Huaibi Chen

Note: Single-shot continuously time-resolved MeV ultrafast electron diffraction

We have demonstrated single-shot continuously time-resolved MeV ultrafast electron diffraction using a static single crystal gold sample. An MeV high density electron pulse was used to probe the sample and then streaked by an rf deflecting cavity. The single-shot, high quality, streaked diffraction pattern allowed structural information within several picoseconds to be continuously temporally resolved with an approximately 200 fs resolution. The temporal resolution can be straightforwardly improved to 100 fs by increasing the streaking strength. We foresee that this system would become a powerful tool for ultrafast structural dynamics studies.

Experimental NMR realization of a generalized quantum search algorithm

A generalized quantum search algorithm, where phase inversions for the marked state and the prepared state are replaced by pi /2 phase rotations, is realized in a 2-qubit NMR heteronuclear system. The quantum algorithm searches a marked state with a smaller step compared to standard Grover algorithm. Phase matching requirement in quantum searching is demonstrated by comparing it with another generalized algorithm where the two phase rotations are pi /2 and 3 pi /2, respectively. Pulse sequences which include non-90 degrees pulses are given

Generation of first hard X-ray pulse at Tsinghua Thomson Scattering X-ray Source

Tsinghua Thomson Scattering X-ray Source (TTX) is the first-of-its-kind dedicated hard X-ray source in China based on the Thomson scattering between a terawatt ultrashort laser and relativistic electron beams. In this paper, we report the experimental generation and characterization of the first hard X-ray pulses (51.7 keV) via head-on collision of an 800 nm laser and 46.7 MeV electron beams. The measured yield is 1.0 × 10(6) per pulse with an electron bunch charge of 200 pC and laser pulse energy of 300 mJ. The angular intensity distribution and energy spectra of the X-ray pulse are measured with an electron-multiplying charge-coupled device using a CsI scintillator and silicon attenuators. These measurements agree well with theoretical and simulation predictions. An imaging test using the X-ray pulse at the TTX is also presented.

In-line phase-contrast imaging based on Tsinghua Thomson scattering x-ray source.

Thomson scattering x-ray sources can produce ultrashort, energy tunable x-ray pulses characterized by high brightness, quasi-monochromatic, and high spatial coherence, which make it an ideal source for in-line phase-contrast imaging. We demonstrate the capacity of in-line phase-contrast imaging based on Tsinghua Thomson scattering X-ray source. Clear edge enhancement effect has been observed in the experiment.

Observation of Field-Emission Dependence on Stored Energy

Field emission from a solid metal surface has been continuously studied for a century over macroscopic to atomic scales. It is general knowledge that, other than the surface properties, the emitted current is governed solely by the applied electric field. A pin cathode has been used to study the dependence of field emission on stored energy in an L-band rf gun. The stored energy was changed by adjusting the axial position (distance between the cathode base and the gun back surface) of the cathode while the applied electric field on the cathode tip is kept constant. A very strong correlation of the field-emission current with the stored energy has been observed. While eliminating all possible interfering sources, an enhancement of the current by a factor of 5 was obtained as the stored energy was increased by a factor of 3. It implies that under certain circumstances a localized field emission may be significantly altered by the global parameters in a system.

A Three-Cell Superconducting Deflecting Cavity Design for the ALS at LBNL

Deflecting RF cavities may be used to generate subpicosecond x-rays by creating correlations between longitudinal and transverse phase space in electron bunches in radiation devices. Up to 2-MV defecting voltage at 1.5-GHz is required for 1.9-GeV electron beam at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory (LBNL). In this paper, we present a conceptual design for a 1.5-GHz three-cell superconducting RF cavity and its couplers. The cavity geometry and its shunt impedance at the deflecting mode are optimized using MAFIA code. The cavity impedances from lower and higher order modes (LOMs and HOMs) are computed as well. Possible schemes for damping most harmful LOM and HOM modes are discussed and simulated.

An X-band disk-and-washer accelerating structure for electron accelerators

X-band electron linacs are an excellent choice for portable accelerators. In this pager we present an X-band disk-and-washer standing wave structure is studied in this paper for a low energy accelerating tube. The DAW cavities are optimized to obtain the largest shunt impedance and quality for the TM020 mode. A set of differential equations is used to describe the optimizing process. The optimized results of a DAW structure working at 9300 MHz are given using SUPERFISH and MAFIA400. Four radial stem supports are adopted. The modes near to the working frequency are given with and without supports. The electromagnetic field building process was analyzed with T3 of MAFIA400. OFC model DAW cavities are built and the first test results are described.

Diffraction based method to reconstruct the spectrum of the Thomson scattering x-ray source

As Thomson scattering x-ray sources based on the collision of intense laser and relativistic electrons have drawn much attention in various scientific fields, there is an increasing demand for the effective methods to reconstruct the spectrum information of the ultra-short and high-intensity x-ray pulses. In this paper, a precise spectrum measurement method for the Thomson scattering x-ray sources was proposed with the diffraction of a Highly Oriented Pyrolytic Graphite (HOPG) crystal and was demonstrated at the Tsinghua Thomson scattering X-ray source. The x-ray pulse is diffracted by a 15 mm (L) ×15 mm (H)× 1 mm (D) HOPG crystal with 1° mosaic spread. By analyzing the diffraction pattern, both x-ray peak energies and energy spectral bandwidths at different polar angles can be reconstructed, which agree well with the theoretical value and simulation. The higher integral reflectivity of the HOPG crystal makes this method possible for single-shot measurement.

Passive energy jitter reduction in the cascaded third harmonic generation process

In free electron laser (FEL) systems with ultraviolet (UV) laser driven injectors, a highly stable UV source generated through cascaded third harmonic generation (THG) from an infrared (IR) source is a key element in guaranteeing the acceptable current jitter at the undulator. In this letter, the negative slope of the THG efficiency for high intensity ultrashort IR pulses is revealed to be a passive stabilization mechanism for energy jitter reduction in UV. A reduction of 2.5 times the energy jitter in UV is demonstrated in the experiment and simulations show that the energy jitter in UV can be reduced by more than one order of magnitude if the energy jitter in IR is less than 3%, with proper design of the THG efficiency curve, fulfilling the challenging requirement for UV laser stability in a broad scope of applications such as the photoinjector of x-ray FELs.

A quantitative method of coupler cavity tuning and simulation

Based on a coupling-cavity chain equivalent circuit model, a quantitative method is developed to determining the coupling coefficient and frequency deviation of a coupler cavity. This method can simplify the tuning and simulation process of coupler cavity. As an example, we derived the analytical expression of the coupling coefficient and frequency deviation of a coupler cavity of a periodic traveling wave accelerating tube. Compared with the method of R.L. Kyhl, it is consistent when the coupling is correct, and much more, it can calculate the coupling coefficient and frequency deviation when coupler cavity is not matched.