Effect of Impurity Impurity on Beam Quality in Arc-arc Sources Tang Pingxi, Shi Lei, Tan Xiaohua, Dai Jingyi (Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621900, Sichuan, China). Single-pulse and low-repetition-frequency pulsed vacuum arc helium ion sources underwent spectral diagnostic experiments and nuclear reaction analysis experiments. The results showed that the cesium ion source plasma does contain more impurities, which can be related to helium ion sources. The quality of the extracted beam creates a major hazard. And suggestions are made on how to reduce the influence of impurities.

The electrode of the vacuum arc crucible ion source is titanium metal that adsorbs germanium. During operation of the ion source, the suction electrode is partially sputtered and vaporized by the bombardment of charged particles and releases the adsorbed helium gas, and the helium gas is further ionized to form a plasma. The helium ion beam generated by the helium ion source is scientifically researched. In clinical medicine, there are a wide range of major impurities used and their effects on the quality of the extracted beam.

1 The composition of the main impurities in the vacuum arc crucible ion source and its influence on the vacuum arc crucible ion source The discharge plasma in the main impurities are: 1) The electrodes and the insulating medium contained in the discharge chamber are related to the elements contained in the vacuum arc discharge process. It will inevitably produce certain ablation and sputtering effects on the electrode and insulating medium. Therefore, the generation of these impurities is unavoidable; 2) Impurity gases adsorbed on the inner wall of the discharge chamber, including 2, N2, H2, H2, CO, CH4, etc. . In the text, H refers to 1H, which is protium.

Among the impurity gases in the vacuum arc helium ion source, H2 has the greatest effect. Although the helium ion source is exposed to the atmosphere during processing, H2 is not the main gas adsorbed by the discharge chamber, but in the further processing, the 2 and N2 adsorbed in the discharge chamber are easier to remove, and the degassing process of H2 and CH4 is easier. Both will be accompanied by the release of H2. On the other hand, titanium as an electrode material has a good ability to absorb hydrogen, it may not only inhale H2 during the early suction saturation, more likely in the process of continuous intake of residual gases H2, and further diffused to a certain depth of the surface layer of the titanium electrode 16. Therefore, after a certain process treatment, in the working state of the vacuum arc helium ion source at a higher degree of vacuum, H2 becomes the main adsorption of the discharge chamber space and the inner wall Impurity gas.

In the vacuum arc crucible ion source which uses the titanium metal as the electrode, the H2 is particularly harmful. This is because helium and neon have similar atomic structures, radiation spectra and atomic radiation constants. In the helium ion source discharge plasma, the probability of being ionized is basically the same. At the same time, since H+ and D+ are similar in quality, the number of charged charges is the same. In a pulsed helium ion source beam, it is difficult to separate the two, and thus the beam quality of the helium ion source has the greatest influence.

The helium pick-up titanium electrode used in the vacuum arc helium ion source is generally saturated in a very high purity helium atmosphere, the formed Ti-D solid solution has a relatively stable atomic ratio, and the D electrode in the titanium electrode is uniformly distributed and highly purified. The H-adsorbed at the later stage is generally distributed only on the surface of the electrode and within a small depth range of the surface layer.

In the working state of the helium ion source, generally a small diameter cathode spot is formed. The release of the helium gas mainly lies in the sputtering under the charged particle bombardment in the vacuum arc discharge and the thermal desorption at the cathode spot. In this work, ANSYS software was used to simulate the instantaneous temperature distribution of the local area of ​​the cathode spot by pulsed charged particle bombardment (). The pulse energy injected at the cathode spot in the numerical simulation is 0.2 J/mm2, and it can be seen that within the delay time interval of about 300 ns, only within the surface layer of 0.8 Mm from the titanium electrode surface, the local temperature exceeds (sucking The titanium electrode releases the temperature of the helium gas). Even taking into account the impact of charged particles and sputtering, the helium gas involved in the discharge of the helium ion source is only derived from the surface of the titanium electrode and the depth of the surface layer is very small, and this region is the impurity H is more The area, therefore, impurity H will inevitably cause greater harm to the performance of the helium ion source.

Titanium electrode surface; â–³ from the titanium electrode surface 0. From the titanium electrode surface 0. From the titanium electrode surface 0.5Mm; From the titanium electrode surface 0. 2 Spectral diagnostic analysis To further understand the impact of impurity H on the cesium ion source performance, using the United States RS's optical multi-channel analysis system performs spectral diagnostic experiments. The system adopts different gratings to separate light, the measurement range covers 190~1100nm, the minimum scale is 0.01nm, and multiple channels can be simultaneously acquired to achieve spatial resolution of the plasma spectrum. Analysis and post-processing of experimental data was done using Winspec32 software.

Since the wavelengths of the erbium and germanium atoms are very close to each other, they are (Da). To separate the two, a 1200-line grating was used to investigate the intensity of the Ha and Da lines near 656 nm, and the relative spectral line intensity was used to characterize the relative contents of H+ and D+ in the helium ion source plasma.

It is the relative line intensity of Ha and D* in the discharge plasma emission spectrum when the vacuum arc helium ion source is operated with a single pulse. From this, it can be calculated that the concentration ratio of H+ and D+ in the plasma is g0.87, which is in the plasma, and the ratio is more than +.

Too much, this is obviously unfavorable to the beam quality of the helium ion source.

In order to discriminate the H+ concentration of impurity H+ in the helium ion source plasma is greater than the D+ concentration, the same as in the single pulse operation, but in the following several discharges, the vacuum arc helium ion source is used due to the change of the surface and degree of the titanium electrode. Placed in a low repetition rate pulse operating state, the repetition frequency is 2s, and the relative spectral line intensities of Ha and 0 in the emission spectrum are analyzed. The experimental results are shown in .

The relative spectral line intensity of H and Liao in the single-pulse operation of the wave vacuum isolated ion helium is relatively low. The relative concentration of D+ and H+ in the pulsed state of the pulse * The total ion concentration after normalization * Relative total argon ion The absolute content of 3D+; the absolute value of 3D+ at the low repetition rate pulse state, the state of its first discharge is the same as the single pulse operation, so the spectrum of the first discharge spectrum is consistent with that of a single pulse, but in the subsequent discharge process. The ion source state has undergone a major change, and the relative content of D+ concentration in the total hydrogen ions has also changed. 1 From the curve 1, one can see that the work has been performed at the beginning of the state. Many, and can not be restored to the previous level within a limited time interval, and D adsorbed inside the titanium electrode can quickly fill up the adsorption voids generated due to the discharge. Therefore, the relative content of D+ in plasma gradually rises, reaching 68% at the highest. At this time, the concentration of D+ is greater than that of H+, and the relative content of D+ is always maintained. From curve 1, it can be seen that in the working state with the repetition frequency of 2 s, the hydrogen (including D and 1H) adsorbed near the discharge spot is not enough to make up, resulting in the reduction of the hydrogen atomic ratio near the arc spot, and therefore the total hydrogen in the plasma. The ion concentration showed a rapid decline. Curve 3 also shows that after 4 or 5 consecutive pulse discharges with low repetition rate, the D+ absolute content in the plasma gradually decreases as the D atomic ratio near the arc spot decreases.

3Nuclear reaction analysis The vacuum arc helium ion source helium ion beam is extracted and bombarded with a helium target, and a 2.5 MeV monochromatic neutron can be obtained. The nuclear reaction formula is: the neutron yield of the helium target can be obtained by measuring the helium ion beam bombardment. Variations of helium ion concentration in the extracted beam of the vacuum arc helium ion source.

The results of the nuclear reaction analysis of the helium ion source extracted beam at a low repetition rate pulse repetition frequency of 2 s are shown in . Since the neutron yield of the D(d,n)He nuclear reaction is proportional to the intensity of the helium ion beam that strikes the target, the result can be approximately equivalent to the variation of the D+ concentration in the vacuum arc helium ion source plasma. Contrast and can be found, the two trends are exactly the same.

4 Results and discussion This work was performed on single-pulse and low-repetition-frequency pulsed vacuum arc helium ion source in the state of the beam quality of the spectrum diagnosis and nuclear reaction analysis, the experimental results can be seen, helium ion source plasma It does contain more impurities H, and it poses a greater hazard to the quality of the extracted beam of the helium ion source. Attention should be paid to the design and processing of the helium ion source.

The D(dn)3He nuclear reaction analysis of the extracted beam from the time/s helium ion source in order to reduce the influence of impurity H in the helium ion source, the helium ion source should take measures as much as possible to increase the inner wall of the discharge chamber and the metal before working. Desorption and outgassing of impurity H in the titanium electrode. The fact that the relative content of D+ gradually rises under the condition of low repetition rate pulse operation also indicates that moderate aging of the helium ion source is also an effective way to reduce the influence of impurity H in the ion source.

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