Femtosecond Time-resolved Transmission Electron Microscopy using an RF Gun

Both the transverse and longitudinal beam dynamics in a photoinjector were investigated experimentally for high-brightness electron beam generation. The transverse emittance growth and the energy spread, both due to the rf and the space-charge effects in the rf gun, were investigated with the laser injection phase. A 31.7 MeV electron beam at 1.0 nC with a normalized rms transverse emittance of 3.2±0.2 mm·mrad, an rms bunch length of 1.8±0.2 ps, and an rms relative energy spread of 0.04±0.01% was obtained in the photoinjector with a 5 ps (full width at half maximum; FWHM) laser. The phase compression in the bunch length was observed in the rf gun. A sub-picosecond electron bunch was observed experimentally in the rf gun at the low injection phase, such as 0.7±0.2 ps at 15° and 0.5±0.1 ps at 10°. The dependences of the transverse emittance, the bunch length and the energy spread on the rf phase of a booster linear accelerator (linac) downstream of the rf gun were investigated experimentally.

We have demonstrated generation and transport of a patterned electron beam from a Diamond Field-Emitter Array (DFEA) cathode in a radio frequency (rf) gun. DFEAs are arrays of micrometer-scale pyramids with nanometer-scale tips. They can be fabricated with base widths ranging from 3 μm to 25 μm and pitches as small as 5 μm. They have an inherent 1:0.7 base to height ratio. DFEAs operate as field-emitter cathodes and potentially produce intrinsically shaped electron beams, which are of interest for a number of accelerator applications. We report on the results of a recent experiment in which a beam, consisting of several beamlets, was produced from a DFEA cathode in an rf gun and transported 2.54 m along a beam line. A macrobunch charge of 60 pC was measured at a cathode field gradient of 15.1 MV/m.

We propose an efficient scheme to produce ultrahigh-brightness tens of MeV electron beams by designing a density-tailored plasma to induce a wakefield in the weakly nonlinear regime with a moderate laser energy of 120 mJ. In this scheme, the second bucket of the wakefield can have a much lower phase velocity at the steep plasma density down-ramp than the first bucket and can be exploited to implement longitudinal electron injection at a lower laser intensity, leading to the generation of bright electron beams with ultralow emittance together with low energy spread. Three-dimensional particle-in-cell simulations are carried out and demonstrate that high-quality electron beams with a peak energy of 50 MeV, ultralow emittance of ∼28 nm rad, energy spread of 1%, charge of 4.4 pC, and short duration less than 5 fs can be obtained within a 1-mm-long tailored plasma density, resulting in an ultrahigh six-dimensional brightness B6D,n of ∼2 × 1017 A/m2/0.1%. By changing the density parameters, tunable bright electron beams with peak energies ranging from 5 to 70 MeV, a small emittance of ≤0.1 mm mrad, and a low energy spread at a few-percent level can be obtained. These bright MeV-class electron beams have a variety of potential applications, for example, as ultrafast electron probes for diffraction and imaging, in laboratory astrophysics, in coherent radiation source generation, and as injectors for GeV particle accelerators.

100-femtosecond MeV electron source for ultrafast electron diffraction

The SuperKEKB commissioning has finally started. The final goal of luminosity is 40 times higher than KEKB. The injector upgrade is required to obtain the low emittance and high charge beam corresponding to the short beam life and small injection acceptance of the SuperKEKB ring. In the injector linac, several new instruments have been installed. Flux Concentrator (FC) was developed for high charge positron beam production. The target bunch charge of positron beam is 4 nC. The new damping ring will be used for positron beam to reduce beam emittance to 10 mm-mrad. However, electron beam must be reached to 20 mm-mrad normalized emittance at 5 nC beam charge without damping ring. Thermionic gun was used for KEKB injector and it was able to generate enough beam charge. However, its emittance is too large. Therefor we developed photo cathode S-band RF gun. This new RF gun has unique accelerating cavity which called quasi-travelling wave side coupled cavity. Laser system for this photo cathode has been also developed. The laser system is constructed with Yb:YAG thin disk for high power and pulse shaping. I will illustrate the RF gun, laser system and several new injector linac systems.

Time-resolved electron diffraction and microscopy of laser-induced processes in thin films

In high energy electron beam therapy, a bolus can be used for the conformal dose to target and achieve the appropriate treatment quality. To evaluate the use of a customized silicone bolus, we analyzed the dosimetric properties of the electron beam according to the silicone bolus thickness. Eight silicone slits were fabricated using an acrylic mold. The size of each slit was 150 × 150 × 5 m3 (width × length × height). Degassing was performed to eliminate bubbles in the slits during the silicone casting process. Depth dose measurements were performed for a 6 MeV and 9 MeV electron beams with 10 × 10 cm2, 6 × 6 cm2 and 5 cm diameter circular fields with various silicone thickness and with the conventional bolus. For the depth dose measurement, the film was placed between two 6 cm solid water phantoms, and the electron beam was delivered parallel to the film. The Rando phantom dose with customized bolus were evaluated with the 10 × 10 cm2 6 MeV electron beam. As the slab thickness increased, the depth dose in the solid water phantom decreased after reaching the maximum dose depth (dmax). Depending on the silicone thickness, the surface dose of the phantom was increased until reaching the electron beam dmax and was decreased after dmax. The coefficient to convert the thickness of the silicone to water were 1.06 in 6 MeV and 1.07 in 9 MeV, respectively. We measured the electron beam dose properties for making patient-specific silicone boluses with optimal thickness. The results of this study can be used to predict the silicone bolus thickness required in order to deliver the appropriate dose to the target.

Research and development of iridium cerium photocathode for SuperKEKB injector linac

We use time-resolved cryogenic transmission electron microscopy (TR-cryo-TEM) on a supersaturated solution of calcium sulfate hemihydrate to examine the early stages of particle formation during the hydration of the hemihydrate. As hydration proceeds, we observe nanoscale amorphous clusters that evolve to amorphous particles and then reorganize to crystalline gypsum within tens of seconds. Our results indicate that a multistep particle formation model, where an amorphous phase forms first, followed by the transformation into a crystalline product, is applicable even at time scales of the order of tens of seconds for this system. The addition of a small amount of citric acid significantly delays the reorganization to gypsum crystals. We hypothesize that available calcium ions form complexes with the acid by binding to the carboxylic groups. Their incorporation into a growing particle produces disorder and extends the time over which the amorphous phase exists. We see evidence of patches of "trapped" amorphous phase within the growing gypsum crystals at time scales of the order of 24 h. This is confirmed by complementary X-ray diffraction experiments. Direct imaging of nanoscale samples by TR-cryo-TEM is a powerful technique for a fundamental understanding of crystallization and many other evolving systems.

All well-established experimental methods for investigating the structure of biological macromolecules have spatial and/or temporal limitations inherent to the technique that limit their applicability to studying macromolecular dynamics. The ability to measure macromolecular dynamics is key to gaining predictive mastery of biological systems. This work demonstrates that sufficient data can be acquired from low-Z soft materials using dynamic transmission electron microscopy (DTEM) to potentially allow measurements of macromolecular motion. While a dynamic measurement of macromolecular motion or structural change requires further effort, it is evident from this work that this is feasible under specific experimental conditions that can be achieved using low-dose, time-resolved transmission electron microscopy.

Atomic-scale contact and noncontact scannings on gold surfaces were performed by gold tips in time-resolved high-resolution transmission electron microscopy (HRTEM) using a piezodriven specimen holder. The structural variations of atomic arrangements in the scanning process were observed in situ with a time resolution of 1/60 s. Possibility of combination-type microscopy of HRTEM and scanning-probe microscopy was shown. Mechanical processing at one atomic-layer scale was demonstrated.

THz-driven resonant structures such as rectangular slits has been widely used for femtosecond temporal characterization of ultrashort electron beams produced in photocathode rf guns. In this work, we show that a simple non-resonant subwavelength pinhole perforated on a metal foil can also provide femtosecond temporal diagnostics to ultrashort electron beams. The streaking effect of the pinhole structure is dominated by the reflected THz field near the surface. A time resolution of 1.5 fs is achieved for a 3 MeV electron beam using a pinhole with diameter of 100 μm. A secondary beam, common in photocathode rf gun but hard to observe with conventional beam diagnostic methods, is also observed with the THz pinhole deflector. In addition to measuring the temporal information of the electron beam, we show that the pinhole may also be used to measure the properties of the THz pulse using the electron beam as the probe. We believe such simple and commercially available pinhole structure is a robust and viable method for femtosecond temporal diagnostics of ultrashort electron beams.

A general microwave electron gun (rf gun) theory and some new ideas about further reducing the emittance of electron beams are reviewed and introduced briefly. Experimental results are presented from a thermionic cathode rf gun which has been developed for Beijing Free-Electron Laser Project (BFELP) at Institute of High Energy Physics (IHEP), Academia Sinica as the injector of 30 MeV constant gradient linac. LaB6 is used as the thermionic emitter with 100 crystal surface and the temperature around 1620 °C. Back bombardment heating effect and another effect connected with the cathode surface temperature was observed. An electron beam with macropulse length 4 μs, repetition rate 12.5 Hz, greater than 500 mA beam current during macropulse and around 1 MeV maximum energy has been obtained. In the last section of this article, two useful formulas used to determine electric field distribution near the cathode surface and coupling coefficient β of a rf gun are introduced.

Modern dual photon energy linear accelerators often come with a few megavoltage electron beams. The megavoltage electron beam has limited range and relative sharp distal falloff in its depth dose curve compared to that of megavoltage photon beam. Its radiation dose is often delivered appositionally to cover the target volume to its distal 90% depth dose (d90), while avoiding the normal ‐ sometimes critical ‐ structure immediately distal to the target. Varian linear accelerators currently offer selected electron beams of 4, 6, 9, 12, 16 and 20 MeV electron beam energies. However, intermediate electron energy is often needed for optimal dose distribution. In this study we investigated electron beam characteristics and implemented two intermediate 7 and 11 MeV electron beams on Varian linear accelerators. Comprehensive tests and measurements indicated the new electron beams met all dosimetry parameter criteria and operational safety standards. Between the two new electron beams and the existing electron beams we were able to provide a choice of electron beams of 4, 6, 7, 9, 11, 12, 16 and 20 MeV electron energies, which had d90 depth between 1.5 cm and 6.0 cm (from 1.5 cm to 4.0 cm in 0.5 cm increments) to meet our clinical needs.PACS number: 87.56.bd.

Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique