A neutral beam probing method is proposed for measurement of plasma current profile in a tokamak. An emulsion plate is used to detect the secondary ions. This method is compared with the ion beam probing in which the primary particles are heavy ions. It has been concluded that a neutral beam is superior to an ion beam for the purpose of determining plasma current profile. From numerical orbit calculations, it has been found that this method is applicable to the tokamak NOVA II, with moderate requirements on the neutral beam source

FUKAO, Masayuki

MASAMUNE, Sadao

NISHIHARA, Hiroshi

English

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:On Measurement of Plasma CurrentProfile by Neutral Beam ProbeMASAMUNE, Sadao; FUKAO, Masayuki; NISHIHARA,HiroshiMASAMUNE, Sadao ...[et al]. On Measurement of Plasma Current Profile by Neutral BeamProbe. Memoirs of the Faculty of Engineering, Kyoto University 1980, 42(1): 76-841980-03-31http://hdl.handle.net/2433/28113576 On Measurement of Plasma Current Profile by Neutral Beam Probe By Sadao MASAMUNE, Masayuki FUKAO and Hiroshi NISHIHARA (Received September 29, 1979) Abstract A neutral beam probing method is proposed for measurement of plasma current profile in a tokamak. An emulsion plate is used to detect the secondary ions. This method is compared with the ion beam probing in which the primary parti-cles are heavy ions. It has been concluded that a neutral beam is superior to an ion beam for the purpose of determining plasma current profile. From numerical orbit calculations, it has been found that this method is applicab-le to the tokamak NOVA II, with moderate requirements on the neutral beam source. 1. Introduction Plasma current distribution has a strong influence on the stability as well as on the power balance of tokamak plasma. The distribution cannot be determined by an electromagnetic measurement from the outside of a current channel. Insertion of a probe electrode into the plasma column is not feasible because of its strong disturbance on the plasma stability. Moreover the electrode will melt down in a moment. In such circumstances, considerable efforts have been made to find methods for determining the current profile or the poloidal magnetic field distribution. The representative techniques that have been developed are (i) particle beam probing1>-5>, (ii) detection of harmonic waves with microwave irradiation6• 7>, (iii) measurement of cyclotron splitted spectrum of scattered laser light8H 0>, and (iv) observation of a change in the polarization of electromagnetic wave propagating through the pla-smaw-w. None of them, however, has been established nor can be used routinely. Hickok and Jobes first tried to determine the plasma current distribution in the ST tokamak1> by injecting a singly ionized thallium beam. They were successful m determining the distributions of electron density and space potential, but not m Department of Nuclear Engineering. On Measurement of Plasma Cumnt Profile by Neutral Beam Probe 77 obtaining the current distribution. This method is attractive because of its simplicity and, after adding an energy analyzer, it is applicable to measuring the space poten-tial and plasma density as well. Technical problems are to obtain a very thin probe beam with a sufficient brightness and to make a highly sensitive particle detector with a good spatial resolution. In the present paper, we propose a method of neutral beam probing with an emulsion plate as the ion detector. We have found that an emulsion plate can detect or record individual tracks of a single helium ion of an energy between 20 and 30 keV, and that the high sensitivity and the excellent spatial resolution greatly reduce the requirement of ideal beam quality. Since the emulsion is not influenced by a magnetic field at all, we can place the detector plate close to the plasma column and make our choice of the species and the energy of neutral particles. In the next section, the principle of the method proposed and the model used in orbit calculations are described. In Section 3, we compare the neutral beam probing with the ion beam method. In Section 4, we discuss the position of detector plate and the divergence of injected beam. Conclusive remarks follow in Section 5. 2. Principle of Measurement and Model for Calculation We use the rectangular coordinate system (x, y, z) shown in Fig. I. The center is at the intersection of the toroidal magnetic axis and a vertical cross section of the torus, positive y-direction is vertically upwards, and positive x-direction radially out-wards. The z-axis is chosen to form a right-hand coordinate system. I I -~~', y ·1/ ~1 ', ·g ', -· I z X Fig. 1. Coordinate system (x, y, z). A neutral particle beam for probing is injected into .the plasma along the y-axis in the negative direction (downwards). A fraction of the primary beam particles is ionized in the plasma either by a charge exchange with plasma ions or by collisions with electrons. The secondary ions begin to deviate from the primary beam path (straight line) 78 Sadao MASAMUNE, Masayuki FuKAO and Hiroshi NISHIHARA at their respective points of birth. The individual secondary moves in its curved orbit determined essentially by the strong toroidal magnetic field and the point of ionization. The secondary orbits are approximately in the xy-plane. However, the secondary beam also undergoes a small deflection in the toroidal direction due to the weak poloidal magnetic field (the z-deflection). The intensity distribution of the poloidal field is uniquely determined by the plasma current density distribution. As the points of ionization are continuously distributed along the primary path, the secondary ion orbits form a curved surface. When the detector plate (emulsion plate) is so fixed that it intersects the orbit.al surface, the curved intersection, which will be called the trace hereafter, is determined by the plasma current distribution. Therefore, the characteristics of the trace can be used to identify the plasma current profile. The equation of motion of ions in an electromagnetic field is given by M!~ =q(E+vxB) ( 1) with dr ilt=V, ( 2) where M and q are the mass and charge of the ion, respectively. The magnetic field B consists of a strong toroidal component B, for the confinement of plasma particles, a vertical component B. for keeping balance with the plasma hoop force, and the poloidal component BP due to the plasma current. Since the electrostatic potential inside the plasma column is of the order of the magnitude of the electron temperature T, which is much lower than the beam energy, we can neglect the electric field E of Eq. (l) in orbit calculations. It should be noted, however, that the electrostatic potential is of great importance and can be determined from the difference in the primary and the secondary beam energies. We will not go into this problem here. The vertical magnetic field B. gives approximately the same z-deflection to all of the secondary ions, irrespective of the poloidal field distribution. Thus, it merely results in a translation of the trace. Besides, the vertical component is much smaller than the toroidal. Therefore, the vertical field B. is neglected in the present analysis. Toroidicity in the toroidal field B, is taken into account by the relation B,=B0R I (R+x), where R is the major radius and B 0 the toroidal field intensity at the minor axis, while the toroidicity in the poloidal field Bp is neglected. Assuming a cylindrical symmetry, the following four types of current distributions are considered: On Measurement of Plasma Current Profile by Neutral Beam Probe (H) hollow distribution (U) uniform distribution (PA) parabolic distribution (PE) peaked distribution j,(r) = (2Ip/rra2) (r/a) 2 }. (r) = Ip/rra2 }. (r) = (2/p/rra2) [1- (r /a) 2] j,(r) = (4/p/rra2)[1-(r/a) 2] 3 79 where IP is the total plasma current and a is the minor radius of the plasma column. The orbits of secondary ions are calculated for these distinct current profiles. Calcu-lations are performed assuming typical plasma parameters of the NOV A II tokamak at Kyoto University. The parameters are shown in Table 1. Table I. Typical Parameters of NOVA II major radius (R,) 30cm minor radius (a) 6cm toroidal field (B,) I0kG vertical field (B,) 0.23kG plasma current (Ip) 20kA 3. Comparison between Neutral-Beam and Ion-Beam Probing A neutral beam of 'He0 atoms of 30 keV and ion beam of 40Ar+ ions of 12 keV are comparatively studied as the primary incident beam for plasma probing. The energies are chosen so that the secondary ions 'He+ and 40Ar++ have the same Larmor radius in the toroidal magnetic field. At the same time, this radius is approximately equal to the plasma radius. Projection of the ion orbits on the xy-plane is shown in Fig. 2. The primary argon ions are injected from the outside in an appropriate direction so that they vertically pass the center of plasma. Figure 3 shows the displacement in the toroidal direction (z-deflection) for the secondary ion produced at the center of plasma, because the z-deflection becomes the largest in this case. It should be noted that the ordinate is tenfold in Fig. 3. Clearly, the z-deflection is larger and more sensitive to the current profile with a neutral injection than with an ion injection. The reason is that, in the case of an ion injection, the primary ions undergo z-deflection in the opposite direction. Conse-quently, the primaries' deflection cancels partly the secondaries' displacement. Moreo-ver, the primary ions have to traverse a long path through the strong toroidal magnetic field before reaching the plasma column. Accordingly, for the ion injection technique to be feasible, the Larmor radius of the primary ions in the toroidal field should be so determined that it is much larger than the minor radius. Thus, we are forced to use heavier ions with a higher beam energy. Consequently, the Larmor radius of the secondaries necessarily becomes considerably larger than the radius of the plasma 80 Sadao MASAMUNE, Masayuki FUKAO and Hiroshi NISHIHARA Hi(30keV) lr(12keV) Fig. 2. Representative ion orbits projected on the xy-plane. Projections are determined by the toroidal field B,. column. As the momentum p that a charged particle gains by traversing a path of length L in a magnetic field B is given by the integral ( 3) and is independent of the velocity of particle, a long flight path is required for the secondary ion to get an appreciable z-deflection after overcoming the reverse deflec-tion of the primary ion. Thus, in a practical arrangement of a tokamak device, both the primary and the secondary ion beam paths will become discouragingly compli-cated. In contrast with this, for a neutral injection, it is enough to place the neutra-lizer at the outside of the plasma. Such an arrangement can be made without diffi. culty. 1.0 0.5 E u On Measurement of Plasma Cu"ent Profile by Neutral Beam Probe 30 cm) ...... N -0.5 ~ -1.0 -1.5 -2.0 Fig. 3. Comparison of displacement in the toroidal direction for 4He0(30 keV) atom and •0Ar+(l2 keV) ion incidence. Four types of plasma current profiles (H), (U), (PA) and (PE) are assumed. 4. Detecting Plane and Beam Divergence 81 Hereafter, we concentrate our attention on the neutral beam probing techniques. Figures 4(a) through 4(c) show the traces of the secondary ions arriving at the detecting planes of the following arrangements: (a) the detector plate is fixed on the plane x=6. 5cm and sensitive to the second-aries in the positive x-direction (left to right), (b) the plate is on the same plane, but sensitive to the secondaries in the negative x-direction (right to left), and (c) the detector is on the horizontal plane y= l. 5cm and sensitive to the second-aries in the positive y-direction (upwards). Each of the figures shows four traces corresponding to the four distinct profiles of the plasma current (H) through (PE) described in section 2. The numericals on the curves show the points of birth of the secondary ions along the initial beam path. The largest z-displacement as well as the largest diffe-rence due to the current profiles appears for the ions born at the center of plasma. The secondary ion born at the point (0, -6, 0) where the primary beam exits the plasma column is entirely insensitive to the current distribution, and its orbit depends only upon the total plasma current. Therefore, its point of detection is fixed on the 82 Sadao MASAMUNE, Masayuki FuKAO and Hiroshi NISHIHARA -10 -~o -2.0 toroidal cfisplacement z (cm) z (cm) -1.0 (0,6.0) -1.0 )( 13 12 10 8 5 6 >, 4 -4 -6 -8 (0,-6.0) -10 Fig. 4. Traces of secondary ions for four different current profiles (H) through (PE) at three different positions of detection (a), (b) and (c). Primary beam is of 4He0 atoms (30 keV). Decector positions are (a) x=6. 5cm and ions hit the plate on the plasma-side, (b) x=6. 5cm but ions hit the oposite side, and (c)y= 1. 5cm. Bars show beam spread brought about by the incident beam divergence of ±5 mrad in z-direction and ±20 mrad in y-direction. detecting plane, irrespective of the current profiles. That is to say, all the traces pass this fixed point. Accordingly, the orbit calculations were started from this point and traced back to the point of ionization. Requirements on the beam quality are of essential importance in this probing method. By the action of the magnetic lens, which the toroidal field makes up in the xy-plane, divergence of beam due to imperfect collimation is suppressed to a On Measurement of Plasma Current Profile by Neutral Beam Probe 83 certain extent. The bars shown in Fig. 4 represent the spread of traces on the detecting plane brought about by the initial divergence vz= ±vy/50 at the point of beam incidence (0, 7. 5, 0). The figure shows that the focussing effect cannot be expected in the toroidal direction. Consequently, the spread of traces in this direc-tion is equal to the length of the total flight path multiplied by the initial angle of beam divergence. There is a suspicion that this large spread limits the applicability of this method. A high temperature plasma emits fast neutral particles bring background on the detector plate. not directly face the plasma. In the cases of (b) should be shielded from background sources. 5. Conclusion particles spontaneously. These Therefore, the detector should and (c) in Fig. 4, the detector A method of neutral beam probing is proposed for determining the profile of plasma current in a tokamak. A neutral beam is superior for this purpose to a heavy ion beam, because the former provides us with a higher sensitivity to the current profile than does the latter. Installation of the equipment is easier with a straight injection of the neutrals rather than with a curved injection of ions. We can choose an optimum beam energy depending upon the plasma parameters, and this involves a mere change of detector position. In order to attain an angle of divergence less than 1/500, it is enough to use a beam collimator consisting of two screens with an ape-rture of 1mm diameter and separated by a distance of 50cm. If a neutral beam with an intensity of 1 nA and a diameter of 1mm, both of the conditions being easily realized, 3 to 5% of the neutrals injected will be ionized in NOV A II plasma and 2 X 108 ions will reach the detector per second. Since the detector of the emulsion plate type is not time-resolving, the incident beam has to be pulsed. With a single pulse of l ms width, 2X 105 ions hit the detector. As the secondary ions are distributed along the trace on the plate almost uniformly, we can fix a trace by only one shot. Numerical orbit calculations with the NOVA II parameters show that discrimination of the uniform distribution (U) from the parabolic distribution (PA) is possible by comparing the traces on the detector plate. References 1) R. L. Hickok and F. C. Jobes, AFOSR TR-72-0018. 2) J. C. Hosea, F. C. Jobes, R. L. Hickok, and A. N. Dellis, Phys. Rev. Lett. 30 (1973) 839. 3) J. Fujita and K. McCormick, Proc. 6th Europ. Conf. on Controlled Fusion and Plasma Physics, Moscow 1 (1973) 141. 4) K. McCormick, M. Kick, and J. Olivain, Proc. 8th Europ. Conf. on Controlled Fusion and 84 Sadao MASAMUNE, Masayuki FUKAO and Hiroshi NISHIHARA Plasma Physics, Prague 1 (1977) 140. 5) R. J. Goldston, Phys. Fluids 21 (1978) 2346. 6) R. Cano, C. Etievant, and J. C. Hosea, Phys. Rev. Lett. 29 (1972) 1302. 7) R. Cano, I Fidone, and J. C. Hosea, Phys. Fluids 18 (1975) 1183. 8) F. W. Perkins, MATT-818 (1970). 9) G. Lehner, Z. Physik 232 (1970) 405. 10) J. Sheffield, Plasma Phys. 14 (1972) 385. 11) F. DeMarco and S. E. Segre, Plasma Phys. 14 (1972) 245. 12) A. D. Craig, ibid. 18 (1976) 777. 13) W. Kuntz and G. Dode!, ibid. 20 (1978) 171. 14) S. E. Segre, ibid. 20 (1978) 295. 

On Measurement of Plasma Current Profile by Neutral Beam Probe

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On Measurement of Plasma Current Profile by Neutral Beam Probe

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