ITBs in plasmas with a very low torque input. The impact of rotational shear on the formation/triggering and the growth of internal ion transport barriers have been investigated.
In these experiments ion ITBs are formed by creating a particular current density and safety factor, q, profile. The q-profile is measured by the MSE diagnostic backed-up by 1 MHD modes and Cascades of Alfvén modes. m/n=5/2 2/1 GC GC
An inner and outer ITB forms just before an integer q surface appears in the plasma Integer q surfaces have been show to play an important role1,2,3 A Cascades of Alfvén modes follows soon afterwards The outer ITB grows until the heating is stepped down or ρ * T ρl T LT 1 GC
JET has the unique capability to increase the toroidal field ripple by independently charging its odd and even number TF coils. It has been shown that increasing the TF ripple has a pronounced effect on the toroidal rotation and its profile1. ITB experiments have been carried out, tuning the TF ripple from its standard value, δbt=0.08% to 1% while keeping the total absorbed power constant2. 1
The JET NBI system is directed in co-current direction Increasing the TF ripple in this scenario caused: Fast ion losses return current in the bulk plasma j B torque1 A reduced the co-current rotation and modified profile shape, Counter-current rotation was seen in the outer part of the plasma. 1
Increasing the TF ripple amplitude: has a detrimental effect on the growth of the ITB. Nevertheless, an ITB triggering event is still visible. GC
A TF ripple scan at constant absorbed power showed: A degradation of the performance of the ITB
Experiments on ion ITBs were furthermore carried out in low torque plasmas with a large ICRH fraction 1 : ICRH coupled to minority H or D usually heats the electrons. In order to the reduce the torque without affecting the ion heat flux, an ITB scenario that uses ICRH coupling to 3 He minority was developed, which heats the ions. The 3 He concentration was kept at 8% in these plasmas ITB scenario with negative central magnetic shear Coupling of up to 7MW ICRH power was achieved, Combined with a small amount of NBI power (Torque~3Nm). 1 th
Even in plasmas with as little as 3Nm toroidal torque ITB triggering events have been observed. However the ITB strength in these plasmas remained very weak compared with similar discharges with dominant NBI T
Strong ITBs are formed with: dv /dr~4-5 10 s Predominant NBI heating and low TF ripple
The rotational shear or shearing rate ωexb has been calculated under the assumption of neo-classical poloidal rotation. ω ExB RBθ E r = B r RBθ 1 P E r = vφ Bθ vθ Bφ + Zne r
The rotational shear or shearing rate ωexb has been calculated under the assumption of neo-classical poloidal rotation. At the time the transport barrier forms/triggers: for high TF ripple or a larger ICRH fractions: ωexb~1-2 104 [s-1] almost one order of magnitude below the ITG growth rate γitg for low TF ripple and high NBI fractions: ωexb~6 104 [s-1] of the order of ITG growth rate γitg The triggering of ion ITBs in JET are usually not predicted from theory based transport models1,2 1
A TF ripple scan at constant absorbed power showed: A degradation of the performance of the ITB The coincides with a reduction in ωexb with TF ripple At the time of triggering / assuming neo-classical poloidal rotation
The ITB will enhance the gradient in toroidal rotation Thus the ITB itself may be able to push up ωexb/γitg. As long as this ratio is high enough at the time of triggering During growth phase Before triggering
In the presence of strong ITBs, poloidal rotation 20x the neoclassical level has been measured1,2. The enhanced poloidal rotation has a large impact on the rotational shear 5 fold of the rotational shear2 [1] CROMBÉ, K., et al., Phys. Rev. Letters 95 (2005) 155003. th
These experiments focused on ion ITBs triggered in plasmas with a reversed central shear. ITBs are triggered even in plasmas with a very low torque and negligible rotational shear. which indicates that the overall rotational shear is not the dominant factor in this triggering mechanism. However, the growth of the ITB was limited if insufficient torque or rotational shear was available when the barrier was triggered. What is the role of poloidal rotation? A better understanding of ITB physics may help us with the development of Advanced Tokamak scenarios.