The familiar principle βmore is more powerfulβ has long been established in many sectors of society, including science and technology. However, in modern realities, more and more often there is a practical implementation of the saying "small, but daring." This manifests itself both in computers that previously took up an entire room, and now fit in the palm of a child, and in particle accelerators. Yes, yes, remember the Large Hadron Collider (LHC), whose impressive dimensions (26 m in length) are literally indicated in its name? Well, this is already in the past, according to scientists from DESY, who have developed a miniature version of the accelerator, which in terms of performance is not inferior to its full-sized predecessor. Moreover, the mini accelerator even set a new world record among terahertz accelerators by doubling the energy of implanted electrons. How was the miniature accelerator developed, what are the main principles of its operation, and what have practical experiments shown? The report of the research group will help us learn about this. Go.
Research basis
According to Dongfang Zhang and his colleagues at DESY (German Electron Synchrotron), who developed the mini-accelerator, ultrafast electron sources play an incredibly important role in modern society. Many of them are manifested in medicine, electronics development and scientific research. The biggest problem with current linacs using RF oscillators is their high cost, infrastructure complexity, and huge appetites for power consumption. And such shortcomings severely limit the availability of such technologies to a wider range of users.
These obvious problems are a great incentive to develop devices that will not be intimidating in terms of size, as well as the degree of power consumption.
Among the relative novelties in this industry, terahertz accelerators can be distinguished, which have a number of "goodies":
- it is expected that short waves and short pulses of terahertz radiation will allow a significant increase in the threshold breakdown*, caused by the field, which will increase the acceleration gradients;
Electrical breakdown* - a sharp increase in current strength when a voltage is applied above the critical one.
- the availability of efficient methods for generating high-field terahertz radiation makes it possible to carry out internal synchronization between electrons and excitation fields;
- classical methods can be used to create such devices, but their cost, production time and size will be greatly reduced.
The scientists believe that their millimeter-scale terahertz accelerator is a compromise between conventional accelerators that are currently available and micro-accelerators that are being developed, but have many disadvantages due to their already very small dimensions.
The researchers do not deny that terahertz acceleration technology has been in development for some time. However, in their opinion, there are still many aspects in this area that have not been studied, tested or implemented.
In their work, which we are considering today, scientists demonstrate the possibilities of STEAM (segmented terahertz electron accelerator and manipulator) is a segmented terahertz electron accelerator and manipulator. STEAM makes it possible to reduce the length of the electron beam to sub-picosecond duration, thus providing femtosecond control over the acceleration phase.
It was possible to achieve an acceleration field of 200 MV/m (MV - megavolt), which leads to a record terahertz acceleration of > 70 keV (kiloelectronvolt) from an implanted electron beam with an energy of 55 keV. In this way, accelerated electrons up to 125 keV were obtained.
Device structure and implementation
Image #1: Scheme of the device under study.
Image No. 1-2: a - a diagram of the developed 5-layer segmented structure, b - the ratio of the calculated acceleration and the direction of electron propagation.
Electron beams (55 keV) are generated from electron gun* and are introduced into the terahertz STEAM-buncher (beam compressor), after which they are transferred to STEAM-linac (linear accelerator*).
Electron gun* β a device for generating an electron beam of the required configuration and energy.
Linear accelerator* - an accelerator in which charged particles pass through the structure only once, which distinguishes a linear accelerator from a cyclic one (for example, the LHC).
Both STEAM devices receive terahertz pulses from a single near infrared (NIR) laser, which also fires the electron gun's photocathode, resulting in internal synchronization between the electrons and the accelerating fields. Ultraviolet pulses for photoemission at the photocathode are generated through two successive stages GVG* fundamental wavelength of near infrared light. This process converts a 1020 nm laser pulse first to 510 nm and then to 255 nm.
GVG* (second harmonic generation) - the process of combining photons with the same frequency during interaction with a nonlinear material, which leads to the formation of new photons with twice the energy and frequency, as well as half the wavelength.
The remaining part of the NIR laser beam is divided into 4 beams, which are used to generate four single-cycle terahertz pulses by generating an intra-pulse frequency difference.
Two terahertz pulses then enter each STEAM device through symmetrical horn structures that direct terahertz energy into the interaction region across the direction of electron propagation.
As electrons enter each of the STEAM devices, they are exposed to electrical and magnetic components. Lorentz forces*.
Lorentz force* is the force with which an electromagnetic field acts on a charged particle.
In this case, the electric field is responsible for acceleration and deceleration, while the magnetic field causes lateral deflections.
Image #2
As we see in the pictures 2a ΠΈ 2b, inside each STEAM device, the terahertz beams are separated transversely by thin metal sheets into several layers of different thickness, each of which acts as a waveguide, transferring part of the total energy to the interaction region. Also, in each layer there are dielectric plates to match the arrival time of the terahertz signal. wave front* with an electron front.
Wavefront* is the surface to which the wave has reached.
Both STEAM devices operate in electrical mode, that is, in such a way as to produce the imposition of an electric field and the suppression of a magnetic field in the center of the interaction area.
In the first device, the electrons are timed to pass through zero crossing* terahertz field, where the temporal gradients of the electric field are maximized and the average field is minimized.
Zero crossing* - the point where there is no tension.
This configuration causes the electron beam tail to accelerate and its head to decelerate, resulting in ballistic longitudinal focusing (2a ΠΈ 2s).
In the second device, the synchronization of the electron and terahertz radiation is set so that the electron beam experiences only the negative cycle of the terahertz electric field. This configuration results in a net continuous acceleration (2b ΠΈ 2d).
The NIR laser resembles a cryogenically cooled Yb:YLF system that produces optical pulses with a duration of 1.2 ps and an energy of 50 mJ at a wavelength of 1020 nm and a repetition rate of 10 Hz. And terahertz pulses with a center frequency of 0.29 terahertz (a period of 3.44 ps) are generated by the tilted pulse front method.
Only 2 x 50 nJ terahertz energy was used to power the STEAM-buncher (beam compressor), while 2 x 15 mJ was required for the STEAM-linac (linear accelerator).
The diameter of the inlet and outlet of both STEAM devices is 120 Β΅m.
The beam compressor is designed with three layers of the same height (0 mm), which are equipped with fused silica plates (Ο΅r =225) of length 4.41 and 0.42 mm for timing control. The equal heights of the compressor layers reflect the fact that no acceleration occurs (2s).
But in the linear accelerator, the heights are already different - 0.225, 0.225 and 0.250 mm (+ fused quartz plates 0.42 and 0.84 mm). The increase in the layer height explains the increase in the speed of electrons during acceleration.
Scientists note that the number of layers is directly responsible for the functionality of each of the two devices. To achieve a higher degree of acceleration, for example, more layers and a different height configuration will be required to optimize interaction.
Results of practical experiments
First of all, the researchers remind that in traditional RF-based accelerators, the influence of the temporal extent of the implanted electron beam on the properties of the accelerated beam is associated with a change in the electric field experienced during the interaction by different electrons within the beam arriving at different times. Thus, it can be assumed that fields with a large gradient and beams with a longer duration will lead to a larger energy spread. Long-duration injected beams can also lead to higher values emittances*.
Emittance* is the phase space occupied by an accelerated beam of charged particles.
In the case of a terahertz accelerator, the period of the excitation field is about 200 times shorter. Hence, tension* supported field will be 10 times higher.
Electric field strength* - an indicator of the electric field, equal to the ratio of the force applied to a fixed point charge placed at a given point in the field to the magnitude of this charge.
Thus, in a terahertz accelerator, the field gradients experienced by electrons can be several orders of magnitude higher than in a conventional device. The time scale on which the curvature of the field is noticeable will be much smaller in this case. It follows from this that the duration of the injected electron beam will have a more pronounced effect.
Scientists in practice decided to test these theories. To do this, they introduced electron beams of different duration, which was controlled by compression due to the first STEAM device (STEAM-buncher).
Image #3
In the case when the compressor was not connected to a power source, electron beams (55 keV) with a charge of βΌ1 fC (femtocoulomb) traveled approximately 300 mm from the electron gun to the linear accelerator device (STEAM-linac). These electrons could expand under the action of space charge forces up to a duration of more than 1000 fs (femtoseconds).
With this duration, the electron beam occupied about 60% of the half-wave of the accelerating field with a frequency of 1,7 ps, which led to an energy spectrum after acceleration with a peak at 115 keV and an energy distribution half-width of more than 60 keV (3a).
To compare these results with the expected ones, the situation of electron propagation through a linear accelerator was simulated, when the electrons were out of sync (ie, do not match) with respect to the optimal injection time. Calculations of such a situation showed that the increase in the electron energy is very dependent on the moment of introduction up to the subpicosecond time scale (3b). That is, with optimal tuning, the electron will experience a full half-cycle of terahertz radiation acceleration in each layer (3s).
If the electrons arrive at different times, then they experience less acceleration in the first layer, from which they need more time to pass through it. The desynchronization then increases in subsequent layers, causing unwanted slowdown (3d).
In order to minimize the negative effect of the temporal length of the electron beam, the first STEAM device operated in compression mode. The electron beam duration at the linac was optimized to a minimum of ~350 fs (half-width) by tuning the terahertz energy supplied to the compressor and switching the linac to hatching mode (4b).
Image #4
The minimum beam duration was set in accordance with the duration of the photocathode UV pulse, the duration of which was ~600 fs. The distance between the compressor and the strip also played an important role, which limited the speed of the thickening force. Taken together, these measures make it possible to ensure the femtosecond accuracy of the injection phase at the acceleration stage.
On the image 4a It can be seen that the energy spread of the compressed electron beam after optimized acceleration in the linear accelerator decreases by a factor of ~4 compared to the uncompressed one. Due to acceleration, the energy spectrum of the compressed beam is shifted towards higher energies, in contrast to the uncompressed beam. The peak of the energy spectrum after acceleration is about 115 keV, and the high-energy tail reaches about 125 keV.
These indicators, according to the modest statement of scientists, are a new acceleration record (before acceleration it was 70 keV) in the terahertz range.
But in order to reduce the energy spread (4a), it is necessary to achieve an even shorter beam.
Image #5
In the case of an uncompressed injected beam, the parabolic current dependence of the beam size reveals the transverse emittance in the horizontal and vertical directions: Ξ΅x,n = 1.703 mm*mrad and Ξ΅y,n = 1.491 mm*mrad (5a).
Compression, in turn, improved the transverse emittance by a factor of 6 to Ξ΅x,n = 0,285 mm*mrad (horizontal) and Ξ΅y,n = 0,246 mm*mrad (vertical).
It is worth noting that the degree of decrease in the emittance is about twice as large as the degree of reduction in the beam duration, which is a measure of the nonlinearity of the interaction dynamics with time, when the electrons experience strong focusing and defocusing of the magnetic field during acceleration (5b ΠΈ 5s).
On the image 5b it can be seen that the electrons introduced at the optimal time experience the entire half-cycle of the electric field acceleration. But electrons that arrive before or after the optimal time point experience less acceleration and even partial deceleration. Such electrons as a result receive less energy, roughly speaking.
A similar situation is observed under the influence of a magnetic field. Electrons injected at the optimum time experience a symmetrical amount of positive and negative magnetic fields. If the introduction of electrons occurred before the optimal time, then there were more positive fields and fewer negative ones. In the case of introduction of electrons later than the optimal time, there are fewer positive and more negative (5s). And such deviations lead to the fact that the electron can deviate to the left, right, up or down, depending on the position relative to the axis, which leads to an increase in the transverse momentum corresponding to the focusing or defocusing of the beam.
For a more detailed acquaintance with the nuances of the study, I recommend looking at ΠΈ to him.
Finale
Summing up, the productivity of the accelerator will increase if the duration of the electron beam is reduced. In this paper, the achievable beam duration was limited by the setup geometry. But, in theory, the beam duration can reach less than 100 fs.
The scientists also note that the quality of the beam can be further improved by reducing the height of the layers and increasing their number. However, this method is not without problems, in particular the increased complexity of manufacturing the device.
This work is the initial stage of a more extensive and detailed study of a miniature version of a linear accelerator. Despite the fact that the tested version already shows excellent results, which can rightly be called a record, there is still a lot of work.
Thank you for your attention, stay curious and have a great week everyone! π
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Source: habr.com