Youichi Sakawa

Plasma production by helicon waves

High density plasma production using m=+1 and m=-1 helicon waves is studied. Characteristics of cylindrical helicon waves including effects of a vacuum gap between the plasma and the conducting wall and of a non-uniform density profile are presented. The m=+1 and m=-1 helicon modes are separately excited by a helical antenna, and the dependences of plasma density and antenna loading resistance on RF power are different for these modes.

Production of Inductively-Coupled Large-Diameter Plasmas with Internal Antenna

Large-diameter RF plasmas at 13.56 MHz have been produced by inductive coupling of an internal-type antenna in a discharge chamber of 400 mm diameter and 200 mm height. For minimization of the electrostatic coupling, (i) a double half-loop antenna of 360 mm diameter was employed for the reduction of antenna inductance and (ii) the antenna conductor was terminated by the insertion of a blocking capacitor to satisfy the impedance balance condition where the RF-voltage amplitude could be minimized to 1/2 the terminal voltage. The plasma source could be operated stably at RF input powers of up to 3 kW to attain plasma densities as high as 1012 cm-3 at Ar pressures around 1 Pa. Furthermore, (iii) full covering of the antenna conductor with an insulator tubing exhibited a significant effect on the suppression of the electrostatic coupling with simultaneous achievement of plasma densities as high as 5×1011 cm-3.

Contribution of slow waves on production of high-density plasmas by m=0 helicon waves

The contribution of the slow waves (SWs) on high-density plasma production by m=0 helicon waves is investigated in H2, D2, He, N2, Ne, Ar, and Xe plasmas. The density-jump is observed in all the gases used at optimum external magnetic fields B0. The measured plasma density np as a function of B0 peaks at a condition close to the lower-hybrid resonance in H2, D2, He, N2, and Ne, whereas, the measured dispersion relation indicates the excitation of helicon wave. Dispersion relation of electromagnetic waves is solved using dielectric tensor elements of a cold uniform plasma including ion terms, which shows two branches: the SW and fast (helicon) wave. The transition from the ionization by SWs, which are excited in the low-density plasma produced by an antenna induction field, to that by helicon waves has been predicted to explain the mechanism to produce the high-density plasma in the helicon wave discharges.

A higher-order radial mode of helicon waves

The axial profile of the axial component of the wave magnetic field has been measured for m = 0 helicon waves. A dip in is observed at cm from the antenna, and is explained as a result of the beat between the fundamental and the second-order radial modes of the m = 0 helicon wave. The measured imaginary part of , which consists only of the wave field, is compared with that of the calculated wave field using the theory of the uniform plasma case. It is found that % of the amplitude arises from the second-order mode and the axial damping length of the fundamental mode .

Proof‐of‐principle experiments of laser wakefield acceleration using a 1 ps 10 TW Nd:glass laser

The principle of laser wakefield particle acceleration was tested by a Nd:glass laser with a power of up to 10 TW and a pulse‐duration of 1 ps. Electrons with 1 MeV/c momentum emitted from a solid target by an intense laser impact were used as test particles. The corresponding field gradient achieved was 1.7 GeV/m in the linear regime and 30 GeV/m in the nonlinear, self‐modulation regime. The results are consistent with simulations.

Compact High-Density Plasma Source Produced by Using Standing Helicon Waves

High-density plasmas of the order of 1013 cm-3 are produced by the m=0 standing helicon wave (SHW) in a short discharge column (10 cm). Unlike conventional helicon wave discharges, high-density plasma production is split into two modes governed by N=1 and 3 (N is the number of half-wavelengths in the plasma column) SHWs. The external static magnetic field B0 dependence of plasma density np shows that N=3 SHW can produce a density higher than N=1 SHW at low magnetic field (~200 G). The standing waves are measured and compared using a two-dimensional wave calculation.

New method to improve He-removal performance of pump limiter by RF ponderomotive force

A new method to improve He-removal performance of a pump limiter by using rf ponderomotive force is proposed, A part of the helium neutralized at the plate in the pump limiter is re-ionized by the plasma in the channel under the limiter blade («scoop») and flows back out of the scoop. The method proposed here is to suppress this back stream and improve the He exhaust rate by an application of rf ponderomotive force. When the frequency is slightly above the cyclotron frequency of low energy re-ionized He + ions, the ponderomotive force is applied only on the He + ions to reduce the back stream and the He pumping efficiency is improved. It is also shown that this method can be extended to improve tritium inventory together with He ash removal by choosing the frequency slightly above the tritium cyclotron frequency

Plasma Production by m = 0 Standing Helicon Waves

m=0 standing helicon waves (SHWs) are used to produce high-density Ar plasmas, of the order of 1013 cm-3, in a bounded cylindrical plasma column. Axial mode number N=1, 3, and 5 (N is the number of half-wavelengths in the plasma column) SHWs are strongly excited for plasma length L=10–26 cm with the rf power Prf≤3.5 kW and external static magnetic field B0=237 G. Hollow and parabolic density profiles that depend on the axial mode of SHW are observed. The standing wave ratio decreases with L, and travelling helicon waves dominate the plasma production for L>26 cm. The measured results are compared with a two-dimensional wave calculation.

Characterization of Plasma Production by m=0 Standing Helicon Waves

Excitation of m=0 standing helicon waves (SHWs) of N=1 and 3 modes (m is the azimuthal mode number and N is the number of half-wavelengths in the axial direction) and production of high-density plasmas of the order of np1013 cm3 have been investigated in a short cylindrical plasma column of 10 cm length [M. Nisoa, Y. Sakawa and T. Shoji: Jpn. J. Appl. Phys. 38 (1999) L777]. Excitation of different N modes of SHWs has caused an abrupt density jump with rf power Prf and axial static magnetic field B0. The power-balance equation is solved using the np dependence of plasma loading resistance R obtained numerically from a two-dimensional wave code. Calculated Prf and B0 dependence of stable np agrees well with measured ones. Measured hollow (parabolic) np profile caused by the excitation of N=1 (N=3, 5 and travelling helicon wave) mode is explained in terms of the perpendicular (parallel) component of wave electric field E⊥ (Ez). Power absorption due to E⊥ and Ez is dominant for the hollow and parabolic np profiles, respectively.

Radio frequency line-plasma source using permanent magnets

A high-density and uniform line-plasma source is developed by an inductive rf discharge using a rectangular discharge chamber (200×100×20mm) with a pair of permanent magnets placed on top and bottom of the chamber. Ion-saturation current-density Jis profile is controlled by varying the width of the magnets and the distance between the antenna and the magnets. A 140-mm-wide plasma [plasma density ≃(1.8-2.5)×1012cm−3 for electron temperature =4–8eV] of a uniformity variation within 90% is produced using a 140-mm-long antenna for an Ar pressure of 20mTorr and a rf power of 3kW. The measured Jis profiles are explained by solving the equation of motion for electrons under a magnetic field structure of longitudinal line cusps.