Dengsheng Wu

One-pulse driven plasma Pockels cell with DKDP crystal for repetition-rate application

Pockels cell (PPC), which can use a thin crystal to perform the uniform electro-optical effect, is ideal component as average-power optical switch with large aperture. In this paper, the key problems in PPC are analyzed for repetition-rate application, and thermo-optical effects are simulated by means of numerical modeling when average power is loaded on the electro-optical crystal. By reformative design and employing a capacity to share the gas discharge voltage, the DKDP PPC driven by one pulse is realized. As gas breakdown delay time is stable, and discharge plasma is uniformly filled the full aperture, it meets the demand of plasma electrode for the repetition-rate PPC with DKDP crystal. The switch efficiency of PPC at the whole aperture is better than 99%.

Power balance on the SG-III prototype facility

With the methods of time-division multiplexing in Frontend and angle detuning in FOA, each beam pulse on SG-III prototype facility is controlled independently and so the systematic variations of power imbalance are eliminated entirely.

Investigating on stress-induced birefringence in vacuum windows of plasma-electrode Pockels cell

An analytical model has been developed to predict the stress-induced birefringence for both circular and square windows. Our main result is that circular windows lead to minimum birefringence because of their symmetric geometry.

Compact plasma Pockels cell for TIL of SGIII laser facility

Compact plasma Pockels cells (PPC) with 70mm aperture driven by one-pulse process have been constructed for technical integration line (TIL) of SGIII laser facility. The experimental results indicate that the working range of gas pressure is wide, and the delay of gas breakdown is steady. Measurements of the optical performance show static transmittance of 93.1%, static extinction ratio of 3900, and average switching efficiency of 99.7%. Eight compact PPCs are used for the second-stage integrating experiments of TIL. By using of parallel driving technology, one driver can work for four PPCs. An analyzer of optical switch is replaced with Brewster-angle Nd-glass slabs in amplifier. Two years application results show that the PPCs can effectively minimize the growth of parasitic-oscillation, and have a high reliability.

Development of large-aperture electro-optical switch for high power laser at CAEP

Large-aperture electro-optical switch based on plasma Pockels cell (PPC) is one of important components for inertial confinement fusion (ICF) laser facility. We have demonstrated a single-pulse driven 4×1 PPC with 400mm×400mm aperture for SGIII laser facility. And four 2×1 PPCs modules with 350mm×350mm aperture have been operated in SGII update laser facility. It is different to the PPC of NIF and LMJ for its simple operation to perform Pockels effect. With optimized operation parameters, the PPCs meet the SGII-U laser requirement of four-pass amplification control. Only driven by one high voltage pulser, the simplified PPC system would be provided with less associated diagnostics, and higher reliability. To farther reduce the insert loss of the PPC, research on the large-aperture PPC based on DKDP crystal driven by one pulse is developed. And several single-pulse driven PPCs with 80mm×80mm DKDP crystal have been manufactured and operated in laser facilities.

Large aperture plasma electrodes Pockels cell for multi-pass amplified scheme of SGII upgrading laser

The multi-pass amplified scheme of SGII upgrading laser is similar as that of NIF. Large aperture plasma electrodes Pockels cell (PEPC) is the key unit of this amplified scheme. The transit time that laser beam passes through the PEPC for the first time and second time is about 270ns. PEPC should switch the state between ON and OFF in 270ns. The response time of the PEPC driven by positive-negative switching pulses can not satisfy the demand of SGII upgrading laser due to the higher generator impedance. In the single-pulse-process, the low-impedance high voltage generator based on double Blumlein pulse-forming line is used to drive the PEPC. The amplitude of single pulse is up to 21kV, while the impedance of the generator is only 6.25Ω. The theoretical charge time of the PEPC with 350mm×350mm aperture is about 54ns, and the response time of PEPC is less than 170ns in the single-pulse-process. The response time is reduced greatly. The switching efficiencies with full aperture are higher than 99.7%. The extinction contrast exceeds 381. The top width of the time window is larger than 160ns, and the bottom width is about 400ns. All the experimental results can meet the specification of SGII upgrading laser.

Single-pulse driven plasma Pockels cell with 350mm×350mm aperture

Large-aperture plasma Pockels cell is one of important components for inertial confinement fusion laser facility. We demonstrate a single-pulse driven PPC with 350mm×350mm aperture. It is different to the PPC of NIF and LMJ for its simple operation to perform Pockels effect. With optimized operation parameters, the PPC meets the optical switching requirement of SGII update laser facility. Only driven by one high voltage pulser, the simplified PPC system would be provided with less associated diagnostics, less the maintenance, and higher reliability.

Switching Characteristics of Large-Aperture Electro-Optical Switches With Plasma Electrodes Part II—Comparison With Experiment

The large-aperture optical switches with plasma-electrode Pockels cell (PEPC) are important elements for the multipass laser amplifiers in the inertial-confinement fusion. In this paper, an optical switch of the PEPC driven by one-pulse process (OPP) is simulated by using a fluid model developed in a previous paper. We computationally study the physical processes of the discharge evolvements and discharge fading away in the large-aperture PEPC switches driven by the OPP. Charging on the surfaces of potassium dihydrogen phosphate crystal in the discharge determines the switching characteristics of the PEPC. The simulation results nearly coincide with the experimental measurement.

The effect of ultraviolet light on one-pulse process Pockels cell gas discharge

Pockels cell driven by one-pulse process (OPP) is a new technique different from the regular plasma electrode Pockels cell (PEPC). In the OPP regime, only the positive and negative switching pulses will be supplied on opposite sides of the crystal. The OPP Pockels cell developed in our laboratory can work well at high working pressure. In this working pressure range, the gas is steady broken at the start of the switching pulse flattop. When the working gas is evacuated to low pressure, the discharge is unsteady. In an invariable electric field, the gas discharge is determined by the initial free electrons. At low pressure, the low density of initial electrons leads to the instability of gas discharge. The ultraviolet light conduces to increasing the density of the initial free electrons, and reducing the randomness of the initial free electrons. In this experiment, the ultraviolet light is used to irradiate the cathode of the Pockels cell. The experimental results show that, at low pressure, the breakdown of the working gas is steady, and the breakdown delay is shorter with ultraviolet irradiation.

Research on 2x1 plasma electrode electro-optical switch with large aperture

In conceptual design of the prototype for SG-III facility, a full aperture electro-optical switch was placed between the cavity mirror and the main amplifier to isolate the reflected beams. The beam on the cavity mirror is 240mm×240mm square. Pockells cells of conversional design with coaxial ring electrodes can not scale to such large square aperture. In the 1980s, a plasma electrode Pockels cell (PEPC) concept was invented at LLNL. It uses transparent plasma electrode formed through gas discharge as the electrodes to apply the voltage across switching crystal to rotate the polarization of a transmitted laser beam. And it can be scaled to large aperture with thin crystal. So the switch which would be used in SG-III is based on this technology. The technical integration line as a prototype of SG-III laser is actually a 4×2 beam bundle. And the full aperture optical switch is mechanically designed four apertures as a removable unit, and electrically two 2×1 PEPC putting together. So we built a 2×1 PEPC to develop the technology first. The 2×1 PEPC is a sandwich structure made of an insulating mid plane between a pair of plasma chambers. The frame of both plasma chambers are machining in duralumin. Each chamber is installed with a planar magnetic cathode and four segments spherical anodes made from stainless steel. The cathode and anode are insulated from the housing with a special shell made from plastic, and plasma is insulated from the housing by an 80-μm-thick anodic coating on the duralumin. The two plasma chambers are separated by a mid plane of glass frame with two square holes. The two holes are filled by two electro-optical crystals with a 240-mm square aperture. With the optimized operating pressure and the electrical parameters, a very good homogeneity and low resistivity plasma electrode is obtained. Finally we tested its switching performance to simulate the case that it will be used in the SG-III prototype facility. It works with a quarter wave delay voltage and the laser beam passes through PEPC twice. The average switching efficiency across the entire aperture is greater than 98.6%, the rising time of the switch is about 83ns, and the transmission of the switch is 86%.