Small, yes, deleted: a miniature linear particle accelerator, setting a new record



    The familiar principle of “more is more powerful” has long been established in many sectors of society, including science and technology. However, in modern realities, the practical implementation of the saying "small, but clever." This is manifested both in computers that previously occupied the whole room, and now are placed in the palm of a child, and in charged particle accelerators. Yes, yes, do you remember the Large Hadron Collider (LHC), whose impressive dimensions (26,659 m in length) are literally indicated in its name? So, this is already in the past, according to DESY scientists who 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, doubling the energy of embedded electrons. How was the miniature accelerator developed, what are the basic principles of its operation, and what have practical experiments shown? This will help us learn the report of the research group. Go.

    Study basis


    According to Dongfang Zhang and his colleagues from DESY (German Electronic Synchrotron), who developed the mini-accelerator, ultrafast electron sources play an incredibly important role in the life of modern society. Many of them are manifested in medicine, the development of electronics and in scientific research. The biggest problem of current linear accelerators using radio frequency generators is their high cost, infrastructure complexity and impressive appetites for power consumption. And such shortcomings greatly limit the availability of such technologies for a wider range of users.

    These obvious problems are a great incentive to develop devices whose sizes will not cause horror, as well as the degree of power consumption.

    Among the relative innovations in this industry, we can distinguish terahertz accelerators, which have a number of "goodies":

    • it is expected that short waves and short pulses of terahertz radiation will significantly increase the breakdown threshold * caused by the field, which will increase the acceleration gradients;
    Electrical breakdown * - a sharp increase in current when a voltage is applied above the critical.
    • the presence of effective methods for generating high-field terahertz radiation allows 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.


    Scientists believe that their terahertz accelerator on a millimeter scale is a compromise between conventional accelerators, which are now available, and micro-accelerators, which are being developed, but have many shortcomings due to their very small dimensions.

    Researchers do not deny that terahertz acceleration technology has been under development for some time. However, in their opinion, there are still a lot of aspects in this area that have not been studied, verified or implemented.

    In their work, which we are considering today, scientists demonstrate the capabilities of STEAM ( segmented terahertz electron accelerator and manipulator) - a segmented terahertz electron accelerator and manipulator. STEAM reduces electron beam lengths to subpicosecond durations, thereby 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 (kiloelectron-volts) from an introduced electron beam with an energy of 55 keV. Thus, accelerated electrons up to 125 keV were obtained.

    The structure of the device and its implementation



    Image No. 1: diagram of the investigated device.


    Image No. 1-2: 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 the electron gun * and embedded in the terahertz STEAM-buncher (beam compressor), and then transferred to the STEAM-linac ( linear accelerator * ).
    Electron gun * - a device for generating an electron beam of the necessary configuration and energy.
    Linear accelerator * - an accelerator in which charged particles pass the structure only 1 time, which distinguishes a linear accelerator from a cyclic one (for example, LHC).
    Both STEAM devices receive terahertz pulses from a single near-infrared (NIR) laser, which also triggers the photocathode of the electron gun, resulting in internal synchronization between electrons and accelerating fields. Ultraviolet pulses for photoemission at the photocathode are generated through two successive stages of SHG * of the main wavelength of the near infrared light. This process converts a laser pulse with a wavelength of 1020 nm, first at 510 nm, and then at 255 nm.
    SHG * (second optical harmonic generation) is the process of combining photons with the same frequency during interaction with non-linear material, which leads to the formation of new photons with doubled energy and frequency, as well as half the wavelength.
    The remainder of the NIR laser beam is divided into 4 beams, which are used to generate four single-cycle terahertz pulses by generating a difference in the in-pulse frequencies.

    Two terahertz pulses then enter each STEAM device through symmetrical horn structures that direct terahertz energy into the interaction region across the electron propagation direction.

    When electrons enter each of the STEAM devices, they are exposed to the electrical and magnetic components of the Lorentz force * .
    Lorentz force * - the force with which an electromagnetic field acts on a charged particle.
    In this case, the electric field is responsible for acceleration and deceleration, and the magnetic field causes lateral deviations.


    Image No. 2

    As we see in images 2a and 2b , inside each STEAM device, terahertz beams are divided across thin metal sheets across into several layers of different thicknesses, each of which acts as a waveguide that transfers part of the total energy to the interaction region. Also, dielectric plates are present in each layer to match the arrival time of the terahertz wave front * with the electron front.
    The wavefront * is the surface to which the wave has reached.
    Both STEAM devices operate in electric mode, that is, in such a way as to superimpose an electric field and suppress a magnetic field in the center of the interaction region.

    In the first device, the electrons are timed so as to pass through the zero crossing * of the terahertz field, where the temporal gradients of the electric field are maximized and the average field is minimized.
    The zero crossing * is the point where there is no voltage.
    Such a configuration causes an acceleration of the tail of the electron beam and a deceleration of its head, which leads to ballistic longitudinal focusing ( 2a and 2c ).

    In the second device, the synchronization of the electron and terahertz radiation is set so that the electron beam experiences only a negative cycle of the terahertz electric field. This configuration leads to pure continuous acceleration ( 2b and 2d ).

    A laser with NIR radiation resembles a cryogenically cooled Yb: YLF system, which emits 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 central frequency of 0.29 terahertz (period of 3.44 ps) are generated by the sloping front of the pulse.

    Only 2 x 50 nJ of terahertz energy was used to power the STEAM buncher (beam compressor), while 2 x 15 mJ were required for the STEAM-linac (linear accelerator).

    The diameter of the inlet and outlet as both STEAM devices is 120 microns.

    The beam compressor is designed with three layers of the same height (0.225 mm), which are equipped with fused quartz plates (ϵ r= 4.41) 0.42 and 0.84 mm long to control time synchronization. The equal heights of the compressor layers reflect the fact that acceleration does not occur ( 2s ).

    But in a linear accelerator, the heights already differ - 0.225, 0.225 and 0.250 mm (+ fused silica plates 0.42 and 0.84 mm). An increase in the layer height explains the increase in the electron velocity 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 are required to optimize the interaction.

    The results of practical experiments


    First of all, the researchers recall that in traditional accelerators based on radio frequencies, the influence of the temporal extent of an embedded electron beam on the properties of an accelerated beam is associated with a change in the electric field experienced during the interaction of various electrons inside 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 greater spread of energies. Introduced long-duration beams can also lead to higher emittances * .
    Emittans * - phase space that occupies an accelerated beam of charged particles.
    In the case of a terahertz accelerator, the period of the excitation field is approximately 200 times shorter. Consequently, the strength * of the supported field will be 10 times higher.
    Electric field strength * is 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, field gradients experienced by electrons can be several orders of magnitude higher than in a conventional device. The time scale at which the curvature of the field is noticeable will be much smaller. It follows from this that the duration of the introduced electron beam will have a more pronounced effect.

    Scientists in practice decided to test the theory. To do this, they introduced electron beams of different durations, which was controlled by compression due to the first STEAM device (STEAM-buncher).


    Image No. 3

    In the case when the compressor was not connected to a power source, electron beams (55 keV) with a charge of ∼1 fC (femtocoulon) passed approximately 300 mm from the electron gun to the linear accelerator device (STEAM-linac). These electrons could expand under the influence of space charge forces up to a duration of more than 1000 fs (femtoseconds).

    With such a 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 the energy spectrum after acceleration with a peak of 115 keV and a half-width of the energy distribution 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 (i.e., did not coincide with) with respect to the optimal introduction time. The calculations of this situation showed that the increase in electron energy very much depends 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 acceleration of terahertz radiation in each layer ( 3c ).

    If the electrons arrive at different times, then they experience less acceleration in the first layer, which requires more time for their passage. Then the desynchronization is amplified in the following layers, which causes an undesirable slowdown ( 3d)

    In order to minimize the negative effect of the temporal length of the electron beam, the first STEAM device worked in compression mode. The electron beam duration at the linear accelerator was optimized to a minimum of ~ 350 fs (half-width) by adjusting the terahertz energy supplied to the compressor and switching the linear accelerator to the hatching mode ( 4b ).


    Image No. 4

    The minimum beam duration was set in accordance with the duration of the UV pulse of the photocathode, the duration of which was ~ 600 fs. The distance between the compressor and the strip also played an important role, which limited the strength of the thickening in speed. Together, these measures make it possible to ensure the femtosecond accuracy of the introduction phase at the acceleration stage.

    On the imageFigure 4a shows that the energy spread of a compressed electron beam after optimized acceleration in a linear accelerator decreases by ~ 4 times compared to uncompressed. Due to acceleration, the energy spectrum of a compressed beam shifts toward higher energies, in contrast to an 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 a modest statement by scientists, are a new record of acceleration (before acceleration 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 No. 5

    In the case of an uncompressed introduced beam, the parabolic dependence of the beam size on the current reveals the transverse emittance in the horizontal and vertical directions: ε x, n = 1.703 mm * mrad and ε y, n = 1.491 mm * mrad ( 5a ).

    The compression, in turn, improved the transverse emittance by a factor of 6 to ε x, n = 0.285 mm * mrad (horizontal) and ε y, n = 0.246 mm * mrad (vertical).

    It should be noted that the degree of decrease in emittance is approximately two times greater than the degree of reduction in the beam duration, which is a measure of the nonlinearity of the dynamics of interaction with time, when the electrons experience strong focusing and defocusing of the magnetic field during acceleration ( 5b and 5c)

    It can be seen in image 5b that the electrons introduced at the optimal time experience the entire half-period of the acceleration of the electric field. But the electrons that arrive before or after the optimal point in time experience less acceleration and even partial deceleration. Such electrons as a result receive less energy, roughly speaking.

    A similar situation is observed when exposed to a magnetic field. Electrons introduced at the optimum time experience a symmetric amount of positive and negative magnetic fields. If the introduction of electrons occurred earlier than the optimal time, then there were more positive fields and fewer negative ones. In the case of the introduction of electrons later than the optimal time, there are fewer positive and more negative ones ( 5 s) 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 that you look into the report of scientists and additional materials to it.

    Epilogue


    Summarizing, the accelerator performance will increase in the case of a decrease in the duration of the electron beam. In this work, the achievable beam duration was limited by the installation geometry. But, in theory, the beam duration can reach less than 100 fs.

    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, increasing the complexity of the production of 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 record, there is still a lot of work.

    Thank you for your attention, remain curious and have a good working week, guys! :)

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