Control Circuit For Neutron Generator Tube

Abstract

In the supply circuit for an ion accelerator having an ion source and an ion extractor electrode, the supply circuit including an energizing supply for the ion source and a power supply for the extractor electrode, there is provided a negative feedback loop connected to cause the extractor electrode current to control the energizing supply output in a sense to maintain constant the extractor current. The described embodiment is a neutron generator tube having a plasma ion source energized by a radio-frequency supply.

Description

BACKGROUND OF THE INVENTION

This invention relates to supply circuits for ion accelerators, in particular for neutron generators in which deuterium and/or tritium ions are extracted from a plasma and accelerated on to a target containing deuterium and/or tritium. A neutron generator tube of this kind is described in our UK Specification No. 1,088,088.

Briefly, in the latter generator a plasma is produced in the ion source portion of the tube by a surrounding coil energized by a radio-frequency oscillator. This portion of the tube is bounded by an apertured boundary electrode, and the plasma is intensified in the region of this aperture by an axial magnetic field derived from a coaxial solenoid.

Positive ions are extracted from the plasma by a negative potential applied to an extractor electrode. A plasma boundary is formed with its perimeter keyed to the edge of the boundary electrode. The ions are extracted into a beam whose profile depends on the electric field configuration produced between this plasma boundary and the potential applied to the extractor. The shape of the boundary and its effect on beam profile depend on the plasma ion density and the value of the applied potential. Optimum operation requires that the ion beam pass through the extractor aperture with minimum interception to prevent undue heating or sputtering of the extractor electrode. The ion beam once through the extractor aperture is accelerated through a target shield aperture by a large negative potential applied thereto, and finally passes through the field-free region inside a suppressor electrode to impinge on the target.

In such tubes there is a small interception of the beam by the extractor electrode under normal running conditions, giving rise to an extractor current. If the interception is too great, the extractor temperature rises and adsorbed gas is released into the tube. This creates an unstable condition, as described in more detail hereafter, which can result in the destruction of the tube. It is particularly likely to occur under start-up conditions, as the extractor potential and radio-frequency power supply to the plasma are increased progressively.

It is an object of the present invention to provide a form of supply circuit which alleviates this problem.

SUMMARY OF THE INVENTION

According to the present invention, in a supply circuit for a particle accelerator having an ion source and an ion extractor electrode, said circuit including an energizing supply for the ion source and a power supply for the extractor electrode, there is provided a negative feedback loop connected to cause the extractor electrode current to control the energizing supply output in a sense to maintain constant said extractor current. The energizing supply may be a radio-frequency supply adapted to energize a plasma ion source.

The present invention also provides, in combination, an ion accelerator having an ion source and an ion extractor electrode, and a supply circuit for said ion accelerator, said circuit including an energizing supply for the ion source and a power supply for the extractor electrode, wherein said circuit comprises a negative feedback loop connected to cause the extractor electrode current to control the energizing supply output in a sense to maintain constant said extractor current. The ion source may be a plasma ion source and said energizing supply a radio-frequency supply.

The ion accelerator may be a sealed neutron generator tube, the ion source being adapted to produce hydrogen isotope ions.

DESCRIPTION OF THE DRAWING

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

FIG. 1 is a schematic diagram of a neutron generator tube connected in a supply circuit embodying the present invention.

FIG. 2 is a graph showing the variation of extractor and total tube current with extractor voltage at constant RF power.

FIG. 3 is a graph showing the variation of total tube current with extractor voltage when the RF power is controlled in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a neutron generator tube of the kind described in detail in UK Specification No. 1,088,088. It comprises an ion-source portion 1 surrounded by an RF coil 2 to energize a plasma therein, and a magnetic solenoid 3. At one end is a backstop 4 and at the other an apertured boundary electrode 5, beyond which are successively an extractor electrode 6, a shield electrode 7, a suppressor electrode 8, and a target 9. The cross-section of the ion beam under normal working conditions is indicated approximately at 10. The gas pressure is maintained constant by a replenisher controlled by a Pirani gauge, both of which are omitted from FIG. 1 for clarity, as is part of the tube envelope. FIG. 1 also shows the connections from the associated supply circuit.

Typical preferred operating conditions for the tube are as follows:

Gas pressure (50/50 D/T mixture) (P) 1.2.times.10.sup..sup.-2 Torr RF energizing power (W) 200 watts RF frequency 14 MHz Magnetic field (H) 90 gauss Extraction potential (V.sub.E) 4.5 kV Extraction interception current (I.sub.E) 0.1 mA Acceleration potential (V.sub.T) 120 kV Total tube current (I.sub.A) 12 mA Target current (I.sub.T) 6 mA Suppressor voltage (V.sub.S) 440 V Suppressor current (I.sub.S) 6 mA Ion beam diameter at target 2.8 cm Neutron output 10.sup.11 neutrons/ second

FIG. 2 illustrates a typical extractor control characteristic for a constant RF energizing power W. With increasing extractor potential V.sub.E, the interception current I.sub.E first rises to a peak corresponding to increasing ion extraction accompanied by beam collection by the extractor. Subsequently ion extraction continues to increase but is associated with a repression of the plasma boundary into the ion-source portion of the tube, the effect of which is closer beam focussing so that the beam passes through the extractor aperture and the interception current falls. The beam profile is well defined with a small interception on the extractor, and this in turn defines the ion beam diameter at the target.

The effect of increasing the RF power W is to shift the whole characteristic to the right as shown. This effect is due to increased ion-source plasma density, which tends to bring the plasma boundary forward into the extraction region and requires an increased extraction potential to restore the initial focus conditions and so reduce the interception current.

A whole series of these characteristics exists, corresponding to the range of possible RF powers, but at the higher levels they are impossible to determine because of instabilities caused at high values of I.sub.E. The maximum continuously acceptable value of I.sub.E is about 0.15 mA.

Increase beyond this firstly causes a rise in the temperature of the extractor electrode, and in the long term, sputtering. Sputtered material is undesirable because eventually conducting deposits can be formed on the insulating walls of the tube with adverse consequences to the high voltage insulation and, less seriously, interference with ion source performance.

More serious and even catastrophic can be the chain of positive feedback events brought about by temperature rises, as follows:

i. Adsorbed gas is driven out of the extractor.

ii. Owing to the finite response time of the pressure control system the gas pressure rises.

iii. With increase in pressure, RF power is more readily coupled to the ion source and the plasma ion density increases.

iv. Following the characteristics of FIG. 3, interception increases, followed by a further temperature rise of the extractor electrode.

v. At the same time ion current to the target increases and if allowed to rise beyond the coolant system capacity will cause rapid target outgassing and further rises in pressure.

vi. The electron current to the backstop also increases with pressure and a further outgassing may ensue from that electrode.

If this sequence is not controlled the power dissipation at the extractor electrode may rise to such a point that the glass-to-metal seal is destroyed. Protection could be achieved by over-current trips in the various circuits, but this can result in an unwelcome and unacceptable number of "shut-downs". Any variation in the tube supply conditions may initiate this chain, and continuous control is important for long-term stable conditions.

In the present invention, any increase of interception current is caused to reduce the RF power, thereby reducing the degree of interception and so inhibiting the runaway chain of events described above. Referring again to FIG. 1, the interception current I.sub.E is made to flow through a resistor R.sub.E and thereby develop a potential across it. This potential is compared with a reference potential derived from a source 11 by a transistorized differential amplifier 12 of conventional construction. The output of amplifier 12 controls the firing angle of a thyristor control circuit 13 connected in the 50 c/s mains supply to the RF oscillator 14, and thus ultimately the RF power supplied to the ion source. To ensure stability of the negative feedback loop so formed, the sensitivity and time-constants are adjusted in a manner familiar to those skilled in the negative feedback control art. The desired value of interception current I.sub.E is set by adjusting the value of R.sub.E.

The above-described supply circuit has enabled long-term stable operation of the tube to be achieved, for example a 12-hour unattended run. In the absence of the negative feedback loop, an operator was required to be in constant attendance.

A further advantage of the invention is that the extractor electrode can be employed as a true control grid, as illustrated in FIG. 3. The extractor potential can be increased or decreased at will to determine the ion current and consequent neutron output. The necessary changes in the RF power supply to the ion source are automatically introduced by the feedback circuit and the plasma boundary is maintained over a wide range in the correct position for stable formation of an ion beam of the desired profile and hence for a stable position on the target.

Although described with reference to a neutron generator tube using an RF plasma ion source, it will be appreciated that the present invention can be applied to neutron generators or other particle accelerators employing an extraction electrode, whatever the process by which the energizing supply causes ion production in the source. For example, in a neutron generator using a PIG-type ion source, the extractor current can be made to control the anode voltage and hence the discharge current in the source. Neutron generators using such ion sources, also known as Penning sources, are described, for example, in UK Specifications Nos. 976,664 and 980,947 and in Nucleonics, December 1960, pp.69-74.

Claims

I claim:

1. A supply circuit for an ion accelerator having an ion source and an ion extractor electrode, said circuit including an energizing supply for the ion source, means for providing a fixed accelerating voltage and a power supply for the extractor electrode, wherein said circuit comprises a negative feedback loop connected to cause the extractor electrode interception current to control the energizing supply output in a sense to maintain constant said extractor interception current at a small fraction of the total current.

2. A circuit as claimed in claim 1 wherein the energizing supply is a radio-frequency supply adapted to energize a plasma ion source.

3. In combination, an ion accelerator having an ion source and an ion extractor electrode, and a supply circuit for said ion accelerator, said circuit including an energizing supply for the ion source, means for providing a fixed accelerating voltage and a power supply for the extractor electrode, wherein said circuit comprises a negative feedback loop connected to cause the extractor electrode interception current to control the energizing supply output in a sense to maintain constant said extractor interception current at a small fraction of the total current.

4. A combination as claimed in claim 3 wherein the ion source is a plasma ion source and said energizing supply is a radio-frequency supply.

5. A combination as claimed in claim 3 wherein the ion accelerator is a sealed neutron generator tube and the ion source is adapted to produce hydrogen isotope ions.