US3373305A - Duoplasmatron source having an offset compressor electrode - Google Patents

Description

March 12, 1968 yL. E. COLLINS ETAL.

DUOPLASMATRON SOURCE HAVING AN OFFSET COMPRESSOR ELECTRODE 3 sheets-'sheet 1 Filed Oct. 18, 1965 March 12, 1968 L. E. COLLINS ETAL 5 Sheets-Sheet 2 Filed Oct. 18, 1965 linfa March 12, 1968 L. E. COLLINS ETAL 3,373,305

DUOPLASMATRON SOURCE HAVING AN OFFSET COMPRESSOR ELECTHODE Filed OC.. 18, 1965 5 Sheets-Sheet 3 United States Patent C 3,373,305 DUQPLASMATRON SOURCE HAVING AN OFFSET COMPRESSOR ELECTRDE Leslie Edward Collins, Reading, and Royston Henry Gobbett, Basingstoke, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Oct. 18, 1965, Ser. No. 497,248 Claims priority, application Great Britain, Oct. 26, 1964, 43,658/ 64 3 Claims. (Cl. 313-230) ABSTRACT F THE DISCLOSURE A duoplasmatron source for producing negative ions including a compressor having an orifice, an anode having an orifice and an extractor having an orifice. The axis of the compressor orice is displaced from the common axis of the anode and the extractor orifices to allow negative ions from the outer region of an arc discharge established between compressor and anode to be extracted through the anode orifice.

This invention relates to ion sources and in particular to sources of the duoplasmatron type.

A full description of a duoplasmatron source can be found, for example, in Rev. Sci. Instrum., vol. 30, p. 694 (1959). Briefiy it comprises a compressor electrode having an outlet orifice, an electron-emitting filament located behind the compressor electrode, an anode located in front of the compressor electrode and having a small orifice coaxial with the outlet orifice thereof, and an extractor electrode located beyond the anode orifice. An axial magnetic field is applied in the space between cornpressor and anode. In operation, electrons from the filament bombard gas behind the compressor to produce a weak plasma which passes through the compressor orifice to the gap between compressor and anode, in which a low-voltage arc discharge is struck. This arc is com- Ipressed by the magnetic field to produce a dense plasma, ions from which pass through the anode orice and are drawn ott by a suitable potential applied to the extractor.

The duoplasmatron source is normally used as a source of positive ions, the extractor being made negative with respect to the anode. For some purposes negative ions are required, but the mere application of a positive eX- traction voltage in existing duoplasmatron sources produces poor results, with a high ratio of electrons to negative ions in the resulting beam.

The present invention provides a duoplasmatron source for the production of negative ions wherein the anode orifice is so oriented in relation to the arc discharge established between compressor and anode as to cause negative ions extracted from said orifice to be drawn from an outer region of said discharge.

The present invention also provides a duoplasmatron source for the production of negative ions including a compressor having an orifice and an anode having an orifice, wherein the axis of the compressor orifice is displaced rom the axis of the anode orifice to allow negative ions from the outer region of an arc discharge established between compressor and anode to be extracted through said anode orifice.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawing wherein:

FIGURE l is a sectional elevation of the electrode structure of an ion source embodying the present invention.

FIGURE 2 is an enlarged sectional elevation of an anode insert used in the embodiment of FIGURE 1.

ice

FIGURES 3-5 are graphs showing the performance of an ion source with various parameters.

FIGURE 6 is a sectional elevation of an embodiment similar to that shown in FIGURE l.

FIGURE 1 shows the filament 1, compressor 2, anode 3 and extractor 4 of a duoplasmatron source. In prior sources of this type, the compressor orifice 5, the anode orifice 6 and the extractor orifice 7 are coaxial with one another. In the present source, however, the axis of the anode and extractor orifices are displaced by a distance d from the axis of the compressor electrode. As in known practice, the anode 3 is not a simple metal sheet as shown in FIGURE l, but is made of copper having a tungsten insert 8 (FIGURE 2) in which the orifice 6 is formed. This composite form of construction is not, however, essential. The axial magnetic field is applied in a known manner by an external circumferential winding shown symbolically at 9.

FIGURE 6 shows the construction of the embodiment to which the graphs of FIGURES 35 relate, in which the tungsten insert 8 is recessed a distance g into the anode 3. The remaining reference numerals and letters correspond to those in FIGURES l and 2, except that hole 6 and dimension a are not marked.

Dimensions which have proved suitable for the production of negative hydrogen ions, referring to the letters shown in FIGURES 1, 2 and 8 are:

a=0.03 inch b=0.l inch c=0.1 inch d=0.0625 inch e=0.2 inch f=0.12 inch g=0.06 inch (FIGURE 6 only) A positive voltage with respect to the anode 3 is applied to the extractor 4.

FIGURE 3 shows how the negative ion yield was found to vary with dimension d above, the several curves being for different values of dimension f. It will be seen that the optimum yield was obtained with d=0.0625 inch and f=0.l2 inch.

FIGURE 4 shows the variation ofthe electron/negative-ion current ratio in the beam as d is varied, for f=0.12 inch. At peak ion current the electron current is increasing more rapidly than the ion current with decreasing d, and hence the above ratio also increases as d decreases. However even at peak ion yield (1120.0625 inch), the ratio is only about 2 to l, which compares favourably with values of about 1000 to 1 reported in the literature. The electron current collected by the extractor at peak ion yield is about 4-5 ma.

The optimum arc discharge current for negative ion production was -found to be about 12 amps, but was not very critical, nor was the 4gas pressure which showed a flat peak at l mm. Hg.

The effect of varying the axial magnetic field is shown in FIGURE 5. The results shown in FIGURE 3 indicate that the arc discharge contains an annular outer region possessin-g optimum conditions for negative ion production. It would therefore be expected that if d were decreased from an initial value, the axial field would have to be increased to apply further compression to the arc, so that the annular region remained aligned with the anode orifice, in order to restore the initial ion yield. In fact, however, the field current can be adjusted to give an increased ion yield as d is decreased. FIGURE 5 shows the magnetic field current required to give maximum yield as d is decreased.

We claim:

1. A duoplasmatron source for the production of negative ions including a compressor having an orice and an anode and 'an extractor each havin-g an orifice, wherein the axis of the compressor oriiice is displaced from the common axis of the anode and extractor orices' to allow negative ions from th'e outer region of an arc discharge established between compressor and anode to be extracted through said anode oriice.

2. A duoplasmatron source for the production of negative ions including a compressor, an 'anode and an extractor each having an orice, wherein the anode and extractor orices have a common axis, the axis of t-he compressor orifice being displaced from said common axis in a direction normal to said common axis and said anode orifice being substantially smaller than said compresser orice so that negative ions from the outer region of an arc discharge established between compressor and anode may be extracted from said discharge.

3. In a duoplasmatron sour-ce comprising a compressor having an orice 'and an anode and an extractor having an orilce, the displacement of the axis of the cornpressor oriiice from the common axis of the anode a'nd extractor orices in a direction normal to said axes t allow the extraction of negative ions from t-he outer region of an 'arc discharge established between compresser and anode.

. References Cited UNITED STATES PATENTS 2,816,243 10/ 1957 Herb 313--63 2,978,580 4/ 1961 Von Ardenne 313-230 JAMES w. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner.

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