Patent Publication Number: US-2021193429-A1

Title: Ultrafast electron diffraction apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority from Korean Patent Application No. 10-2019-0172374 filed on Dec. 20, 2019, the disclosures of which are incorporated herein in its entirety by reference for all purposes. 
     TECHNICAL FIELD 
     The present disclosure relates to an ultrafast electron diffraction apparatus, and more particularly, to an ultrafast electron diffraction apparatus for controlling a state of an electron beam generated in a radio-frequency (RF) photoelectron gun. 
     BACKGROUND 
     When an electromagnetic wave or a matter wave having a short wavelength such as an X-ray or an electron beam is used, a user can directly observe a structure of an atom or a molecule. Representative technology for this is an electron microscope. 
     In addition to this technology, by generating and using both pumping light stimulating the sample and an electron beam or X-ray which are probe beams that observe dynamics of molecular structure of a sample, in an ultra-short period of time, a structural change of the molecules in the sample may be measured. When measuring the molecule&#39;s structural change of the sample, time-resolved diffraction technology is used, and pumping light generally uses an ultrashort laser light source, and the probe beam uses high energy electromagnetic waves or matter waves such as X-rays or electron beams. 
     Recently, in the case of using ultrafast electron diffraction (UED) technology that uses relativistic electrons and X-ray free-electron laser (X-FEL), which is the 4-th generation synchrotron radiation, the structural change of the sample&#39;s molecules may be observed with time accuracy of about 100 femtoseconds (fs) in atomic or molecular units. 
     The above-described X-FEL and UED technology use ultrashort X-rays or ultrashort electron beams, respectively, as tools for reading structural changes of atoms and molecules. X-FEL technology is appropriate for studies of large molecules or thick samples such as biological or complex materials, and UED technology is appropriate for studies of samples with small-sized molecules, thin films, and gas phases. That is, the X-FEL and UED technology may have complementary roles. 
     The main technical challenges of the above technologies are to improve time accuracy and brightness of the beam. In order to improve the time accuracy, it is necessary to further shorten pulse duration of x-ray or electron beam and reduce timing jitter between pump and probe pulses. Further, in order to improve the brightness of the x-ray or electron beam, the output power of X-ray pulse and the charge of the electron beam should be increased. 
     In UED, shortening of the electron pulse, which is used as a probe, is limited by space charge force, which is a repulsive force among negatively charged electrons gathered in the pulse. When more electrons are used for increasing brightness of the electron beam, the space charge force of the electron beam increases, and thus the pulse duration of the electron beam increases rapidly even if the electron beam travels a small distance. 
     By increasing the kinetic energy of the electron beam, the velocity of electrons approaches the velocity of light and thus the space-charge effect decreases. When acceleration energy is too high, a size of an apparatus increases rapidly and thus it is known that the energy of a few millions of electron volts (MeV) of electron beam is appropriate for the UED experiment. In conventional technologies, in order to minimize the pulse duration increase of electron beam by the space charge force, the ultrafast electron diffraction apparatus uses a short linear structure. Further, some ultrafast electron diffraction apparatuses may additionally use an RF cavity in order to compress the electron beam. When the electron beam is compressed using the RF cavity, there is a problem that a phase of the RF in the cavity is fluctuated due to the temperature change of the RF cavity and the instability of the RF electric field. In this case, there is a problem that time accuracy is generally deteriorated because timing jitter in which the electron beam reaches the sample increases. 
     An ultrafast electron diffraction apparatus using an RF photoelectron gun and a linear structure has a problem that low electron pulse charge of 10 femtocoulombs (fC) or less should be used for obtaining an electron beam pulse of about 100 femtoseconds. Further, in the case of using such a linear structure, as the ultrafast electron diffraction apparatus is configured with one beamline, there is a problem that an experiment apparatus should be newly installed and used each time when a plurality of users use the ultrafast electron diffraction apparatus. 
     PRIOR ART DOCUMENT 
     Japanese Laid-Open Patent No. 2018-146265 (Sep. 20, 2018) 
     SUMMARY 
     In view of the above, the present disclosure provides an ultrafast electron diffraction apparatus capable of solving limitations such as time accuracy, beam brightness, and number of beamlines, and controlling a state of an electron beam generated in an RF photoelectron gun. 
     In accordance with a first embodiment of the present disclosure, there is provided an ultrafast electron diffraction apparatus including: a photoelectron gun configured to emit an electron beam; a bending portion for emitting the electron beam emitted from the photoelectron gun by changing a travel direction of the electron beam by a predetermined angle; and a sample portion including a sample to be analyzed by the electron beam emitted from the bending portion, wherein the electron beam reaches the sample portion in a state that a pulse of the electron beam is compressed as the travel direction of the electron beam is changed by the predetermined angle through the bending portion. 
     In accordance with a second embodiment of the present disclosure, there is provided an ultrafast electron diffraction apparatus including: a photoelectron gun configured to independently emit an electron beam multiple times at time intervals; a bending portion for emitting the electron beam emitted from the photoelectron gun by changing a travel direction of the electron beam by a predetermined angle; and a sample portion including a sample to be analyzed by the electron beam emitted from the bending portion, wherein a plurality of electron beams independently emitted to the sample portion and having different average energies reach with a reduced timing jitter to less than a few femtosecond according to selection of an RF phase of the photoelectron gun and an angle change of the electron beam through the bending portion. 
     According to the present disclosure, it is possible to simultaneously measure a structure and motion of atoms or molecules of a substance, and while improving time accuracy compared to the prior art, brightness of an electron beam can be improved by about 100 times compared to the prior art. 
     In particular, when using an RF photoelectron gun, there is an effect of fundamentally arriving timing jitter of the relative difference of a time in which an electron beam and a laser beam reach a sample. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the principle of ultrafast electron diffraction (UED) technology using a photoelectron gun. 
         FIG. 2  is a diagram illustrating technology for compressing an electron beam using an RF cavity in conventional UED technology. 
         FIG. 3  is a diagram illustrating a configuration of an ultrafast electron diffraction apparatus on the same plane according to an embodiment of the present disclosure. 
         FIG. 4  is a diagram illustrating a state in which a beamline with 90° bend in the ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
         FIGS. 5A to 5C  are graphs illustrating a state of compressing an electron beam in the ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
         FIGS. 6A to 6C  are graphs illustrating a state of relative timing in which an electron beam generated in a photoelectron gun reaches a sample in the ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
         FIG. 7  is a diagram illustrating removal of a dark current and colinear matching of pumping light with electron beam in the ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
         FIG. 8  is a diagram illustrating a configuration of an ultrafast electron diffraction apparatus on a space according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, configurations and operations of embodiments will be described in detail with reference to the accompanying drawings. The following description is one of various patentable aspects of the disclosure and may form a part of the detailed description of the disclosure. 
     However, in describing the present disclosure, detailed descriptions of known configurations or functions may be omitted to clarify the present disclosure. 
     The disclosure may be variously modified and may include various embodiments. Specific embodiments will be exemplarily illustrated in the drawings and described in the detailed description of the embodiments. However, it should be understood that they are not intended to limit the disclosure to specific embodiments but rather to cover all modifications, similarities, and alternatives which are included in the spirit and scope of the disclosure. 
     The terms used herein, including ordinal numbers such as “first” and “second” may be used to describe, and not to limit, various components. The terms simply distinguish the components from one another. 
     When it is said that a component is “coupled” or “linked” to another component, it should be understood that the former component may be directly connected or linked to the latter component or a third component may be interposed between the two components. 
     Specific terms used in the present application are used simply to describe specific embodiments without limiting the disclosure. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. 
     A preferred embodiment of the present disclosure will be described in more detail with reference to the accompanying drawings. 
       FIG. 1  is a diagram illustrating the principle of ultrafast electron diffraction (UED) technology using a photoelectron gun.  FIG. 2  is a diagram illustrating technology for compressing an electron beam using an RF cavity in conventional UED technology.  FIG. 3  is a diagram illustrating a configuration of an ultrafast electron diffraction apparatus on the same plane according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , ultrafast electron diffraction technology will be described in more detail. First, X-ray free-electron laser technology uses a very large scientific facility that has a length of about 1 km or more and that costs one billion USD in a construction cost. However, ultrafast electron diffraction technology may be installed in laboratories with dimensions of tens of meters. The ultrafast electron diffraction apparatus can be constructed at a very low cost compared to the X-ray free-electron laser facility because equipment to implement ultrafast electron diffraction technology does not exceed 10 millions of USD. The key to the development of ultrafast electron diffraction is to obtain equivalent time and brightness performance, compared to those of X-ray free-electron lasers. 
     As illustrated in  FIG. 1 , an ultrafast electron diffraction apparatus  1  includes a photoelectron gun  110  that emits an electron beam  12  toward a sample and a light source unit  170  that emits light. In this case, light emitted from the light source unit  170  is diverged into two femtosecond laser pulses, and one femtosecond laser pulse is wavelength-converted into ultraviolet rays  21  and enters the photoelectron gun  110 . The photoelectron gun  110  generates an electron beam pulse used as a probe using the incident ultraviolet rays  21  to emit the electron beam  12 . The other femtosecond laser pulse is radiated to the sample to be used for pumping (exciting or stimulating) the sample. By adjusting a path of pumping light  23  radiated to the sample in this way, it is possible to observe temporal changes in a structure of atoms and molecules in the sample after pumping. 
     In both the X-ray free-electron laser technology and the ultrafast electron diffraction technology, it is important to improve time accuracy and brightness of the electron beam  12 , and time accuracy or a time resolution τ is represented by Equation 1. 
       τ=√{square root over (τ pump   2 +τ probe   2 +τ jitter   2 +τ VM   2 )}  (Equation 1)
 
     Here, τ pump  is a pulse duration (a length of a pulse in a time unit) of the pumping light  23 , τ probe  is a pulse duration of the electron beam (or X-ray)  12  as a probe, τ jitter  is timing jitter in which the pumping light  23  and the electron beam  12  as the probe reach a sample, and τ VM  indicates an increase in an effective pulse duration by the difference in speed between the electron beam  12  and the ultraviolet ray  21  while passing through the sample. 
     It is known that it is difficult to reduce τ jitter  in X-ray free-electron laser technology using an RF accelerator and relativistic ultrafast electron diffraction technology. 
     The brightness of the electron beam  12  is determined according to the number of photons or electrons included in the pulse. In general, since, in the X-ray free-electron laser technology, about 10 12  photons are included in a single pulse, it is possible to observe the molecular dynamics of a material with a single pulse. 
     Since electrons, which are charged particles, have scattering power that is about 100,000 to 1 million times more stronger than X-rays, when about 10 6  electrons per pulse are used for exploring the materials, they may exhibit similar performance as that of X-ray free-electron laser technology. However, when electrons, which are charged particles, are gathered in a very small space-time with a certain amount of charges or more, a space charge force that pushes against each other occurs strongly, and for this reason, a problem may occur that the characteristics of the electron beam  12  deteriorate and that the pulse width rapidly increases. 
     Accordingly, in relativistic ultrafast electron diffraction technology developed so far, by generating an electron beam  12  with a pulse width of about 100 femtoseconds in the RF photoelectron gun  110 , and locating the sample as close as possible in a linear path of the electron beam  12 , an apparatus developed to minimize expanding phenomenon of the electron beam  12  is used. Nevertheless, in an apparatus according to such ultra-fast electron diffraction technology, the degree in which a pulse width of the electron beam  12  increases and how many electrons are included in a single pulse act as important factors. 
     When the electron beam  12  includes about 10 5  or more electrons per pulse or when the electron beam  12  is dozens of femtocoulombs (fC) or more, the electron beam  12  rapidly increases a pulse width even if it travels about 1 m. Accordingly, as illustrated in  FIG. 2 , technology of compressing the electron beam  12  using an RF cavity  30  is used. A first RF cavity  1100  and a second RF cavity  30  are illustrated in  FIG. 2 , and an electric field (E) formed in the second RF cavity  30  is used for decelerating front side electrons moving at the front side in a direction of travel of the electron beam  12  by changing the front side electrons into a relatively low-energy eletrons (L), and accelerating back side electrons moving at the back side in the direction of travel of the electron beam  12  by changing the front side electrons into a relatively high-energy eletrons (H). In this way, the degree of acceleration and deceleration of electrons in the electron beam  12  is formed linearly according to a position of a longitudinal axis in the pulse of the electrons. Such a velocity distribution of the electron beam  12  is referred to as a positive chirp. 
     Accordingly, while the electron beam  12  travels for a predetermined distance, electrons traveling in the front move relatively slowly, and electrons traveling in the back move relatively quickly and thus electrons may gather at the center position of the electron pulse. Such gathering of electrons is referred to as velocity compression or ballistic compression. 
     As described above, the phase of the RF in the second RF cavity  30  used for velocity compression or ballistic compression may fluctuate according to the changes of temperature or environment, thereby increasing timing jitter in which the electron beam  12  reaches the sample. 
     Accordingly, as illustrated in  FIG. 3 , the ultrafast electron diffraction apparatus  1  according to the embodiment of the present disclosure is used. Referring to  FIG. 3 , the ultrafast electron diffraction apparatus  1  according to the embodiment of the present disclosure includes a photoelectron gun  110 , bending portions  120  and  140 , a first quadrupole magnet module  130 , a second quadrupole magnet module  132 , a sample portion  150 , a detection unit  160 , and a light source unit  170  (not shown). In this case, the bending portions  120  and  140  include a main bending portion  120  and an auxiliary bending portion  140 . A plurality of auxiliary bending portions  140  may be provided and be interlocked with one main bending portion  120 . Further, the ultrafast electron diffraction apparatus  1  may include a plurality of beamlines. 
     The photoelectron gun  110  emits an electron beam having predetermined energy using a pulsed RF signal. In the present embodiment, the electron beam emitted from the photoelectron gun  110  has energy of about 2 MeV to 4 MeV, and has a bunch charge of about 1 pC and bunch duration of about 100 fs. In the present embodiment, when a diameter of the electron beam emitted from the photoelectron gun  110  is about 150 μm to 500 μm and when bunch duration is about 50 fs to 300 fs, a bunch charge may be about 0.1 pC to 5 pC. 
     The light source unit  170  emits ultraviolet rays  21  for generating electron beam pulses used as probes in the photoelectron gun  110 , and also emits pumping light  23  for pumping (excitating, stimulating) a sample toward the sample portion  150 . In this case, the light source unit  170  diverges light. Further, the light source unit  170  converts a part of the diverged light into ultraviolet rays  21  to emit it toward the photoelectron gun  110 , and applies another part of the diverged light as pumping light to the auxiliary bending portion  140  to travel toward the sample portion  150 . 
     In the present embodiment, a light diverged from the light source unit  170  and emitted as the ultraviolet ray  21  may be directly radiated to the photoelectron gun  110 . Another light among the diverged lights may be emitted toward the sample portion  150  as pumping light  23 , and the pumping light  23  may be emitted toward the sample portion  150  through the auxiliary bending portion  140 . A detailed description thereof will be described later. 
     The main bending portion  120  is disposed at the rear end of the photoelectron gun  110 , and the electron beam emitted from the photoelectron gun  110  is moved to the main bending portion  120 . The main bending portion  120  is configured with a dipole magnet, and may adjust magnetic field strength of the dipole magnet. Thus, the dipole magnet included in the main bending portion  120  may be a permanent magnet or an electromagnet. Further, a shape of the main bending portion  120  may be formed in a circular shape, a square shape, or other shapes. 
     The main bending portion  120  changes an emitting angle of the electron beam coming from the photoelectron gun  110  according to the strength of the applied magnetic field. In the present embodiment, the main bending portion  120  may change the electron trajectories with a bending angle θ such as about 0°, 22.5°, 45°, and 67.5° on a predetermined plane. Here, the bending angle θ changed on the plane is an angle between an incident direction (e.g., a fourth beamline BL 4 ) in which the electron beam is incident and an emitting direction of the electron beam whose emitting direction is changed, as illustrated in  FIG. 3 . 
     The first quadrupole magnet module  130  is disposed so that one side faces the main bending portion  120  and the other side faces the auxiliary bending portion  140 . The first quadrupole magnet module  130  may include three quadrupole magnets disposed between the main bending portion  120  and the auxiliary bending portion  140 , as illustrated in  FIGS. 3 and 4 , and serves to focus electron beams emitted from the main bending portion  120 . That is, the first quadrupole magnet module  130  serves as a focusing or a defocusing lens. 
     The auxiliary bending portion  140  is connected to the first quadrupole magnet module  130 , receives the electron beam emitted from the first quadrupole magnet module  130 , bends an angle of the electron beam, and then emits the electron beam. In this case, the auxiliary bending portion  140  is configured with a dipole magnet, as in the main bending portion  120 , and may adjust magnetic field strength of the dipole magnet. To this end, the dipole magnet included in the auxiliary bending portion  140  may be configured with a permanent magnet or an electromagnet. A shape of the auxiliary bending portion  140  may be formed in a circular shape, a square shape, or other shapes. Further, a plurality of auxiliary bending portions  140  may be provided, and at least some auxiliary bending portions  140  of the plurality of auxiliary bending portions  140  may be configured to place on different virtual planes. 
     The auxiliary bending portion  140  changes an emitting angle of the electron beam emitted from the first quadrupole magnet module  130  according to the strength of the applied magnetic field. In the present embodiment, the auxiliary bending portion  140  may change an emitting angle of the electron beam to an emitting angle of, for example, about 0°, 22.5°, 45°, and 67.5° on a predetermined plane. In the present embodiment, the emitting angle of the electron beam changed in the auxiliary bending portion  140  may be the same as that of the electron beam changed in the main bending portion  120 . 
     For example, when the main bending portion  120  changes a bending angle of the electron beam to an angle of 22.5°, the auxiliary bending portion  140  may also equally change the bending angle of the electron beam to an angle of 22.5°. Accordingly, a beamline in which the sample portion  150  is disposed may have a bending angle of 45° to the direction of the electron beam emitted from the photoelectron gun  110 . 
     In the present embodiment, the auxiliary bending portion  130  may be omitted, as needed. In this case, a direction of travel of the electron beam emitted from the main bending portion  120  becomes a final direction of travel of the electron beam. 
     The second quadrupole magnet module  132  has one side connected to the auxiliary bending portion  140  and the other side connected to the sample portion  150 . In the present embodiment, three second quadrupole magnet modules  132  are provided and serve to focus the electron beam emitted from the auxiliary bending portion  140 . 
     The sample portion  150  has a sample disposed therein, and an electron beam emitted through the second quadrupole magnet module  132  is incident to the sample portion  150 . In the present embodiment, the above-described pumping light  23  together with the electron beam  12  may enter the sample portion  150 . 
     The detection unit  160  detects an electron beam  12  that has passed through the sample portion  150 . To this end, the detection unit  160  may analyze the sample included in the sample portion  150  using a screen for the scattered electron beam  12 . 
     In the present embodiment, beamlines BL 1  to BL 7  are defined as lines in which the auxiliary bending portion  140 , the second quadrupole magnet module  132 , the sample portion  150 , and the detection unit  160  are disposed, and the beamlines BL 1  to BL 7  may have a linear shape. In other words, the beamlines BL 1  to BL 7  may be configured such that the electron beam emitted from the auxiliary bending portion  140  travels in a straight line toward the sample portion  150 . Therefore, the electron beams emitted from the photoelectron gun  110  are changed in the angles at the main bending portion  120  and the auxiliary bending portion  140 , respectively, and then travel to the detection unit  160  through the sample portion  150  in a linear shape through the beamlines BL 1  to BL 7 . 
     In the present embodiment, for example, when the beamline BL 6  is bent by 90°, electron beams generated in the photoelectron gun  110  have about 3.1 MeV, and a length of the beamline BL 6  is 3.2 m, energy distribution of the electron beams should satisfy −61 eV/fs. In this case, the length of the beamline BL 6  may be 2 m to 5 m, and in this case, in order to compress a predetermined pulse of the electron beam, an energy distribution range of the electron beam may be −60 eV/fs±15%. 
     This is because the electron beams are compressed and timing jitter is compensated (or significantly reduced) during the electron beams emitted from the photoelectron gun  110  rotate in a direction of travel through the main bending portion  120  and the auxiliary bending portion  140 , and thus the sample portion should be disposed at a position in which timing jitter is compensated (or significantly reduced) while the electron beams are maximally compressed. Accordingly, a length of the beamline may be set according to an energy distribution range of the electron beam and the degree of bending of the bending portions  120  and  140 . 
     The ultrafast electron diffraction apparatus  1  having the above configuration is configured to have a plurality of beamlines, as illustrated in  FIG. 3 , and in this case, compression of electron beams emitted from the photoelectron gun  110  will be described with reference to  FIGS. 4 and 5 . 
       FIG. 4  is a diagram illustrating a state in which a beamline is 90° in an ultrafast electron diffraction apparatus according to the embodiment of the present disclosure.  FIGS. 5A to 5C  are graphs illustrating a state of compressing an electron beam in an ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
     When the light emitted from the light source unit  170  is diverged and wavelength-converted into ultraviolet rays  21  to be entered into the photoelectron gun  110 , electron beams are generated, and the electrin beams can be a negative chirp in pulse energy distribution dut to a space charge force by the charge density of the electron beams. For example, when a charge of about 1 pC is generated in an electron beam space with a pulse duration of 100 fs and a diameter of 0.5 mm, electrons located in the front of the electron beam move faster (increase energy) because of a repulsive force thereof, and electrons located in the back thereof move more slowly (decrease energy). Such velocity distribution is referred to as a negative chirp state. 
     In this case, as illustrated in  FIG. 4 , when the electron beam passes through the bending portions  120  and  140  having a predetermined emitting angle (e.g., 90°), fast-speed electrons disposed at the front move in a relatively long path (a path with a large radius of curvature) outside the bending portions  120  and  140 , and the slow-speed electrons disposed at the back move in a relatively short path (a path with a small radius of curvature) inside the bending portions  120  and  140 . Therefore, the relative position distribution of the electron beam changes, and the slow-speed electrons move in a relatively short path and thus they are located in front of the fast-speed electrons, and the fast-speed electrons move in a long path to be located behind the slow-speed electrons. Such a state is referred to as a positive chirp state. In other words, according to an embodiment of the present disclosure, when the electron beam passes through the bending portions  120  and  140  without applying an RF electric field as in a bunching RF cavity, the electron beam may be in a positive chirp state. 
     Accordingly, as the electron beams move along a linear beamline through the main bending portion  120  and the auxiliary bending portion  140 , all electrons naturally move to be the same position because of the difference in speed of the electrons. In other words, immediately after being emitted from the bending portions  120  and  140 , the slow-speed electrons travel in front of the fast-speed electrons, but as they travel through the linear beamline, the fast-speed electrons catch up the slow-speed electrons and thus they travel with little difference in position between each other. Such a movement of electrons so that there is no positional difference between them is referred to as velocity compression or ballistic compression. In other words, the electrons pass through the bending portions  120  and  140  and then travel through the linear beamline, thereby moving with the same position or a relative position within a predetermined range of error. The sample portion  150  including the sample is disposed at a point where electrons of the electron beam (pulse of the electron beam) are maximally compressed. 
     In this case, a graph illustrated in  FIG. 5A  shows a negative chirp state of the electron beam at a position A where the electron beam is emitted from the photoelectron gun  110 , and a graph illustrated in  FIG. 5B  shows a positive chirp state of the electron beam at a position B at which a bending angle of the electron beam is changed through the main bending portion  120  and the auxiliary bending portion  140 . A graph illustrated in  FIG. 5C  shows a state in which the pulse of the electron beam is maximally compressed at the position C of the sample portion  150 . 
     The electron beam passing through the sample portion  150  increases in pulse width thereafter. However, electrons are diffracted according to structure information of the sample at the point where the pulse width is maximally compressed (shortest), and are detected by the detection unit  160  in that state. 
     Further, in the present embodiment, a function in which the ultrafast electron diffraction apparatus  1  compensates (or reduces significantly) timing jitter generated in the photoelectron gun  110  will be described with reference to  FIGS. 4 and 6A to 6C . 
       FIGS. 6A to 6C  are graphs illustrating a compensating mechanism of timing jitter in which an electron beam generated in a photoelectron gun reaches a sample in an ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
     The photoelectron gun  110  is configured to independently emit an electron beam multiple times at time intervals. These independent electron beams should theoretically have constant average kinetic energy, but actually have different average energies. When the electron beams have different average energies in this way, a flight time, at which the electron beam generated in the photoelectron gun  110  reaches the sample, may also vary. When the photoelectron gun  110  generates an electron beam, a phase of the RF and an incident time of the ultraviolet ray  21  may fluctuate due to various factors. In particular, a change in temperature of the photoelectron gun  110 , fluctuation of a phase and power of the incident RF pulse, etc. may be main causes in which average kinetic energy of the electron beam pulse changes. When the ultrafast electron diffraction apparatus  1  has a linear structure, such a change in the average kinetic energy becomes timing jitter in which the electron beam pulse reaches the sample, and as a length of the linear section increases, the timing jitter increases. Due to the change in average kinetic energy of the electron beam, a time varies at which the electron beam pulse exits the photoelectron gun  110 . However, as in the present embodiment, the ultrafast electron diffraction apparatus having a bending structure may completely compensate the timing jitter in which the electron beam reaches the sample even if there is a change in the average kinetic energy of the electron beam pulses. The electron beam having high average energy generated in the photoelectron gun  110  quickly exits the photoelectron gun, but conversely, the electron beam takes a relatively longer time because it passes a longer path in the bent structure. Therefore, as these two times cancel each other, when a phase slope of the RF in the photoelectron gun  110  is adjusted, even if there is a change in the average kinetic energy generated in the photoelectron gun  110 , it is possible to realize a condition where there is no timing jitter in which the electron beam reaches the sample. 
     As illustrated in  FIG. 6A , at a position A where an electron beam is emitted from the photoelectron gun  110 , comparing emission time points at a reference time point of #1 in which the average energy distribution of the electron beam is a relatively intermediate state, #2 in which the average energy distribution of the electron beam is a relatively high state, and #3 in which the average energy distribution of the electron beam is a relatively low state, it can be seen that the electron beam of #2 was emitted at a relatively fast time point and that the electron beam of #3 was emitted at a relatively slow time point and that the electron beam of #1 was emitted at the reference time point. 
     A graph illustrated in  FIG. 6B  represents the energy distribution of the electron beam at a position B where an emitting angle of the same electron beam illustrated in  FIG. 6A  is changed through the main bending portion  120  and the auxiliary bending portion  140 . When comparing emission time points at the reference time point of #1 where the average energy distribution of the electron beam is a relatively intermediate state, #2 where the average energy distribution of the electron beam is a relatively high state, and #3 where the average energy distribution of the electron beam is a relatively low state, it can be seen that the electron beam of #2 is arrived at a relatively slow time point and that the electron beam of #3 is arrived at a relatively fast time point and that the electron beam of #1 is arrived at the reference time point. 
     According to the embodiment of the present disclosure, when electron beams are independently emitted from the photoelectron gun  110  multiple times, even if there is a change in the average kinetic energy by a change in RF phase between each electron beam, as illustrated in  FIGS. 6A and 6B , by compensating the time difference in which the electron beams exit the photoelectron gun  110  and the time difference in which the electron beams pass through beamlines passing through the bending portions  120  and  140  and having the linear beamline, timing jitter in which each electron beam reaches the sample may be fundamentally eliminated. In other words, because the electron beam emitted at a time point earlier than the reference time point has a high energy distribution on average, the electron beam moves along a long path (a path with a large radius of curvature) while passing through the bending portions  120  and  140  to be emitted relatively late at the bending portions  120  and  140 . Accordingly, an electron beam emitted at a time point earlier than the reference time point may take the same time as the time taken for the electron beam emitted at the reference time point to reach the sample portion  150  by being emitted from the photoelectron gun  110 . Further, because an electron beam emitted at a time point slower than the reference time point has a low energy distribution on average, the electron beam moves along a short path (a path with a small radius of curvature) while passing through the bending portions  120  and  140  to be emitted relatively quickly from the bending portions  120  and  140 . Accordingly, the electron beam emitted at a time point slower than the reference time point may take the same time as the time taken for the electron beam emitted at the reference time point to reach the sample portion  150  by being emitted from the photoelectron gun  110 . 
     As described above, when the average kinetic energy of the electron beam emitted from the photoelectron gun  110  changes, the time in which the electron beam exits the photoelectron gun  110  changes, but the electron beam is bent in the main bending portion  120  and the auxiliary bending portion  140 , respectively and thus a difference in time for the electron beam to reach the sample portion  150  may be canceled. 
     In the present embodiment, a function of removing a dark current  33  generated in the photoelectron gun  110  will be described with reference to  FIG. 7 . 
       FIG. 7  is a diagram illustrating removal of a dark current and matching of incident trajectories of pumping light with the electron beam at the sample position in an ultrafast electron diffraction apparatus according to the embodiment of the present disclosure. 
     The photoelectron gun  110  generates an electron beam only when the ultraviolet pulse  21  enter. However, even if the ultraviolet pulse  21  do not enter, when there is an accelerating electric field, electrons having a low current may be generated due to microstructure or impurities of a photocathode surface. The low current electron beam generated in the photoelectron gun  110  is the dark current  33 . The dark current  33  has a lower current value than that of the photoelectron beam, but is emitted for much longer time, and thus the dark current  33  may have a level of noise that cannot be ignored. 
     A photographing time of an intensified charge coupled device (ICCD) or an electron multiplying charge coupled device (EMCCD), which are parts of the detection unit  160  for photographing an electron diffraction pattern, is several nanoseconds to several microseconds, and the photoelectron beam occurs in a time shorter than 1 picosecond, but the dark current  33  may be continuously emitted. Accordingly, in order to remove the dark current  33 , as illustrated in  FIG. 7 , a filter unit  122  capable of passing through only an electron beam may be disposed. In the present embodiment, as the electron beam rotates through the main bending portion  120  and the auxiliary bending portion  140 , energy and spatial filter functions may be included in the filter unit  122 . Therefore, when the filter unit  122  is disposed in an appropriate position, most of the dark current  33  may be removed. In the present embodiment, it is described that the filter unit  122  is disposed at the rear end of the main bending portion  120 , but the filter unit  122  may be additionally installed at the front end of the sample portion  150 , as needed. Further, the filter unit  122  may be disposed between the photoelectron gun  110  and the bending portions  120  and  140 , and may be disposed between the main bending portion  120  and the auxiliary bending portion  140 , as needed. 
     Further, a description of improving a time performance by matching incident trajectories of the pumping light  23  and the electron beam  12  will be described with reference to  FIG. 7 . 
     As illustrated in  FIG. 7 , when rotating the electron beam  12 , it is necessary to match a direction of travel of the electron beam  12  and a direction of travel of the pumping light  23 . To this end, when an incident angle of the pumping light  23  is matched to the direction of travel of the electron beam  12 , the directions of travel of the electron beam  12  and the pumping light  23  match, so that when passing through the sample, an element that enables time accuracy to be poor may be removed. 
     In particular, in the case of using an electron beam of relativistic energy, because a velocity of the electron beam almost reaches a velocity of light, it is possible to minimize the effect of expanding a pulse width of the electron beam caused by the velocity difference. 
     A configuration of an ultrafast electron diffraction apparatus  1  on a space according to another embodiment of the present disclosure will be described. 
       FIG. 8  is a diagram illustrating a configuration of an ultrafast electron diffraction apparatus on a space according to another embodiment of the present disclosure. 
     As illustrated in  FIG. 8 , one main bending portion  120  is disposed under one photoelectron gun  110 . A plurality of auxiliary bending portions  140  may be disposed below the main bending portion  120 , and a plurality of sample portions  150  are disposed in an outer direction of the plurality of auxiliary bending portions  140 , respectively, so that a plurality of beamlines may be disposed. Accordingly, as illustrated in  FIG. 8 , virtual lines which are parallel to the plurality of beamlines may be not parallel to one another, and may have an angle of about 90° or a different angle with respect to a virtual line passing through the photoelectron gun  110  and the main bending portion  120  in a three-dimensional space. 
     In this case, a disposition of the photoelectron gun  110 , the main bending portion  120 , one auxiliary bending portion  140 , and one sample portion  150  may be the same as that on the plane illustrated in  FIG. 4 . In the present embodiment, it is described that a plurality of beamlines have an angle of 90°, but each beamline may be deformed to have a different angle, as needed, and as the beamline is disposed on a three-dimensional space, an angle of each beamline may be set independently. 
     Further, as all of the plurality of beamlines have an angle of 90°, dispersion of electron beams in each beamline may be completely compensated. 
     While the configuration and features of the present disclosure has been shown and described with respect to the embodiments in accordance with the present disclosure, the present disclosure is not limited thereto. It will be apparent to those skilled in the art to make various changes or modifications within the spirit and scope of the present disclosure, and thus, such changes or modifications are found to belong to the appended claims.