Picosecond High Voltage Switching of a Pressurized Spark Gap

Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of (low energy) electrons into a laser wakefield puts extreme demands on the shortness and timing, i.e. a fraction of a plasma period, typically less than 100 fs. In order to meet these requirements, we have revisited the concept of pulsed DC acceleration. Simulations have shown that this concept can be successful if high voltage pulses (of the order MV) can be switched with picosecond precision. As a fust step towards this goal, a IO kV laser triggered pressurized spark gap was designed and built. One of the limitations on risetime and jitter in high voltage laser triggered spark gaps is the initial breakdown process. Since this is a stochastic process it will cause jitter, and the growth rate of the plasma will determine the fastest possible risetime of the pulse. A way to overcome this limitation is to create a line focus between the electrodes, using a high power femtosecond laser. At laser intensities above approximately 10" W/m2 near- threshold or tunneling ionization causes near-instantaneous ionization of. a complete plasma channel between the electrodes, much like a photoconductive semi-conductor switch. Because of the instantaneous ionization and the high degree of ionization in the plasma channel, jitter and risetime are reduced considerably. We will present the fust results from switching of a 10 kV spark gap with 3 mm inter-electrode distance, using a femtosecond Ti:Sapphire laser. A line focus of the laser is created, using cylindrical optics. Folded-wave interferomeny will he described to study the development of the plasma channel on femtosecond timescales

Picosecond high voltage switching for pulsed DC acceleration

Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of a (low energy) electron bunch into a laser wakefield requires acceleration gradients of the order GV/m. In principle DC acceleration can achieve GV/m acceleration gradients. If high voltage pulses of the order MV can be switched with picosecond precision, the performance of such an accelerator would be greatly enhanced and even multistage DC acceleration would become feasible. Presently risetime and jitter of high voltage pulses in high voltage laser triggered spark gaps are limited to the nanosecond regime by the initial stochastic breakdown process in the gap. A way to overcome this limitation is to create a line focus between the electrodes with an intensity above 1018 W/m2 using a high power femtosecond Ti:Sapphire laser. Because of the instantaneous ionization and high degree of ionization in the plasma channel, picosecond switching precision can be achieved and jitter is reduced significantly. A spark gap test setup with 3 mm interelectrode distance has been build and the first measurements have been done. Femtosecond diagnostics for characterization of the laser induced plasma and electro-optic diagnostics for the high voltage pulse have been developed

Single-cycle surface plasmon polaritons on a bare metal wire excited by relativistic electrons

Terahertz (THz) pulses are applied in areas as diverse as materials science, communication and biosensing. Techniques for subwavelength concentration of THz pulses give access to a rapidly growing range of spatial scales and field intensities. Here we experimentally demonstrate a method to generate intense THz pulses on a metal wire, thereby introducing the possibility of wave-guiding and focussing of the full THz pulse energy to subwavelength spotsizes. This enables endoscopic sensing, single-shot subwavelength THz imaging and study of strongly nonlinear THz phenomena. We generate THz surface plasmon polaritons (SPPs) by launching electron bunches onto the tip of a bare metal wire. Bunches with 160 pC charge and ≈6 ps duration yield SPPs with 6-10 ps duration and 0.4±0.1 MV m-1 electric field strength on a 1.5 mm diameter aluminium wire. These are the most intense SPPs reported on a wire. The SPPs are shown to propagate around a 90° bend.</p

Hydrogen content of plasma deposited a-Si:H

Amorphous hydrogenated Si is deposited using a remote Ar/H plasma. The plasma is generated in a d.c. thermal arc and expands into a low pressure chamber (20 Pa). Pure silane is injected into the plasma jet immediately after the arc source in a typical flow mixt. of Ar:H2:SiH4 = 55:10:6 scc/s. At the low Te in the jet (0.3 eV), silane radicals are produced mainly by H abstraction. In-situ ellipsometry yields refractive indexes of 3.6-4.2 at 632.8 nm and growth rates of 10-20 nm/s. FTIR anal. yields a H content of 9-25 at.% and refractive indexes of 2.7-3.3 in the IR. The SiH d. decreases with increasing H content, whereas the SiH2 d. increases, indicating a deterioration of the microstructure. The optical bandgap remains const. at .apprx.1.72 eV. The photocond. is of the order 10-6 (Wcm)-1 and the photoresponse 106. [on SciFinder (R)

An expanding thermal plasma for deposition of a-Si:H

A remote argon/hydrogen plasma is used to deposit amorphous hydrogenated silicon. The plasma is generated in a DC thermal arc (typical operating conditions 0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H2:SiH4=55:10:6 scc/s. The electron temperature in the jet is low (typ. 0.3 eV): silane radicals are thought to be produced mainly by hydrogen abstraction, but also by a sequence of dissociative charge exchange and consecutive dissociative recombination. In-situ ellipsometry yields refractive indices of 3.6-4.2 at 632.8 nm and growth rates of 10-20 nm/s. FTIR analysis yields a hydrogen content of 9-25 at.% and refractive indices of 2.7-3.3 in the infrared. The SiH density decreases with increasing hydrogen content, whereas the SiH2 density increases. Above 11 at.%, the majority of hydrogen is bonded in the SiH2 configuration. The optical bandgap remains constant at approximately 1.72 eV. The photoconductivity is of the order 10-6 (Ωcm)-1 and the photoresponse 106.</p