Tornado type closed magnetic trap for an ECR source

Abramova K.B.*, Smirnov A.N. **, Voronin A.V. *, Zorin V.G. **
* Ioffe Physico-Technical Institute, St.-Petersburg, Russia

** Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia

We propose to use a Tornado type closed magnetic trap for creation of a source of mul-ticharged ions with plasma heating by microwave radiation. Plasma loss in closed traps is deter-mined by diffusion across the magnetic field, which increases substantially plasma confinement time as compared to the classical mirror trap [1]. We propose to extract ions with the aid of additional coils which partially destroy the closed structure of the magnetic lines in the trap, but don not influence the total confinement time. This allows for producing a controlled plasma flux that depends on the magnetic field of the additional coil. The Tornado trap also possesses merits such as an opportunity to produce high magnetic fields up to 3 T, which makes possible heating and confinement of plasma with a high density of electrons; plasma stability to magneto-hydrodynamic perturbations because the magnetic field structure corresponds to the "min B" configuration; and relatively low costs.

All estimates and calculations were carried out for the Tornado 322 pulse installation [2]. The trap had the following parameters: maximal magnetic field 2.8 T, trap diameter 350 mm, magnetic field pulse duration (halfwidth) 7 ms, and pulse repetition rate 0.01. We propose a scenario of plasma heating using sequential switching on of two generators at the frequency of 2.45 GHz for producing initial plasma and at the frequency of 30 GHz for its heating [3]. Ion distribution over charge states was calculated for Argon plasmas within the framework of a 0-dimensional nonstationary set of differential equations for ionization balance, which took into account transverse plasma diffusion as well as recombination and charge exchange in the case of a uniform source of neutral gas. It is shown that, by the end of the magnetic field pulse, ion dis-tribution over charge states may reach a maximum at Ar+15 for the plasma density of 1013 cm-3.

To conclude, an ECR source of multicharged ions based on a Tornado type trap is a highly promising and feasible pulse source that may be fabricated in the near future.

[1] G. A. Galechyan and B. P. Peregood, Sov. Phys. Tech. Phys., 14, 1272 (1970); K. B. Abramova et al, Tech. Phys., 42, (2), 134 (1997).
[2] K.V. Abramova et al, Trans. of Fusion Technology, 35, 263, (1999).
[3] A.L. Goldenberg, A.G. Litvak, Physics of Plasmas, 2, 2562 (1995).

Presenting Author : Zorin V.G.
Presentation : Poster
Wednesday Poster Session at 14:45

Formation of multicharged ions at quasi-gasdynamic plasma confinement in a mirror magnetic trap

Golubev S.V. , Razin S.V. , Smirnov A.N. , Vodopyanov A.V. , Zorin V.G.
Institute of Applied Physics, Russian Academy of Sciences, Russia

It was shown in [1] that an increase in plasma density Ne in sources of multicharged ions leads to a substantial increase of ion current and improves slightly the ion distribution over charge states. Validity of this statement was verified in experiments with plasma densities not exceeding several units of 1012 cm-3. It was revealed [2] that, for the electron densities exceeding 1013 cm-3, the regime of plasma confinement in a trap changes significantly, the scaling described in [1] is no longer valid, and the quasi-gasdynamic regime of plasma confinement is realized. The plasma confinement time ti in this regime weakly depends on electron density. Consequently, the parameter governing formation of multicharged ions, Neti , grows as the electron density is increased. This means that an increase in plasma density results not only in an increase in the total ion current but also in the shift of the ion charge state distribution towards higher charge states. The present work concerns experimental investigation of the quasi-gasdynamic regime of confinement of a hot plasma in a direct magnetic trap and formation of multicharged ions in this regime.

Experiments were conducted on the setup described in detail in [3]. Millimeter wave radiation with maximum power W=130 kW, frequency f=37.5 GHz, and pulse duration up to 1.5 ms was focused along magnetic field lines into a simple mirror trap with mirror ratio 3.4, length 25 cm, and maximum magnetic field 2.5 T.

The temperature and density of the electrons were determined from spectral analysis of X-ray bremsstrahlung of plasma in the 2-20 keV range and from the transmission factor of diag-nostic microwave radiation through the plasma. It is concluded that a quasi-gasdynamic regime of plasma confinement is realized.

Ion distribution over charge states in the quasi-gasdynamic regime is calculated and the re-sults obtained are compared with experimental data. A strong effect of anisotropy of the electron distribution function over energies on the efficiency of plasma confinement and formation of multicharged ions is considered. Problems of plasma stability in an axisymmetric mirror trap un-der powerful microwave pumping are addressed.

Possible ways for developing a quasi-gasdynamic ECR source of multicharged ions are analyzed.


Presenting Author : Zorin V.
Presentation : Poster
Wednesday Poster Session at 14:45