Ion source including a filter electrode

Provide an ion source for outputting ion beam with high purity of polyvalent positive ion.The ion source 10 includes: a target 12 from which electron and positive ion are generated by plasma formed by laser 13 irradiation; a first power supply source (first voltage E1) that sets an electric potential of the target 12 higher than that of a destination of the positive ion (corresponding to an acceleration channel 18 in FIG. 1); and a second power supply source (second voltage E1) that sets an electric potential of on a pass (corresponding to a filter electrode 15 in FIG. 1) from the target 12 to the destination 18 higher than that of the target 12.

TECHNICAL FIELD

The present invention relates to an ion source for outputting ion beam through generating plasma by laser irradiation on a target.

BACKGROUND ART

A laser utilizing ion source generates plasma through irradiating the condensed laser beam to a solid target and then evaporates and ionizes the element of the target by the laser energy. The generated plasma maintain their state and are transported to the entrance of an accelerator, where only ions are input to the accelerator by differential electric potential and then outputted as the ion beam (refer to Patent document 1, 2). While it is known that the ion acceleration of the accelerator is superior if the valence of the positive ion is higher or the mass thereof is smaller. Also the laser utilizing ion source is effective in generating a polyvalent positive ion.

CITATION LIST

Patent Literature

DESCRIPTION OF THE INVENTION

Problems to be Solved by the Invention

However, the ion beam outputted from the laser utilizing ion source contains high ratio of impurities such as a cluster ion with large mass and positive ion with a low valence other than the polyvalent positive ion. For this reason, there is some problem that the linear accelerator (RFQ) is polluted by the impurities if the ion beam consisted of the low purity polyvalent ion enters into the linear accelerator.

The present invention has been made in view of such circumstances, and provides the ion source which can output ion beam with high purity of polyvalent positive ion.

DESCRIPTION OF EMBODIMENTS

First Embodiment

As shown inFIG. 1an ion source10includes: a target12from which electron and positive ion are generated by plasma formed by laser13irradiation; a first power supply source (first voltage E1) that sets an electric potential of the target12higher than that of a destination of the positive ion (corresponding to an acceleration channel18inFIG. 1); and a second power supply source (second voltage E2) that sets an electric potential of on a path (corresponding to a filter electrode15inFIG. 1) from the target12to the destination18higher than that of the target12.

An ionization chamber11accommodates the target12in evacuated internal space, the ionization chamber11having an electric potential set to the same electric potential as that of the target12.

A laser irradiation member (not shown) is arranged outside of the ionization chamber11. The laser13passing through a transparent window provided on the surface of the ionization chamber11, and entering into the internal space to irradiate surface of the target12. A condenser (not shown) is installed inside or outside of the ionization chamber11. The laser13is condensed by the condenser before or after passing through the transparent window.

The element of the target12evaporates, ionizes, and then generating plasma14by the energy of the irradiated laser13. The plasma14is in the state where the evaporated element of the target12ionizing into positive ion and electron, and become electrically neutral as a whole.

The plasma14contains impurities such as cluster ion with large mass and a positive ion with a low valence other than the desired polyvalent positive ion.

The positive ion in the plasma14has a larger initial velocity when it is emitted from the surface of the target12when the valence of the positive ion is higher or the mass thereof is smaller. The plasma14is emitted from the laser irradiating point and spread toward the beam direction X perpendicularly with the target.

The filter electrode15is provided on the path of the beam direction X from downstream side of the target12to upstream side of the linear accelerator17. The form of the filter electrode15takes tubed shape, flat plate shape, etc., the form is not especially limited if having a passing mouth at the center for the positive ion.

The plasma14generated in the ion source10passes through the communicating path16, and enters into the linear accelerator17. The communicating path16is insulated electrically because electric potential differs between the ionization chamber11and the linear accelerator17. Entering the plasma14into the linear accelerator17, electron is separated, and the positive ion is accelerated in the acceleration channel18.

In the power supply circuit shown inFIG. 1, the target12is applied the target voltage (E0+E1) in which first voltage E1was added to bias voltage E0. The filter electrode15is applied the filter voltage (E0+E1+E2) in which second voltage E2added to the target voltage (E0+E1). Meanwhile the bias voltage E0may be sufficient equal to 0V.

Cluster ions with large mass and low valence ions among the positive ions14contained in the plasma emitted from the target12cannot pass through the filter electrode15in the beam direction X due to their slow initial velocity. Thus disposing the filter electrode15on a path from the target12to the acceleration channel18, the purity of the desired polyvalent positive ions outputted from the ion source10can be improved.

The ratio and quantity of the desired polyvalent positive ions outputted from the ion source10can be adjusted by adjusting the second voltage E2.

The acceleration channel18is applied the accelerating voltage (E0+E*) in which superimposing high-frequency-voltage E* on the bias voltage E0.

Since the electric potential of entrance of the acceleration channel18is set low rather than that of the target12and the filter electrode15. Thereby the polyvalent positive ion outputted from the ion source10adding speed rather than initial velocity and entering into the entrance of the acceleration channel18.

Herewith the polyvalent positive ion entered into the acceleration channel18will be further accelerated.

Second Embodiment

As shown inFIG. 2, the ion source10according to a second embodiment further includes a plasma transfer duct19having both end portions opened to the target12and the acceleration channel18respectively, the plasma transfer duct19having an electric potential set to a same electric potential as that of the target12.

In addition, the same portion to which a mark is common withFIG. 1andFIG. 2, overlapping explanation is omitted.

As the result of arranging the plasma transfer duct19, the plasma generated from the target12can be led to the entrance of the acceleration channel18without spreading.

The filter electrode15is arranged on the path of the plasma transfer duct19. Thereby the cluster ion with big mass and the low valence ion cannot pass the plasma transfer duct19, the ion source10can output the polyvalent positive ion with high purity and with high efficient.

With reference toFIG. 3effect of the present invention is described.

FIG. 3Ais a graph showing the distribution of the ion current outputted from an ion source10versus every valence of the ion under the condition of the second power supply source is set to 0 (E2=0V).

FIG. 3Bis a graph showing the distribution of the ion current outputted from an ion source versus every valence of the ion under the condition of the second power supply source is operated (E2≠0V).

The ion source10used for the experiment having the composition shown in the second embodiment, and the target12made of graphite. The property of the time of flight (TOF) of a carbon ion differs depending on the valence (+1 to +6). Based on such the property the graph shows the measurement value of the ion current for every valence of the ion. Note that the valence of ion becomes higher the time of flight (TOF) becomes shorter.

As shown inFIG. 3A, by setting out of the second voltage E2=0, the ion current value of the low valence carbon ion is observed with high intensity as shown in region (a).

On the other hand, as shown inFIG. 3B, by setting out of second voltage E2≠0, the ion current value of the polyvalent carbon ion is observed with high intensity, as shown in region (b).

As mentioned above, at least one embodiment of the ion source10, by setting electric potential of the filter electrode15disposed on a pass from the target12to the acceleration channel18higher than that of the target12, the purity of the desired polyvalent positive ion outputted from the ion source10can be improved by confining the cluster ion with big mass and the low valence ion among the positive ions14in the ionization chamber11.