Journal of Nuclear and Particle Physics

p-ISSN: 2167-6895    e-ISSN: 2167-6909

2017;  7(1): 1-5

doi:10.5923/j.jnpp.20170701.01

 

Current Independent Approximation of Tokamak Plasma Asymmetry Factor

A. Salar Elahi, M. Ghoranneviss

Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran

Correspondence to: A. Salar Elahi, Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran.

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Abstract

In this contribution we have presented a current independent approximation of tokamak plasma asymmetry factor based on magnetic probes measurement in IR-T1 tokamak. The main advantage of this technique is that it based only on the one diagnostic (only magnetic probes and not need to plasma current measurement). Based on this method, four magnetic pickup coils were designed, constructed, and installed on outer surface of the IR-T1 tokamak chamber and then plasma asymmetry factor was measured from them. Also the result of this technique was compared with other technique (flux loops technique) and found in agreement with each other.

Keywords: Tokamak, Plasma Asymmetry Factor, Magnetic Probes

Cite this paper: A. Salar Elahi, M. Ghoranneviss, Current Independent Approximation of Tokamak Plasma Asymmetry Factor, Journal of Nuclear and Particle Physics, Vol. 7 No. 1, 2017, pp. 1-5. doi: 10.5923/j.jnpp.20170701.01.

1. Introduction

The tokamaks are the most promising candidate for a viable commercial fusion reactor. Before this very promising energy source can be harnessed, many scientific and technological problems must first be resolved. One of the principal problems facing magnetic confinement schemes such as the tokamak configuration is the issue of stably maintaining the plasma column within the discharge chamber. In order to accomplish this, a suitably shaped magnetic field structure must be produced.
The tokamak plasma is subjected to several forces in the major radial direction that must be dynamically counterbalanced by an appropriate magnetic Lorentz force in order to maintain plasma equilibrium in the horizontal direction [1-20]. In general, in the low beta tokamaks, radial pressure balance is achieved by the poloidal field, and also toroidal force balance is achieved by interaction of the external vertical field with toroidal current (when inward Lorentz force equal with sum of the three outward forces (hoop force, tire tube force, and 1/R force) due to the toroidal configuration of the tokamak). But, in toroidal force balance problem, the two opposite forces may not be equal, and therefore plasma intend to shift inward or outward, which is a very dangerous for tokamak plasma equilibrium. Therefore, plasma equilibrium study is one of the fundamental problems of the magnetically confined plasmas. Measurement of the tokamak plasma asymmetry factor (Λ) is essential for tokamak experiments. Very of plasma information can be deduced from this parameter, such as the plasma equilibrium state, plasma energy, plasma confinement time, plasma toroidal current profile, and Magnetohydrodynamics (MHD) instabilities [21-50]. In this contribution we presented a current independent approximation of tokamak plasma asymmetry factor only based on magnetic probes measurement in IR-T1 tokamak, which is a small, air core, low beta and large aspect ratio tokamak with a circular cross section, (see Table 1). This technique based on magnetic probes measurement for determination of plasma asymmetry factor will be discussed in section 2. In order to compare of the result, other method is also experimented. Experimental results and comparison between them will be presented in section 3. Summary and conclusion are also will be discussed in section 4.

2. Current Independent Approximation of Tokamak Plasma Asymmetry Factor based on Magnetic Probes Measurement

Because of dependence of the plasma position and plasma current distribution to the magnetic fields distributions around the plasma, therefore magnetic pickup coils give us information about the plasma equilibrium parameters such as the plasma position and plasma asymmetry factor. Poloidal and normal magnetic fields distributions around the plasma are [1, 2]:
(1)
(2)
where are the plasma radius, chamber minor and major radiuses, free space permeability, and plasma current, respectively, and where:
(3)
Therefore, by rearranging the above equations, a current independent relation for the plasma asymmetry factor obtained:
(4)
where we used the quasi-cylindrical coordinates . Indeed, equations (1) and (2) accurate for the low , large aspect ratio and circular cross section tokamaks as IR-T1.
Based on this technique, in the IR-T1 tokamak four magnetic probes were designed, constructed and installed. Two magnetic probes were located on the circular contour of the radius in angles of and to detect the tangential component of the magnetic field and two magnetic probes are also located above, , and below, , to detect the normal component of the magnetic field , as shown in the Fig. (1). The magnetic fields measured by the magnetic probes consists of the desired magnetic fields produced by the plasma current as well as unwanted magnetic fields such as that produced by the toroidal field coils. This happens primarily as a result of misalignments of the probes. In order to eliminate, or at least to reduce, these stray magnetic fields, the waveforms to which they correspond are added with suitable polarity to the measured signals via an adjustable gain passive mixer. To accomplish this, the gains are adjusted in the absence of the plasma, while all other fields are present, until the coil signals are zero or as close to zero as possible. After compensation and integration of magnetic probes output, and by substituting the poloidal and normal components of the magnetic fields in Equation (4), plasma asymmetry factor was determined. Experimental results will be presented in the next section.
Figure (1). Positions of the four magnetic probes on outer surface of the IR-T1 tokamak chamber

3. Experimental Result and Comparison with Other Technique Result

In order to determination of the IR-T1 tokamak plasma asymmetry factor using this technique, only we needed to determination of the magnetic fields distributions around the plasma. Therefore we designed, and constructed a four magnetic pickup coils, and installed them on outer surface of the IR-T1 chamber. Then, we measured the asymmetry factor by substitution of output of magnetic probes after compensation and integration into Eq. (4).
The result of this technique and comparison with other method (flux loops technique) are presented in the Fig. (2). These figures show that the results of two techniques are in good agreement with each other. The acceptable differences between them are because of (1) approximation in measurements of magnetic fields distribution around the plasma because of discrete probes measurements and (2) large aspect ratio approximation, and (3) possible error in compensation of excessive magnetic flux.
Table 1. The range of plasma parameters of the IR-T1 Tokamak
     
Figure (2). Time evolution of (a) Plasma current, (b) differences of poloidal magnetic fields at high field side and low field side of tokamak, (c) IR-T1 tokamak plasma asymmetry factor which determined by the flux loop technique, and (d) plasma asymmetry factor which measured using the current independent method

4. Summary and Conclusions

We presented a current independent approximation of tokamak plasma asymmetry factor based on magnetic probes measurement in IR-T1 tokamak. The main advantage of this technique is that it based only on the one diagnostic (only magnetic probes and not need to plasma current measurement). Based on this method, four magnetic pickup coils were designed, constructed, and installed on outer surface of the IR-T1 tokamak chamber and then plasma asymmetry factor was measured from them. Also the result of this technique was compared with flux loops technique and found in agreement with each other. The acceptable differences between them are because of (1) approximation in measurements of magnetic fields distribution around the plasma because of discrete probes measurements and (2) large aspect ratio approximation, and (3) possible error in compensation of excessive magnetic flux.

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