Presentation on theme: "Density profile changes are the result of the profile consistency effect. K.Razumova The very important feature of tokamak plasma behavior is its abilities."— Presentation transcript:
Density profile changes are the result of the profile consistency effect. K.Razumova The very important feature of tokamak plasma behavior is its abilities for self- organization. About 20 years ago it was shown that plasma aspire to form the pressure profile corresponding to the best confinement for given conditions and then try to keep it, when we put any external forces.
Self-consistent profiles in OH and L modes realise for the relative plasma pressure and radius, normalized on any rational magnetic surface a= (q=5 in a given case) Results for different tokamaks: T-10, T-11, TFR, TM-3, PLT, ASDEX, PDX. Yu.V.Esipchuk, K.A.Pazumova, Plasma Phys. and Contr. Fusion, V.28 (1986) p. 1273
In spite of substantial changes in Te and ne profiles during on-axis ECRH and gas puffing, the normalized pressure profile remains to be the same. We can conclude that “pump-out” under on-axis ECRH is the result of the pressure conservation effect.
OFF AXIS + on AXIS ECRH B=2.33T; I p =180kA; off axis ECRH P=0.75MW; on axis P=055MW Density profiles are received from radio- interferometer together with reflectometer At the beginning of off axis ECRH central density increase is seen.
This result is supported by SXR diagnostic Off axis ECRH. Change of line averaged SXR intensity profile at moment t 2 in relation to that at moment t 1 before heating (I SXR (t 2,r)-I SXR (t 1,r)). (no Abel inversion)
We can interpret this data in such a way: I SXR Z eff n 2 (T e ). Just in the heating region the main term is (T e ). But decrease of n e leads to a strong decrease of I SXR in more central regions. As the central T e does not changed during this time we can conclude that the central I SXR increase must be due to the central density increase (Z eff n 2 ).
In spite of strong change of T e profile, the pressure peakedness is permanent within the experimental accuracy
Conclusion Plasma property to keep its pressure profile lead to density decrease in the heating region. So n(r): 1)flatten under on axis heating, that we call the “Pump Out”; 2)has steep gradient in the central zone under the off axis heating.
1.As it was demonstrated theoretically [9-11], the self-consistent profiles are in agreement with the principle of minimum free energy in the plasma. The plasma tries to establish profiles of its parameters, in accordance to the minimum energy principle, which is connected with plasma instabilities (best confinement). The external influences like boundary conditions, heating power deposition profile, and so on, do not permit to stabilize the instabilities completely. The OH process has the least constraints, so it leads to the best confinement, albeit that Ohm’s law is also a limitation of plasma freedom. The shape of the plasma profiles is governed by the pressure profile p(r)=ne(r) T(r), but not by the electron temperature Te(r), or the density profiles ne(r) separately.
We can conclude that the density pump-out is the result of p(r)/p(0) conservation, or (which is the same) p/p conservation This process may take place very rapidly, because it is a loss of equilibrium. In ITB regions, the higher p is permitted since the instabilities that normally restore the pressure profile are suppressed.
. All experimental results may be explained as follows: Rational magnetic surfaces are the cells with a high transport, which is determined by the before mentioned instabilities. When these cells touch each other, the high transport takes place in a wider region. This decreases the local value of the pressure gradient p. When the self-consistent pressure profile is distorted by auxiliary heating, pellets or other means, the distances between cells are increased (or decreased); this changes the transport and keeps the relative pressure profile p(r) practically constant.
If we limit the maximum number mMax near the rational surfaces with low m and n values, a gap with no rational surfaces will be seen, and so the ITBs could be preferably formed there. Decreasing dq/dr, we make better confinement in this ITB region. Wider the zone dq/dr=0, wider the enhanced confinement zone. What does happen, when we try to increase the pressure gradient p somewhere in a plasma? The density of rational magnetic surfaces increases in this region, therefore the transport coefficients increase also.
Confinement will depend not only on the density of rational surfaces, but on the dimension of cells. Of course this explanation needs further experimental checks. Rational surfaces have a number of peculiarities, like Geodesic Acoustic Modes (GAM), supra-thermal electron generation, and so on. One must take into account that ITER will have self-consistent profile
Abel recnstruction of the interferometr results