304L 6.35*1mm Stainless steel coiled tubing suppliers ,Demonstration of an intense lithium beam for generating pulsed direct neutrons

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As an alternative to the study of nuclear reactors, a compact accelerator-driven neutron generator using a lithium-ion beam driver may be a promising candidate because it produces little unwanted radiation. However, it was difficult to deliver an intense beam of lithium ions, and the practical application of such devices was considered impossible. The most acute problem of insufficient ion flow was solved by applying a direct plasma implantation scheme. In this scheme, a high-density pulsed plasma generated by laser ablation of a lithium metal foil is efficiently injected and accelerated by a high-frequency quadrupole accelerator (RFQ accelerator). We have achieved a peak beam current of 35 mA accelerated to 1.43 MeV, which is two orders of magnitude higher than conventional injector and accelerator systems can provide.
Unlike X-rays or charged particles, neutrons have a large penetration depth and unique interaction with condensed matter, making them extremely versatile probes for studying the properties of materials1,2,3,4,5,6,7. In particular, neutron scattering techniques are commonly used to study the composition, structure, and internal stresses in condensed matter and can provide detailed information on trace compounds in metal alloys that are difficult to detect using X-ray spectroscopy8. This method is considered a powerful tool in basic science and is used by manufacturers of metals and other materials. More recently, neutron diffraction has been used to detect residual stresses in mechanical components such as rail and aircraft parts9,10,11,12. Neutrons are also used in oil and gas wells because they are easily captured by proton-rich materials13. Similar methods are also used in civil engineering. Non-destructive neutron testing is an effective tool for detecting hidden faults in buildings, tunnels and bridges. The use of neutron beams is actively used in scientific research and industry, many of which have historically been developed using nuclear reactors.
However, with the global consensus on nuclear non-proliferation, building small reactors for research purposes is becoming increasingly difficult. Moreover, the recent Fukushima accident has made building nuclear reactors almost socially acceptable. In connection with this trend, the demand for neutron sources at accelerators is growing2. As an alternative to nuclear reactors, several large accelerator-splitting neutron sources are already in operation14,15. However, for a more efficient use of the properties of neutron beams, it is necessary to expand the use of compact sources at accelerators, 16 which may belong to industrial and university research institutions. Accelerator neutron sources have added new capabilities and functions in addition to serving as a replacement for nuclear reactors14. For example, a linac-driven generator can easily create a stream of neutrons by manipulating the drive beam. Once emitted, neutrons are difficult to control and radiation measurements are difficult to analyze due to the noise created by background neutrons. Pulsed neutrons controlled by an accelerator avoid this problem. Several projects based on proton accelerator technology have been proposed around the world17,18,19. The reactions 7Li(p, n)7Be and 9Be(p, n)9B are most frequently used in proton-driven compact neutron generators because they are endothermic reactions20. Excess radiation and radioactive waste can be minimized if the energy chosen to excite the proton beam is slightly above the threshold value. However, the mass of the target nucleus is much larger than that of protons, and the resulting neutrons scatter in all directions. Such close to isotropic emission of a neutron flux prevents efficient transport of neutrons to the object of study. In addition, to obtain the required dose of neutrons at the location of the object, it is necessary to significantly increase both the number of moving protons and their energy. As a result, large doses of gamma rays and neutrons will propagate through large angles, destroying the advantage of endothermic reactions. A typical accelerator-driven compact proton-based neutron generator has strong radiation shielding and is the bulkiest part of the system. The need to increase the energy of driving protons usually requires an additional increase in the size of the accelerator facility.
To overcome the general shortcomings of conventional compact neutron sources at accelerators, an inversion-kinematic reaction scheme was proposed21. In this scheme, a heavier lithium-ion beam is used as a guide beam instead of a proton beam, targeting hydrogen-rich materials such as hydrocarbon plastics, hydrides, hydrogen gas, or hydrogen plasma. Alternatives have been considered, such as beryllium ion-driven beams, however, beryllium is a toxic substance requiring special care in handling. Therefore, a lithium beam is the most suitable for inversion-kinematic reaction schemes. Since the momentum of lithium nuclei is greater than that of protons, the center of mass of nuclear collisions is constantly moving forward, and neutrons are also emitted forward. This feature greatly eliminates unwanted gamma rays and high angle neutron emissions22. A comparison of the usual case of a proton engine and the inverse kinematics scenario is shown in Figure 1.
Illustration of neutron production angles for proton and lithium beams (drawn with Adobe Illustrator CS5, 15.1.0, https://www.adobe.com/products/illustrator.html). (a) Neutrons can be ejected in any direction as a result of the reaction due to the fact that moving protons hit the much heavier atoms of the lithium target. (b) Conversely, if a lithium-ion driver bombards a hydrogen-rich target, neutrons are generated in a narrow cone in the forward direction due to the high velocity of the system’s center of mass.
However, only a few inverse kinematic neutron generators exist due to the difficulty of generating the required flux of heavy ions with a high charge compared to protons. All of these plants use negative sputter ion sources in combination with tandem electrostatic accelerators. Other types of ion sources have been proposed to increase the efficiency of beam acceleration26. In any case, the available lithium-ion beam current is limited to 100 µA. It has been proposed to use 1 mA of Li3+27, but this ion beam current has not been confirmed by this method. In terms of intensity, lithium beam accelerators cannot compete with proton beam accelerators whose peak proton current exceeds 10 mA28.
To implement a practical compact neutron generator based on a lithium-ion beam, it is advantageous to generate high-intensity completely devoid of ions. The ions are accelerated and guided by electromagnetic forces, and a higher charge level results in more efficient acceleration. Li-ion beam drivers require Li3+ peak currents in excess of 10 mA.
In this work, we demonstrate the acceleration of Li3+ beams with peak currents up to 35 mA, which is comparable to advanced proton accelerators. The original lithium ion beam was created using laser ablation and a Direct Plasma Implantation Scheme (DPIS) originally developed to accelerate C6+. A custom-designed radio frequency quadrupole linac (RFQ linac) was fabricated using a four-rod resonant structure. We have verified that the accelerating beam has the calculated high purity beam energy. Once the Li3+ beam is effectively captured and accelerated by the radio frequency (RF) accelerator, the subsequent linac (accelerator) section is used to provide the energy needed to generate a strong neutron flux from the target.
The acceleration of high performance ions is a well established technology. The remaining task of realizing a new highly efficient compact neutron generator is to generate a large number of completely stripped lithium ions and form a cluster structure consisting of a series of ion pulses synchronized with the RF cycle in the accelerator. The results of experiments designed to achieve this goal are described in the following three subsections: (1) generation of a completely devoid of lithium-ion beam, (2) beam acceleration using a specially designed RFQ linac, and (3) acceleration of analysis of the beam to check its contents. At Brookhaven National Laboratory (BNL), we built the experimental setup shown in Figure 2.
Overview of the experimental setup for accelerated analysis of lithium beams (illustrated by Inkscape, 1.0.2, https://inkscape.org/). From right to left, laser-ablative plasma is generated in the laser-target interaction chamber and delivered to the RFQ linac. Upon entering the RFQ accelerator, the ions are separated from the plasma and injected into the RFQ accelerator through a sudden electric field created by a 52 kV voltage difference between the extraction electrode and the RFQ electrode in the drift region. The extracted ions are accelerated from 22 keV/n to 204 keV/n using 2 meter long RFQ electrodes. A current transformer (CT) installed at the output of the RFQ linac provides non-destructive measurement of the ion beam current. The beam is focused by three quadrupole magnets and directed to a dipole magnet, which separates and directs the Li3+ beam into the detector. Behind the slit, a retractable plastic scintillator and a Faraday cup (FC) with a bias of up to -400 V are used to detect the accelerating beam.
To generate fully ionized lithium ions (Li3+), it is necessary to create a plasma with a temperature above its third ionization energy (122.4 eV). We tried to use laser ablation to produce high-temperature plasma. This type of laser ion source is not commonly used to generate lithium ion beams because lithium metal is reactive and requires special handling. We have developed a target loading system to minimize moisture and air contamination when installing lithium foil in the vacuum laser interaction chamber. All preparations of materials were carried out in a controlled environment of dry argon. After the lithium foil was installed in the laser target chamber, the foil was irradiated with pulsed Nd:YAG laser radiation at an energy of 800 mJ per pulse. At the focus on the target, the laser power density is estimated to be about 1012 W/cm2. Plasma is created when a pulsed laser destroys a target in a vacuum. During the entire 6 ns laser pulse, the plasma continues to heat up, mainly due to the reverse bremsstrahlung process. Since no confining external field is applied during the heating phase, the plasma begins to expand in three dimensions. When the plasma begins to expand over the target surface, the center of mass of the plasma acquires a velocity perpendicular to the target surface with an energy of 600 eV/n. After heating, the plasma continues to move in the axial direction from the target, expanding isotropically.
As shown in Figure 2, the ablation plasma expands into a vacuum volume surrounded by a metal container with the same potential as the target. Thus, the plasma drifts through the field-free region towards the RFQ accelerator. An axial magnetic field is applied between the laser irradiation chamber and the RFQ linac by means of a solenoid coil wound around the vacuum chamber. The magnetic field of the solenoid suppresses the radial expansion of the drifting plasma in order to maintain a high plasma density during delivery to the RFQ aperture. On the other hand, the plasma continues to expand in the axial direction during the drift, forming an elongated plasma. A high voltage bias is applied to the metal vessel containing the plasma in front of the exit port at the RFQ inlet. The bias voltage was chosen to provide the required 7Li3+ injection rate for proper acceleration by the RFQ linac.
The resulting ablation plasma contains not only 7Li3+, but also lithium in other charge states and pollutant elements, which are simultaneously transported to the RFQ linear accelerator. Prior to accelerated experiments using the RFQ linac, an offline time-of-flight (TOF) analysis was performed to study the composition and energy distribution of ions in the plasma. The detailed analytical setup and observed state-of-charge distributions are explained in the Methods section. The analysis showed that 7Li3+ ions were the main particles, accounting for about 54% of all particles, as shown in Fig. 3. According to the analysis, the 7Li3+ ion current at the ion beam output point is estimated at 1.87 mA. During accelerated tests, a 79 mT solenoid field is applied to the expanding plasma. As a result, the 7Li3+ current extracted from the plasma and observed on the detector increased by a factor of 30.
Fractions of ions in laser-generated plasma obtained by time-of-flight analysis. The 7Li1+ and 7Li2+ ions make up 5% and 25% of the ion beam, respectively. The detected fraction of 6Li particles agrees with the natural content of 6Li (7.6%) in the lithium foil target within the experimental error. A slight oxygen contamination (6.2%) was observed, mainly O1+ (2.1%) and O2+ (1.5%), which may be due to oxidation of the surface of the lithium foil target.
As previously mentioned, the lithium plasma drifts in a fieldless region before entering the RFQ linac. The input of the RFQ linac has a 6 mm diameter hole in a metal container, and the bias voltage is 52 kV. Although the RFQ electrode voltage changes rapidly ±29 kV at 100 MHz, the voltage causes axial acceleration because the RFQ accelerator electrodes have an average potential of zero. Due to the strong electric field generated in the 10 mm gap between the aperture and the edge of the RFQ electrode, only positive plasma ions are extracted from the plasma at the aperture. In traditional ion delivery systems, ions are separated from the plasma by an electric field at a considerable distance in front of the RFQ accelerator and then focused into the RFQ aperture by a beam focusing element. However, for the intense heavy ion beams required for an intense neutron source, non-linear repulsive forces due to space charge effects can lead to significant beam current losses in the ion transport system, limiting the peak current that can be accelerated. In our DPIS, high-intensity ions are transported as a drifting plasma directly to the exit point of the RFQ aperture, so there is no loss of the ion beam due to space charge. During this demonstration, DPIS was applied to a lithium-ion beam for the first time.
The RFQ structure was developed for focusing and accelerating low energy high current ion beams and has become the standard for first order acceleration. We used RFQ to accelerate 7Li3+ ions from an implant energy of 22 keV/n to 204 keV/n. Although lithium and other particles with a lower charge in the plasma are also extracted from the plasma and injected into the RFQ aperture, the RFQ linac only accelerates ions with a charge-to-mass ratio (Q/A) close to 7Li3+.
On fig. Figure 4 shows the waveforms detected by the current transformer (CT) at the output of the RFQ linac and the Faraday cup (FC) after analyzing the magnet, as shown in fig. 2. The time shift between the signals can be interpreted as the difference in the time of flight at the location of the detector. The peak ion current measured at CT was 43 mA. In the RT position, the registered beam can contain not only ions accelerated to the calculated energy, but also ions other than 7Li3+, which are not sufficiently accelerated. However, the similarity of the ion current forms found by means of QD and PC indicates that the ion current mainly consists of accelerated 7Li3+, and the decrease in the peak value of the current on PC is caused by beam losses during ion transfer between QD and PC. Losses This is also confirmed by the envelope simulation. To accurately measure the 7Li3+ beam current, the beam is analyzed with a dipole magnet as described in the next section.
Oscillograms of the accelerated beam recorded in the detector positions CT (black curve) and FC (red curve). These measurements are triggered by the detection of laser radiation by a photodetector during laser plasma generation. The black curve shows the waveform measured on a CT connected to the RFQ linac output. Due to its proximity to the RFQ linac, the detector picks up 100 MHz RF noise, so a 98 MHz low pass FFT filter was applied to remove the 100 MHz resonant RF signal superimposed on the detection signal. The red curve shows the waveform at FC after the analytical magnet directs the 7Li3+ ion beam. In this magnetic field, apart from 7Li3+, N6+ and O7+ can be transported.
The ion beam after the RFQ linac is focused by a series of three quadrupole focusing magnets and then analyzed by dipole magnets to isolate impurities in the ion beam. A magnetic field of 0.268 T directs the 7Li3+ beams into the FC. The detection waveform of this magnetic field is shown as the red curve in Figure 4. The peak beam current reaches 35 mA, which is more than 100 times higher than a typical Li3+ beam produced in existing conventional electrostatic accelerators. The beam pulse width is 2.0 µs at full width at half maximum. The detection of a 7Li3+ beam with a dipole magnetic field indicates successful bunching and beam acceleration. The ion beam current detected by FC when scanning the magnetic field of the dipole is shown in Fig. 5. A clean single peak was observed, well separated from other peaks. Since all ions accelerated to the design energy by the RFQ linac have the same speed, ion beams with the same Q/A are difficult to separate by dipole magnetic fields. Therefore, we cannot distinguish 7Li3+ from N6+ or O7+. However, the amount of impurities can be estimated from neighboring charge states. For example, N7+ and N5+ can be easily separated, while N6+ may be part of the impurity and is expected to be present in about the same amount as N7+ and N5+. The estimated pollution level is about 2%.
Beam component spectra obtained by scanning a dipole magnetic field. The peak at 0.268 T corresponds to 7Li3+ and N6+. The peak width depends on the size of the beam on the slit. Despite broad peaks, 7Li3+ separates well from 6Li3+, O6+, and N5+, but poorly separates from O7+ and N6+.
At the location of the FC, the beam profile was confirmed with a plug-in scintillator and recorded with a fast digital camera as shown in Figure 6. The 7Li3+ pulsed beam with a current of 35 mA is shown to be accelerated to a calculated RFQ energy of 204 keV/n, which corresponds to 1.4 MeV , and transmitted to the FC detector.
Beam profile observed on a pre-FC scintillator screen (colored by Fiji, 2.3.0, https://imagej.net/software/fiji/). The magnetic field of the analytical dipole magnet was tuned to direct the acceleration of the Li3+ ion beam to the design energy RFQ. The blue dots in the green area are caused by defective scintillator material.
We achieved the generation of 7Li3+ ions by laser ablation of the surface of a solid lithium foil, and a high current ion beam was captured and accelerated with a specially designed RFQ linac using DPIS. At a beam energy of 1.4 MeV, the peak current of 7Li3+ reached on the FC after analysis of the magnet was 35 mA. This confirms that the most important part of the implementation of a neutron source with inverse kinematics has been implemented experimentally. In this part of the paper, the entire design of a compact neutron source will be discussed, including high energy accelerators and neutron target stations. The design is based on results obtained with existing systems in our laboratory. It should be noted that the peak current of the ion beam can be further increased by shortening the distance between the lithium foil and the RFQ linac. Rice. 7 illustrates the entire concept of the proposed compact neutron source at the accelerator.
Conceptual design of the proposed compact neutron source at the accelerator (drawn by Freecad, 0.19, https://www.freecadweb.org/). From right to left: laser ion source, solenoid magnet, RFQ linac, medium energy beam transfer (MEBT), IH linac, and interaction chamber for neutron generation. Radiation protection is provided primarily in the forward direction due to the narrowly directed nature of the produced neutron beams.
After the RFQ linac, further acceleration of the Inter-digital H-structure (IH linac)30 linac is planned. IH linacs use a π-mode drift tube structure to provide high electric field gradients over a certain range of speeds. The conceptual study was carried out based on 1D longitudinal dynamics simulation and 3D shell simulation. Calculations show that a 100 MHz IH linac with a reasonable drift tube voltage (less than 450 kV) and a strong focusing magnet can accelerate a 40 mA beam from 1.4 to 14 MeV at a distance of 1.8 m. Energy distribution at the end of the accelerator chain is estimated at ± 0.4 MeV, which does not significantly affect the energy spectrum of neutrons produced by the neutron conversion target. In addition, the beam emissivity is low enough to focus the beam into a smaller beam spot than would normally be required for a medium strength and size quadrupole magnet. In medium energy beam (MEBT) transmission between the RFQ linac and the IH linac, the beamforming resonator is used to maintain the beamforming structure. Three quadrupole magnets are used to control the size of the side beam. This design strategy has been used in many accelerators31,32,33. The total length of the entire system from the ion source to the target chamber is estimated to be less than 8 m, which can fit in a standard semi-trailer truck.
The neutron conversion target will be installed directly after the linear accelerator. We discuss target station designs based on previous studies using inverse kinematic scenarios23. Reported conversion targets include solid materials (polypropylene (C3H6) and titanium hydride (TiH2)) and gaseous target systems. Each goal has advantages and disadvantages. Solid targets allow precise thickness control. The thinner the target, the more accurate the spatial arrangement of neutron production. However, such targets may still have some degree of unwanted nuclear reactions and radiation. On the other hand, a hydrogen target can provide a cleaner environment by eliminating the production of 7Be, the main product of the nuclear reaction. However, hydrogen has a weak barrier ability and requires a large physical distance for sufficient energy release. This is slightly disadvantageous for TOF measurements. In addition, if a thin film is used to seal a hydrogen target, it is necessary to take into account the energy losses of gamma rays generated by the thin film and the incident lithium beam.
LICORNE uses polypropylene targets and the target system has been upgraded to hydrogen cells sealed with tantalum foil. Assuming a beam current of 100 nA for 7Li34, both target systems can produce up to 107 n/s/sr. If we apply this claimed neutron yield conversion to our proposed neutron source, then a lithium-driven beam of 7 × 10–8 C can be obtained for each laser pulse. This means that firing the laser just twice per second produces 40% more neutrons than LICORNE can produce in one second with a continuous beam. The total flux can be easily increased by increasing the excitation frequency of the laser. If we assume that there is a 1 kHz laser system on the market, the average neutron flux can easily be scaled up to about 7 × 109 n/s/sr.
When we use high repetition rate systems with plastic targets, it is necessary to control the heat generation on the targets because, for example, polypropylene has a low melting point of 145–175 °C and a low thermal conductivity of 0.1–0.22 W/m/K. For a 14 MeV lithium-ion beam, a 7 µm thick polypropylene target is sufficient to reduce the beam energy to the reaction threshold (13.098 MeV). Taking into account the total effect of ions generated by one laser shot on the target, the energy release of lithium ions through polypropylene is estimated at 64 mJ/pulse. Assuming that all the energy is transferred in a circle with a diameter of 10 mm, each pulse corresponds to a temperature rise of approximately 18 K/pulse. Energy release on polypropylene targets is based on the simple assumption that all energy losses are stored as heat, with no radiation or other heat losses. Since increasing the number of pulses per second requires the elimination of heat buildup, we can use strip targets to avoid energy release at the same point23. Assuming a 10 mm beam spot on a target with a laser repetition rate of 100 Hz, the scanning speed of the polypropylene tape would be 1 m/s. Higher repetition rates are possible if beam spot overlap is allowed.
We also investigated targets with hydrogen batteries, because stronger drive beams could be used without damaging the target. The neutron beam can be easily tuned by changing the length of the gas chamber and the hydrogen pressure inside. Thin metal foils are often used in accelerators to separate the gaseous region of the target from vacuum. Therefore, it is necessary to increase the energy of the incident lithium-ion beam in order to compensate for the energy losses on the foil. The target assembly described in report 35 consisted of an aluminum container 3.5 cm long with an H2 gas pressure of 1.5 atm. The 16.75 MeV lithium ion beam enters the battery through the air-cooled 2.7 µm Ta foil, and the energy of the lithium ion beam at the end of the battery is decelerated to the reaction threshold. To increase the beam energy of lithium-ion batteries from 14.0 MeV to 16.75 MeV, the IH linac had to be lengthened by about 30 cm.
The emission of neutrons from gas cell targets was also studied. For the aforementioned LICORNE gas targets, GEANT436 simulations show that highly oriented neutrons are generated inside the cone, as shown in Figure 1 in [37]. Reference 35 shows the energy range from 0.7 to 3.0 MeV with a maximum cone opening of 19.5° relative to the direction of propagation of the main beam. Highly oriented neutrons can significantly reduce the amount of shielding material at most angles, reducing the weight of the structure and providing greater flexibility in the installation of measurement equipment. From the point of view of radiation protection, in addition to neutrons, this gaseous target emits 478 keV gamma rays isotropically in the centroid coordinate system38. These γ-rays are produced as a result of 7Be decay and 7Li deexcitation, which occurs when the primary Li beam hits the input window Ta. However, by adding a thick 35 Pb/Cu cylindrical collimator, the background can be significantly reduced.
As an alternative target, one can use a plasma window [39, 40], which makes it possible to achieve a relatively high hydrogen pressure and a small spatial region of neutron generation, although it is inferior to solid targets.
We are investigating neutron conversion targeting options for the expected energy distribution and beam size of a lithium ion beam using GEANT4. Our simulations show a consistent distribution of neutron energy and angular distributions for hydrogen targets in the above literature. In any target system, highly oriented neutrons can be produced by an inverse kinematic reaction driven by a strong 7Li3+ beam on a hydrogen-rich target. Therefore, new neutron sources can be implemented by combining already existing technologies.
The laser irradiation conditions reproduced ion beam generation experiments prior to the accelerated demonstration. The laser is a desktop nanosecond Nd:YAG system with a laser power density of 1012 W/cm2, a fundamental wavelength of 1064 nm, a spot energy of 800 mJ, and a pulse duration of 6 ns. The spot diameter on the target is estimated at 100 µm. Because lithium metal (Alfa Aesar, 99.9% pure) is quite soft, the precisely cut material is pressed into the mold. Foil dimensions 25 mm × 25 mm, thickness 0.6 mm. Crater-like damage occurs on the surface of the target when a laser hits it, so the target is moved by a motorized platform to provide a fresh portion of the target’s surface with each laser shot. To avoid recombination due to residual gas, the pressure in the chamber was kept below the range of 10-4 Pa.
The initial volume of the laser plasma is small, since the size of the laser spot is 100 μm and within 6 ns after its generation. The volume can be taken as an exact point and expanded. If the detector is placed at a distance xm from the target surface, then the received signal obeys the relationship: ion current I, ion arrival time t, and pulse width τ.
The generated plasma was studied by the TOF method with FC and an energy ion analyzer (EIA) located at a distance of 2.4 m and 3.85 m from the laser target. The FC has a suppressor grid biased by -5 kV to prevent electrons. The EIA has a 90 degree electrostatic deflector consisting of two coaxial metal cylindrical electrodes with the same voltage but opposite polarity, positive on the outside and negative on the inside. The expanding plasma is directed into the deflector behind the slot and deflected by the electric field passing through the cylinder. Ions satisfying the relationship E/z = eKU are detected using a Secondary Electron Multiplier (SEM) (Hamamatsu R2362), where E, z, e, K, and U are the ion energy, state of charge, and charge are EIA geometric factors. electrons, respectively, and the potential difference between the electrodes. By changing the voltage across the deflector, one can obtain the energy and charge distribution of ions in the plasma. The sweep voltage U/2 EIA is in the range from 0.2 V to 800 V, which corresponds to an ion energy in the range from 4 eV to 16 keV per charge state.
The distributions of the charge state of the ions analyzed under the conditions of laser irradiation described in the section “Generation of fully stripped lithium beams” are shown in Figs. 8.
Analysis of the distribution of the state of charge of ions. Here is the ion current density time profile analyzed with EIA and scaled at 1 m from the lithium foil using the equation. (1) and (2). Use the laser irradiation conditions described in the “Generation of a Completely Exfoliated Lithium Beam” section. By integrating each current density, the proportion of ions in the plasma was calculated, shown in Figure 3.
Laser ion sources can deliver an intense multi-mA ion beam with a high charge. However, beam delivery is very difficult due to space charge repulsion, so it was not widely used. In the traditional scheme, ion beams are extracted from the plasma and transported to the primary accelerator along a beam line with several focusing magnets to shape the ion beam according to the pickup capability of the accelerator. In space charge force beams, the beams diverge non-linearly, and serious beam losses are observed, especially in the region of low velocities. To overcome this problem in the development of medical carbon accelerators, a new DPIS41 beam delivery scheme is proposed. We have applied this technique to accelerate a powerful lithium-ion beam from a new neutron source.
As shown in fig. 4, the space in which the plasma is generated and expanded is surrounded by a metal container. The enclosed space extends to the entrance to the RFQ resonator, including the volume inside the solenoid coil. A voltage of 52 kV was applied to the container. In the RFQ resonator, ions are pulled by potential through a 6 mm diameter hole by grounding the RFQ. The non-linear repulsive forces on the beam line are eliminated as the ions are transported in the plasma state. In addition, as mentioned above, we applied a solenoid field in combination with DPIS to control and increase the density of ions in the extraction aperture.
The RFQ accelerator consists of a cylindrical vacuum chamber as shown in fig. 9a. Inside it, four rods of oxygen-free copper are placed quadrupole-symmetrically around the beam axis (Fig. 9b). 4 rods and chambers form a resonant RF circuit. The induced RF field creates a time-varying voltage across the rod. Ions implanted longitudinally around the axis are held laterally by the quadrupole field. At the same time, the tip of the rod is modulated to create an axial electric field. The axial field splits the injected continuous beam into a series of beam pulses called a beam. Each beam is contained within a certain RF cycle time (10 ns). Adjacent beams are spaced according to the radio frequency period. In the RFQ linac, a 2 µs beam from a laser ion source is converted into a sequence of 200 beams. The beam is then accelerated to the calculated energy.
Linear accelerator RFQ. (a) (left) External view of the RFQ linac chamber. (b) (right) Four-rod electrode in the chamber.
The main design parameters of the RFQ linac are the rod voltage, resonant frequency, beam hole radius, and electrode modulation. Select the voltage on the rod ± 29 kV so that its electric field is below the electrical breakdown threshold. The lower the resonant frequency, the greater the lateral focusing force and the smaller the average acceleration field. Large aperture radii make it possible to increase the beam size and, consequently, increase the beam current due to the smaller space charge repulsion. On the other hand, larger aperture radii require more RF power to power the RFQ linac. In addition, it is limited by the quality requirements of the site. Based on these balances, the resonant frequency (100 MHz) and aperture radius (4.5 mm) were chosen for high-current beam acceleration. The modulation is chosen to minimize beam loss and maximize acceleration efficiency. The design has been optimized many times to produce an RFQ linac design that can accelerate 7Li3+ ions at 40 mA from 22 keV/n to 204 keV/n within 2 m. The RF power measured during the experiment was 77 kW.
RFQ linacs can accelerate ions with a specific Q/A range. Therefore, when analyzing a beam fed to the end of a linear accelerator, it is necessary to take into account isotopes and other substances. In addition, the desired ions, partially accelerated, but descended under acceleration conditions in the middle of the accelerator, can still meet lateral confinement and can be transported to the end. Unwanted rays other than engineered 7Li3+ particles are called impurities. In our experiments, 14N6+ and 16O7+ impurities were of the greatest concern, since the lithium metal foil reacts with oxygen and nitrogen in the air. These ions have a Q/A ratio that can be accelerated with 7Li3+. We use dipole magnets to separate beams of different quality and quality for beam analysis after the RFQ linac.
The beam line after the RFQ linac is designed to deliver the fully accelerated 7Li3+ beam to the FC after the dipole magnet. -400 V bias electrodes are used to suppress secondary electrons in the cup to accurately measure the ion beam current. With this optics, the ion trajectories are separated into dipoles and focused in different places depending on the Q/A. Due to various factors such as momentum diffusion and space charge repulsion, the beam at the focus has a certain width. The species can only be separated if the distance between the focal positions of the two ion species is greater than the beam width. To obtain the highest possible resolution, a horizontal slit is installed near the beam waist, where the beam is practically concentrated. A scintillation screen (CsI(Tl) from Saint-Gobain, 40 mm × 40 mm × 3 mm) was installed between the slit and the PC. The scintillator was used to determine the smallest slit that the designed particles had to pass through for optimal resolution and to demonstrate acceptable beam sizes for high current heavy ion beams. The beam image on the scintillator is recorded by a CCD camera through a vacuum window. Adjust the exposure time window to cover the entire beam pulse width.
Datasets used or analyzed in the current study are available from the respective authors upon reasonable request.
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