Observation of slowing-down of ion-beam velocity and heating of ions

OBSERVATION

18 December 1972

PHYSICS LETTERS

Volume 42A, number 4

OF SLOWING-DOWN

OF ION-BEAM

VELOCITY

AND HEATING

OF IONS

T. FUJITA, T. OHNUMA and S. ADACHI Department of Electrical Engineering,

Tohoku University, Sendai, Japan

Received 14 November 1972

In the measurements of spatial evolutions of ion beams in an ion beam-plasma system, the slowing-down of the ion-beam velocity and the thermalization of the ion beams have been observed

In ion beam-plasma systems, many theoretical and numerical investigations on the instabilities [l-3] and the relaxations of ion beams [4,5] have been reported. Also, some experiments on these problems have been performed [6-81. In this letter, we report the experimental results on the spatial evolution of ion beams when ion beams are injected into plasmas. The experiments are conducted in the same device previously used for investigating oscillations in the ion beam-plasma system [6]. Two non-magnetized plasmas produced by d.c. discharge are separated by a negatively-biased mesh. The application of voltage between two anodes results in injection of ion beams into plasmas. Argon gas is used in the neutral gas-pressure of 5 X low4 torr. The diameters of plasmas and ion beams are 17 cm and 5 cm, respectively. The plasma density is n = 2-5 X 1O8 cme3. The electron and ion temperatures are T, z 4-7 eV and T, x = 0.4-0.8 eV, respectively. The velocity of ion beams is changed from 3 V to 100 V. A Faraday cup, which is movable along the plasma column, is used for the energy analyzer of ion components. The spatial evolution of a double humped ion distribution function is shown in fig. 1(b) with the distance from the ion-beam source as a parameter. The left and right humps are the plasma ions and the ion beams, respectively. In the distribution function, the interval between the peaks of two humps corresponds to the ion-beam velocity vb(V). In fig. l(b), where T/b0 is the initial velocity of ion beams, the ion-beam velocity is shown to slow down gradually along the plasma column. Fig. 1(a) shows the slowing-down of the ion-beam velocity, when the initial velocity I&, is changed. The slowing-down in lower ion-beam velocity is observed stronger than that in higher ion-beam

Fig.

l-(a)

I

01

h

5

8

11

’

2v

2 (cm)

Fig. 1. The spatial evolutions of ion beams. Fig. 1 (b) shows the spatial evolution of ion beams with the initial velocity PbO = 10 V. Fig. 1 (a) shows the spatial changes of the ionbeam velocity Pb(V) and the thermal energy AVb(V), which is the half-width of the distribution function of ion beams. The symbols of VW, A VW and Z are the initial ion-beam velocity, the initial thermal energy of ion beams and the distance from the ion-beam source, respectively. In fig. 1 (a), the VahIeSOf VW - (A vb - A VbO) are plotted instead Of those Of AVb.

velocity. Fig. 1(b) shows, on the other hand, that the thermal energy of ion beams AI’,(V), which is the half-width of the distribution function of ion beams, increases as the Faraday cup becomes far from the ion-beam source. When the half-width of the 319

Volume 42A, number 4

distribution function is separated into the lower (A VL) and the higher (A VI,) energy sides by its peak. the value of AvL is found to be about twice as much as that of AvH. The half-width which is the sum of AIJL and AP’H is called the thermal energy of ion beams in this letter. As shown in fig. I (a), where the symbol of -x-x- is the difference between the initial velocity vbO and the increment of the thermal cnergy AV, from the initial thermal ener_gy AVbo_ i.e., vb() ~~(A vbPA vbO), the thermalizing effect of ion beams with lower velocity is observed stronger than that with higher one?. Fig. t (a) shows that the decrease of ion-beam velocity approximately corresponds to the increase of the thermal energy of ion beams. That is, the most (~60~ 100%) of the kinetic energy lost by the slowing-down of ion beams is transfered to the thermal energy of ion beams. In conclusion, the slowing-down of the ion-beam velocity and the heating of ion beams are observed when ion beams are injected into plasmas. The experimental results show that their effects in lower ionbeam velocity are stronger than those in higher one. Furthermore, the ~6&100% of the kinetic energq t In the preparation

of this letter, we had a chance to read the report by Taylor et al. [ 91. They show the heating of ion beams, which is similar to (but, not the same as) our rcsuits, under different experimental conditions.

320

IX December

PHYSICS LETTERS

I972

lost by the slowing-down of Ion-beam velocity is found to be transfered to the thermal energy of ion beams. Furthermore experimental and theoretical investigations on the mechanism of the slowing-down of the ion-beam velocity are considered presently. in which also the relations of the observed oscillations to the thermalizatio~~ of ion beams are taken into accollIl

t.

The authors would like to thank Professor \i. Hatta for his encoul-agement.

III T. Ohnuma

and Y. Iiarta. .I. I’hyy. Sot. Japan 21 (I 966) 986. PI B.D. Fried and A.Y. Wang, Phys. Fluids 9 (1966) 1084. [31 D.W. Forslund and C.R. Shonk, Phys. Rev. Lett. 25 (1970) 281. I41 Y. Itikawa

and 0. Aono,

Phys. Fluids

9 (1966)

1259.

I51 I:.H. Ree and R.E. Kidder, Phys. Fluids 6 (1963) 857. [61 T. Ohnuma, T. Fujita and Y. Hatta, Phys. Lett. 36A (1971)

265.

171 I.L. Klavan et al., Phys. Rev. Lett. 28 (1972) 1254. IS1 A.G. Borishcnko, G.S. Kirichenko and B.G. Fmaryk, The 4th Intern. Conf. on Plasma physics and controlled nuclear fusion (1971) IAEA/CN-28/E] 191 R.J. Taylor and F.V. Coroniti, Phys. Rev. Letters (1972) 34.

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