NIFS-115

FULL TEXT (PDF, 1573 KB)


Author(s):

Y. Okabe

Title:

Study of Au^- Production in a Plasma-Sputter Type Negative Ion Source

Date of publication:

Oct. 1991

Key words:

Au^-, work function, plasma, negative ion source, photoelectric effect, negative ion beam, energy spread, electrostatic energy analyzer, heavy ion beam probe, plasma potential

Abstract:

A negative ion source of plasma-sputter type has been constructed for the purpose of studying physical processes which take place in the ion source. Negative ions of gold are produced on the gold target which is immersed in an argon discharge plasma and biased negatively with respect to the plasma. The work function of the target surface was lowered by the deposition of Cs on the target. An in-situ method has been developed to determine the work function of the target surface in the ion source under discharge conditions by measuring the photoelectron currents induced by two lasers (He-Ne, Ar^+ Iaser) simultaneously injected onto the target. The reflectivity at each wavelength on a cesiated gold surface was measured and taken into account in deriving the work function from the photoelectron current measured. The dependence of Au^- production rate, defined as the ratio of Au^- current to the target current, on the work function of the target surface was obtained from simultaneous measurements of both target surface work function and Au^- production rate while the Cs coverage was changed due to the plasma ion sputtering. The observed minimum work function of a cesiated gold surface in an argon plasma was 1 .3 eV, when the negative ion production rate took the maximum value. The production rate increased monotonically and saturated when the surface work function was reduced from I .9 eV to I .3 eV. This range of the change in the work function was limited by the wave lengths of lasers used for the determination of the work function. The dependence of Au^- production rate on the incident ion energy was deduced by changing the target bias voltage while the work function was kept constant. The production rate increased with the target bias voltage, and showed a similar dependence on the incident ion energy as the sputtering rate of gold with Ar^+ bombardment. The dependence of Au^- production rate on the number of the incident ion was studied by changing the plasma discharge current, while the work function was again kept constant. The ion density increases linearly with the discharge current. Both negative ion current and target current increased linearly with discharge current, but the negative ion production rate remained constant in this range of discharge current. From the experimental results, it is shown that the sputtering process is an important physical process for the negative ion production in the plasma-sputter type negative ion source. The energy distribution function was also measured with an electrostatic energy analyzer. When the bias voltage was smaller than about 280 V, the high energy component in the distribution decreased as the target voltage was decreased. Therefore, the energy spread Delta E, which was defined as the full width at half maximum (FWHM) of the observed negative ion energy distribution also decreased. This tendency is also seen in the energy spectrum of Cu atoms sputtered in normal direction by Ar^+ ions. The energy spread Delta E was between 4 and 10 eV when the target voltage was smaller than 300 V. From the consideration of the negative ion production rate and the energy spread, the Au^- beam produced in the plasma-sputter type negative ion source is found applicable to the heavy ion beam probing (HIBP) which is a diagnostic technique for the plasma potential in a magnetically confined plasma.

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