مقاله A special survey on Plasma Diagnosticin Tokamaks by electro magnetic waves word
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A special survey on Plasma Diagnostic in tokamaks by electromagnetic waves1 This work is financially supported by I.A.U.Mashad: under contract no :820 66/301/2
2 school board scientific member of I.A.U.Mashad
Emami Zahra2
Abstract
In this paper through using mirnov oscillations by mirnov coils the magnetic bursts out side the plasma media in tokamaks, and during current rise have been taken to in account. The resultant data for Damavand tokamak (AEOI) and IR-T1 tokamak (PPRC) compared, and a classification of the cause and effect mechanisms introduced.
1. Introduction
Now a day the most promising method of supplying the energy is thermonuclear fusion in which the nuclear rearrangement results in a reduction of of total mass and a conseqment release of energy in the form of kinetic energy of reaction products. Then the thermonuclear reaction is the driving force behind plasma researchs, which deals with the problems like temperature, density, confinement and so on. Obviously the plasma parameters are well different with those of familiar states of matter. Any how, during only a few past decades, a great deals of research has been under taken in plasma physics, so that plasma diagnostics plays the key role to extract a thorough knowledge of plasma ready to go under fusion.
In this paper the shown figure 1 is magnetic measurement made by sensing magnetic fields directly in various places inside and or outside the plasma, using special coils and probes of various type.
Figure 1: oscillation of poloidal magnetic field measured outside the plasma (JET) [1]
During the current rise it is often found that bursts of magnetic oscillation occur. Following the discovery of this activity using magnetic coils around the plasma surface, magnetic oscillation, and so-called mirnov oscillation come into consideration [2]. In other hand the causative mechanism for magnetic fluctuation in to kamaks may be due to current gradients tend to the strongest instabilities which could be described by MHD model of plasma. The shown figure 2 is an example of magnetic fluctuation found to occur during current rise.
Figure 2: Development of magnetic fluctuations during current rise, together with their spatial dependence. [2]
2
In the case of circular cross-section large aspect-ratio tokamak (R/a=3-4) the modes take the form exp i (m-n) with m and n being poloidal and toroidal mode numbers. The azimuthal variation of the magnetic field allowed them to be identified with a decreasing sequence of m number. It seems likely that these modes are tearing modes having resonant surfaces q=m close to the surface of the plasma [3], [1].
The plasma emits soft X-rays with intensity dependent on the electron temperature and density, which at sufficient large current; this emission goes under oscillation with saw tooth time dependence. Although these instabilities do not prevent of satisfactory operation of the tokamak , the resulted disruption can in volve a sudden loss of confinement and a rapid decay of the whole current leading to an end of the discharge .The disruption instability usually involves the growth of an m=2 mode to a larges amplitude . The tearing modes in their non linear form tend to magnetic topology in with the plasma with simple nesting of metric axisym surfaces broken to so-called magnetic islands [1]. The islands interactions with different modes prevent the field lines map out a surface, but instead follow a space filling trajectory called erjodic , all affecting enhanced mode of transport.The table 1summarized some plasma parameters in typical tokamak, and the temporal rising of instability during a precursor phase of a disruption is shown in figure 3.
Table 1.Typical tokamak plasma [4]
Plasma volume 1-100 m3
Total plasma mass 10-4-10-2 gr
Ion concentration 1019-1020 m3
Temperature 1-40kev
Pressure 0.1-5 AT
Ion thermal vetecity 100-1000kms-1
Electron thermal velocity 0.01c-0.1c
Magnetic field 1-10T
Total plasma current 0.1-7MA
Fig 3: The evolution of MHD instability in a disruption, with expanded sample. The bottom trace gives the time deviation of Br signal [5].
2. Experimental approach
The data presented have related to the following sources.