1. INTRODUCTION
Radiation-beam modification of metal devices by pulsed ion beams (PIB) provides a high heating speed and the subsequent cooling of their near-surface layer, exceeding 108–109 K/s. It allows the realization of compounds and structures that are not possible to obtain with the use of traditional metallurgical methods. In the course of this process, solid solutions and secondary phases are obtained, which are not typical of the stable diagram of the phase state. The combination of high satiety solid solution with dispersive, structural and sub-structural composition produces the unique effects of increasing surface strength and durability, and improving other material properties (Bystritskii & Didenko, Reference Bystritskii and Didenko1984). This method offers more possibilities to modify structures and near-surface layer properties in comparison with heat treatment, surface plastic deformation, etc. For modification, high-thermal conductivity materials we need to use PIB with an energy density higher than 4–5 J/cm2 and pulse duration of not more than 100–150 ns. The increase in pulse duration leads to an increase in the heated layer thickness in a bombarded target with an inessential temperature increase. For example, when an ion beam with a current density 30 A/cm2 with a carbon ions energy of 300 keV and pulse duration 150 ns is absorbed in a silicon target, the near-surface layer temperature decreases from 1600 K to 800 K in 400 ns due to thermal conductivity, and the heated layer increases to 4–5 micrometers (Bleycher et al., Reference Bleycher, Krivobokov and Pashchenko1999). The ion range does not exceed 2 micron in this case.
It is possible to increase ion beam energy density without increasing pulse duration by increasing the accelerating voltage and (or) ion current density. However, along with ion generation an essential amount of electrons with the same energy is formed. With an electron absorption of energy over 500 keV, X-ray radiation increases rapidly and it is necessary to provide extra radiation shielding. With electron and ion currents' densities over 20 A/cm2 a collective effect appears, and full-load current is limited by their charge in the A-K gap. In this case, the ion current density is determined by the A-K gap and accelerating voltage. The energy density of PIB, along with an accelerating voltage of 200–300 kV and A-K gap of 7–8 mm, does not exceed 0.3–0.5 J/cm2 (Pushkarev & Isakova, Reference Pushkarev and Isakova2012a). The use of high-current electron beams of nanosecond duration for material modification requires reasonably higher energy density (>20 J/cm2), due to having a wide electron range, exceeding hundreds of microns in metals.
It is possible to obtain a high energy density of PIB in the target with the help of effective focusing of the ion beam and elimination of its scattering while drifting. In ion diodes, geometric (ballistic) focusing is often used. Incidentally, the anode and cathode are semi-cylindrical or hemispherical. However, while ions are transferred to the focus their deviation from the initial path occurs due to Coulomb repulsion, influence of electromagnetic fields, diffusive scattering, etc. (Bystritskii & Didenko, Reference Bystritskii and Didenko1984). The influence of different instabilities and critical currents on the transfer of PIB is smaller than for the high-current electron beams because of the huge mass of charge carriers.
Detailed analysis of the ion beam focusing process, formed by diodes of various geometries, is presented in the work of Olson (Reference Olson1982). It is shown that magnetically insulated diodes have a rather small PIB divergence (ratio of PIB radius at half-height to the distance towards the diode), amounting to 1–4°, in contrast to reflex diodes and pinch-diodes. Work by Yatsui et al. (Reference Yatsui, Tokuchi, Tanaka, Ishizuka, Kawai, Sai, Masugata, Ito and Matsui1985) contains an analysis of the reasons for the focusing problems of PIB, formed by a magnetically insulated diode with hemispherical electrodes. The research was conducted with the accelerator ETIGO-1 (voltage 1.2 MV, current 240 kA, pulse duration 50 ns, PIB compound — protons). The authors noted that PIB focusing diameter is, generally, determined by an aberration because of the heterogeneous thickness of the anode plasma and distortion of the electric field near the cathode. When the cathode geometry was changed, it decreased the divergence from 6° to 2.5°. The results of research in PIB focusing on an applied B r magnetically insulated diode are presented in the work of Davis et al. (Reference Davis, Bartsch, Olson, Rej and Waganaar1997). With an accelerating voltage of 400 kV and pulse duration 0.5 µs the proton beam divergence amounted to 8°. Research on PIB focusing in a self-magnetically insulated diode was conducted with the accelerator PARUS (0.8 MB, 60 ns) (Bystritskii et al., Reference Bystritskii, Glyshko, Sinebryukhov and Kharlov1991). The diode is made in a hemispherical form with an anode radius of 90 mm. The authors point out that beam divergence is determined, mostly, by the sagging of magnetic field lines in cathode exit slits and the corresponding distortion of a virtual cathode surface. The half-angle of divergence, measured by a camera-obscura, amounted to 3°. In the self-magnetically insulated ion diode, developed by Zieher (Reference Zieher1984a), PIB divergence was about 1.5°–3.6° for different anodes. A necessary condition for current neutralization of an ion beam from a self-magnetically B θ-insulated ion diode is derived (Zieher, Reference Zieher1984b). The requirement for which is the following: the magnetic field of the cathode outside the diode gap is small enough to still allow electrons to flow from the cathode along with the beam.
In spite of numerous studies into the focusing process and a detailed analysis of the reasons for ion beam scattering formed by a magnetically insulated diode, there are no experimental data about the influence of electromagnetic fields in the area of ion drifting on PIB focusing in self-magnetically insulated diodes. This type of diode has a great resource of continuous work in contrast to outer magnetically insulated diodes, which is extremely important for the process application of PIB. In comparison with outer magnetically insulated diodes, in this type of diode, a magnetic field is formed by the current flowing through the cathode. This field penetrates a skin-layer depth in the plasma of a compensated ion flow (in transfer area), which causes the exit of low-energy electrons and PIB scattering as a result of Coulomb repulsion. If the concentration of ions (and thermal electrons) is low, a skin-layer depth may compose the essential part of a beam diameter. Furman et al. (Reference Furman, Stepanov and Furman2007) propose the use of a metal shield for improving the ion beam focusing, formed by an ion diode with applied magnetic insulation. The aim of the presented work is to research the influence of the metal shield on the passive focusing of intense ion beams in self-magnetically insulated diodes.
2. EXPERIMENTAL SET-UP
Investigations were conducted at the TEMP-4M accelerator (Pushkarev & Isakova, Reference Pushkarev and Isakova2012a) set in a double pulse mode: the first pulse being negative (300–500 ns, 100–150 kV) and the second positive (150 ns, 200–250 kV). The shot to shot variation in the accelerating voltage for 100–200 pulses does not exceed 6–7% (Pushkarev et al., Reference Pushkarev, Isakova and Khailov2012b). The beam is composed of C+ ions (80–85%) and protons; the beam energy density is 0.5–5 J/cm2 (for different types of diode); the pulse frequency is 5–10 pulses per minute. Diode current was measured with the help of a Rogowski coil. The voltage on the potential electrode was measured by a high-frequency high-voltage divider, which was installed in front of the diode connection. The electrical signals emanating from the sensors were recorded with a Tektronix 3052B oscilloscope (500 MHz, 5 GSPS). Diagnostics of PIB parameters were also conducted, with the use of a thermal imaging method adapted for double-pulse operation mode (Isakova & Pushkarev, Reference Isakova and Pushkarev2013), using a Fluke TiR10 thermal imagery device. As a target, stainless steel foil was used with a thickness of 100 micrometers. Diode connection, diagnostic equipment arrangement and the calibration of the TEMP-4M accelerator are considered in detail in our paper (Isakova, Reference Isakova2011).
The main part of the research is conducted with a strip focusing diode, of size 22 cm × 4.5 cm, and a focus of 15 cm. The gap spacing between the potential and grounded electrodes was chosen to match the diode impedance with that of the Blumline (4.9 ohms). In the optimized arrangement, the gap increased from 8 mm at the bolted end of the grounded electrode to 10 mm at the open end. The potential electrode was made from graphite, the grounded electrode was made from stainless steel with 2 cm × 0.5 cm slits and 70% transparency. The electrodes of the focusing ion diode have a semi-cylindrical configuration, and geometrical focusing is possible just in the vertical section of the PIB. The scheme of the diode assembly of the TEMP-4M accelerator and typically occurring waveforms are show in Figure 1.
(Color online) Scheme of a diode assembly, waveforms of the accelerating voltage and total current in the diode.
The part of the research is conducted with a conical self-magnetically insulated ion diode (Pushkarev & Isakova, Reference Pushkarev and Isakova2012c). The outer diameter of the conical diode grounded electrode is 20 cm, the inner diameter is 9.2 cm, and the cone angle is 37°. The mean length of coil turn is 46 cm, and the surface area equals 220 cm2. The potential electrode is made from graphite; the A-K gap is 8–10 mm, and the focus equals 12 cm. Figure 2 shows the cone diode and waveforms, characterizing its work.
(Color online) Photo of a conical diode and waveforms of the accelerating voltage and total current in the diode.
The studies on the metal shield influence on PIB transfer are also conducted with a spiral self-magnetically insulated ion diode (Pushkarev & Isakova, Reference Pushkarev and Isakova2012d). Figure 3 shows the spiral diode and waveforms, illustrating its work in a double-pulsed operation mode.
(Color online) Photo of a spiral diode, waveforms of the accelerating voltage, and total current in the diode.
The grounded electrode is made from steel wire with a diameter of 3 mm in the form of an Archimedean spiral with an outer diameter of 14 cm and inner diameter of 5 cm. The helix pitch is 8–10 mm, and the length of the helix is 150–170 cm. The potential electrode is a flat graphite disk of 20 cm in diameter and 4 cm in height. The 7–8 mm A-K gap was constant throughout the diode's length. As with the diodes with explosive emission cathodes, the strip focusing diode, cone diode and spiral diode worked effectively under a pressure of 0.1 Pa with a lifespan of more than 107 pulses. The frequency of pulse generation was only limited by the heating of the diode.
3. RESEARCH ON PIB CHARGE NEUTRALISATION
For effective ion beam transfer it is necessary to provide its charge neutralization. A high density of intense ion beams (1010–1013 ion/cm3) leads to a change in potential that may reach hundreds of kilovolts. A corresponding field (105–106 V/cm) causes the scattering of such a beam along its length. This needs a full charge neutralization of ion beams in the outlet from the A-K gap. The most effective ways of full charge neutralization of ion beams are ion transfer through dense plasma and the auto neutralization of the ion flow, while PIB transiting through a transparent cathode (Humphries, Reference Humphries1990).
For measuring the charge neutralization of PIB, formed by a focusing diode, we used a Faraday cup without an electron cut-off. This method was used in the work of Yatsui et al. (Reference Yatsui, Tokuchi, Tanaka, Ishizuka, Kawai, Sai, Masugata, Ito and Matsui1985). The Faraday cup is made from a demountable connector CP50-812, the collector diameter is 8 mm, and the diameter of the collimating aperture in the cover is 4 mm. An electron current was synchronically registered by two Faraday cups placed in a focal plane. The construction of the Faraday cup and waveforms are given in Figure 4.
(Color online) The construction of a Faraday cup without electron cut-off and waveforms of the accelerating voltage (the second pulse), ion current density, compensated PIB current density, current density, measured by a Faraday cup, covered by Al foil with a thickness of 10 micrometers.
Ion current density was measured with the help of a collimated Faraday cup with electron magnetic cut-off (B = 0.4 T). The investigation showed that the electron concentration in a focus exceeds the ion concentration 1.3–1.5 times in a strip focusing self-magnetically insulated diode. In the self-magnetically insulated ion diode ions acceleration appears between the layer of explosive emission plasma on the anode surface and the layer of drifting electrons near the grounded electrode surface. In the investigated diodes the drift layer thickness is 0.3–0.5 mm, and the electron concentration in the drifting area is (3–5)·1014 cm−3. In this case, electron energy does not exceed 50 keV. With an ion current density of 30–40 A/cm2 and accelerating voltage of 250 kV, the ion concentration in PIB does not exceed 1012 cm−3. These ions pass through a thick layer of drifting low-energy electrons, which provides effective neutralization of the PIB.
4. USE OF A SHIELD IN A FOCUSING DIODE
4.1. Investigation of a Strip Focusing Diode
For increasing focus effectiveness and preventing ion loss in the course of transfer to the target, we used a metal shield installed on a grounded electrode. Figure 5 shows the strip focusing diode with a shield and density distribution of the PIB in vertical section in a focal plane.
(Color online) Photo of a strip focusing diode with a shield and energy-density distribution of PIB, formed by a diode with a shield and without any shield.
The shield is made from stainless steel with a thickness of 1 mm. Skin-layer thickness in the shield for a magnetic field, formed by the current passing through the diode's electrodes, is 120 micrometers. Figure 6 shows an infrared image on the target of an ion beam, formed by a strip focusing self-magnetically insulated diode. The distance to the diode is 15 cm. Each heat pattern is obtained for one pulse generation of PIB.
(Color online) Thermograms of the PIB, formed by a focusing diode without shield (a) and with shield (b).
The studies performed showed that PIB width on the half-height decreases from 60 mm to 40–42 mm with the use of a shield. PIB divergence decreases from 11° to 7.5–8°. At the same time, a metal shield does not change the effectiveness of ion beam generation. Figure 7 shows the energy balance in the diode assembly.
(Color online) Energy dependence of the extracted beam on the energy supplied to the diode, for a spiral diode and for a strip diode with a shield and without any shield.
The total energy of the beam was calculated by integrating the energy density profile of the beam, which we obtained from the thermal imprint of the beam on a thin metal target monitored with an infrared camera. The energy supplied to the diode was calculated by multiplying the measured diode current and diode voltage, and integrating the product over time (see Fig. 1). Figure 7 shows data for a strip focusing diode with and without a shield (experimental points). Performed studies showed that the increase in PIB energy density, formed by a strip focusing diode with a shield, is obtained because of PIB focus improving without any changes in full beam energy in a pulse.
In the ion diode, operating in a double-pulse mode, a thermal image on the target may be formed by accelerated electrons having reached the target in the first pulse, and ions during the second pulse. An additional shield may not only improve ion beam focusing, but also contribute to the transfer of electrons to the target. Thermal imaging diagnostics do not allow dividing the ion/electron contribution into target heating. For measuring the electron current density, accelerated in the A-K gap of a strip diode and having reached the target, we used a Faraday cup without a magnetic cut-off (see Fig. 4). The electron current was synchronically registered by two Faraday cups, placed in a focal plane. Typical oscillograms of the accelerating voltage and current are shown in Figure 8.
(Color online) Waveforms of the accelerating voltage, first pulse and current density, registered by a Faraday cups in a focal plane in the PIB center and at a distance of 7 cm from the center.
The mean electron energy density in the diode focus (integral of voltage multiplied by the electron current density in the course of the first pulse) does not exceed 0.04 J/cm2. This is less than the PIB energy density in the focus of a strip focusing diode (4–5 J/cm2), see Figure 5.
4.2. Ion Beam Focusing in a Cone Diode
The use of a metal shield allows eliminating the scattering of PIB, formed by a cone diode. Figure 9 shows an infrared image on the target of an ion beam, formed by a cone diode of different constructions, and Figure 10 demonstrates the energy density distribution in a horizontal section (one pulse).
(Color online) Thermograms of the PIB, formed by a cone diode without shield (a) and with shield (b).
(Color online) Energy density distribution of PIB, formed by a cone diode with a shield and without any shield.
The characteristic property of a cone diode construction is a near placement at the beginning and end of the diode (see Fig. 2). For effective PIB focusing it is necessary to close a metal shield. But with a short circuit of the self-magnetically insulated diode's beginning and end, the electron component of the full-load current increases considerably, and it passes in a short-circuit condition (Pushkarev et al., Reference Pushkarev, Isakova and Guselnikov2011). If the shield is open, PIB is formed with high section heterogeneity (Fig. 9a). Effective ion beam focusing, formed by a cone diode, was obtained by joining the shield's ends by a thin wire. Its inductivity limited the current in the shield and allowed optimizing the transfer conditions of PIB.
5. METAL SHIELD INFLUENCE ON ION BEAM TRANSFER IN A SPIRAL DIODE
The use of a metal shield allows improving the transference of PIB, formed by a spiral self-magnetically insulated diode. Figure 11 shows a diode construction without a shield and an infrared image on the target of an ion beam (one pulse).
(Color online) Photo of a spiral diode and PIB infrared image, formed by a diode without any shield.
With the absence of a metal shield in a spiral diode, PIB is formed with high cross-section heterogeneity. Energy distribution is unstable from pulse to pulse. The effectiveness of PIB generation in a spiral ion diode without any shield equals 20% (Fig. 7).
With the use of a shield in a spiral diode (Fig. 12) the ion beam becomes more homogeneous. Figure 12 shows an infrared image of typically occurring PIB, formed by a spiral diode with a shield. The shield is made from brass foil with a thickness of 220 micrometers (skin-layer is 84 micrometers).
(Color online) Infrared image of PIB, formed by a spiral diode with a shield with a first pulse duration of 410 ns (a) and 540 ns (b).
Figure 13 shows the energy density distribution of PIB formed by a spiral diode.
(Color online) Energy density distribution of PIB in horizontal section with first pulse durations of 410 ns and 540 ns.
If the metal shield has slits, the effect of PIB focusing gets worse. Figure 14 shows the construction of a spiral diode and an ion beam heat pattern. The distance from the target is 9 cm, and the A-K gap is 7.5 mm.
(Color online) Photo of a spiral diode with a shield and infrared image of a PIB.
As well as with the absence of a metal shield, in a spiral diode PIB is formed with high section heterogeneity, and energy distribution is unstable from pulse to pulse.
6. DISCUSSION
The conducted researches showed that an ion beam formed by a self-magnetically insulated diode was fully neutralized by thermal electrons and n e » n i. Therefore we may consider it as a plasma object. With an electromagnetic field frequency less than the frequency of the Langmuir oscillations, a magnetic field penetrates into the plasma by a skin-layer depth, the amount of which equals (if n e in cm−3) (Morozov, Reference Morozov2006):
$${\rm \lambda}_e = \displaystyle{5.31 \cdot 10^5 \over \sqrt{n_e}}\comma \; \quad cm$$is possible to calculate the electron concentration, which is equal to the ion concentration in the PIB, according to the relation:
$$n_e = n_i = \displaystyle{\,j \over z \cdot v_i}$$where j is the ion current density, z is an ion charge, v i is the ion velocity.
It is possible to determine the ion velocity after acceleration in an A-K gap from the following relation:
$$v_i = \sqrt{\displaystyle{2Uz \over m_i}}$$In this case, the skin-layer thickness equals:
$${\rm \lambda}_e = 134 \displaystyle{U^{1/4} \over \sqrt{\,j}}\comma \; \quad mm$$with the accelerating voltage in volts and ion current density in A/m2.
Figure 15 shows the calculation results for skin-layer thickness changes during PIB generation.
(Color online) Waveforms of the accelerating voltage, the second pulse and skin-layer thickness changes while PIB generating.
The performed analyses showed that in our experimental conditions a magnetic field penetrates the plasma of a neutralized ion beam to a considerable depth, pulling down in 2.7 times only in the depth of 6 mm with PIB diameter of 40–45 mm. Therefore it is important to provide the absence of the electromagnetic field in the ion drifting area for effective transfer and ion beam focusing. In the initial construction of a self-magnetically insulated diode a grounded electrode had side shields with slits (Figs. 1 and 14). These shields only provided a partial decay of the magnetic field, formed around the grounded electrode of the diode in the ion transfer area. The use of an additional metal shield improves the shielding of the ion transfer area, prevents the loss of low-energy electrons from the PIB and disbalances its electrical neutrality.
7. CONCLUSION
Complex research was conducted into the influence of metal shields with different constructions on PIB focusing, formed by various diodes with self-magnetic insulation. It is observed that electron concentration in a strip focusing diode exceeds the ion concentration by 1.3–1.5 times. It indicates the effective neutralization of PIB and absence of its scattering due to Coulomb repulsion. The energy of the following electrons does not exceed 40–50 keV. The study showed that in a focal plane the width of PIB, formed by a strip focusing diode, decreases its half-height from 60 mm (a diode without any shield) to 40–42 mm with the use of the shield. PIB divergence decreases from 11° to 7.5°–8°. The modeling showed that under our experimental conditions a magnetic field penetrates into the plasma of a neutralized ion beam to a considerable depth, pulling down in 2.7 times only in the depth of 6 mm with a PIB diameter of 40–46 mm. The use of a metal shield improves the shielding of the ion transfer area, prevents the loss of low-energy electrons from the PIB and breaks down its electrical neutrality.
ACKNOWLEDGMENT
The research is conducted with the support of the Russian Fundamental Research Fund, within scientific project no. 12-08-00118-a.
