Edmund Wyndham

Measurements on electron beams in pulsed hollow-cathode discharges

Two different regimes of electron beams have been reported in the pulsed hollow cathode discharge-a low-current, high-energy beam, and a relatively high-current beam of low energy. The high-energy beam is related to the hollow cathode geometry and is found to be present even in the absence of subsequent gas breakdown, while the low-energy beam is always associated with voltage breakdown. Detailed measurements of the spatial and temporal distribution of the electron beam transported beyond a semitransparent anode associated with gas breakdown are reported. In particular, a high-energy component is observed after electrical breakdown. Low-energy electron beams are observed to be transported beyond the anode throughout the main discharge period. >

High current electron beam generation in a vacuum transient hollow cathode discharge

Experimental observations are presented of prebreakdown electron beam generation in a transient hollow cathode discharge (THCD) in a vacuum. The discharge driver consists of a 400-kV maximum voltage, 25-nF Marx operated at 450-J stored energy coupled to a 120-ns, 1.5- Omega coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd:YAG laser is used to produce a plasma at the back of the cathode surface, inside the hollow cathode region (HCR). It is found that a plasma density of a few 10/sup 18/ cm/sup -3/ in a volume of a few mm/sup 3/ is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10-200 mtorr range. >

High Performance Next Generation EUV Lithography Light Source

EUVL solution for HVM at the 22 nm node requires a high power long-term EUV source operation with hundreds of watts at the intermediate focus output. EUV mask blank and mask defects inspections require at-wavelength tools with high brightness. Theoretical analysis with a 2-D radiation MHD code Z* has been performed to address key issues in EUV plasma sources with radiation transfer. The study shows that self-absorption defines the limiting brightness of a single EUV source, which cannot meet the requirements of the HVM tool with high efficiency and is not sufficient for critical metrology applications, given the limiting etendue of the optics. It is shown that the required irradiance can be achieved by spatial multiplexing, using multiple small sources. We present here details of the study, as well as experimental results from a novel EUV light source with an intrinsic photon collector demonstrating high brightness, the i-SoCoMo concept, where an impulse micro discharge plasma source is integrated to a photon collector based on an active plasma structure. The small physical size and low etendue properties of the i-SoCoMo unit allows a large number of such sources to be put together in one physical package and be operated in a multiplexed fashion to meet necessary power requirements.

X ray emission from X pinch experiments on the Llampüdkeñ Generator

The results from the first plasma physics experiments on the Llampüdkeñ Generator (1MA, 250 ns) are presented. X Pinch experiments have been undertaken at current levels of 400 kA with a rise time of ∼250 ns. X pinches were produced mainly from aluminium wires of different diameters and with varying numbers of wires. Results from X-ray diagnostics characterising the emitted radiation are presented. The diagnostics include filtered PIN diodes and a pinhole and slit-wire camera. Radiation of energy greater then 2.5 keV was emitted from hot spots in timescales of a few nanoseconds. Using the results from the slit-wire camera, the diameter of the hot spots is shown to be less than 5 µm.

X-ray and plasma dynamics of an intermediate size capillary discharge

A small pulsed power generator, 150 kA and 120 ns, is used to form a plasma in a 5-mm diameter alumina ceramic tube. A hollow cathode geometry is used and a preionized plasma is formed in an initial vacuum background by focussing a pulsed Nd:YAG laser onto a metallic target in the hollow cathode volume. The evolution of the preionizing plasma and its expansion into the main discharge volume may be assisted by applying a current of order Amps for a variable time before the main discharge current is applied. Strong electron beams are observed both during the preionizing stage and during the start of the main current. The plasma species and temporal evolution during the main discharge is observed using X-ray spectroscopy and X-ray pinhole imaging. On varying the rate of rise of the current in the pinching phase, the transient hollow cathode effect was found to be significant at early times in the discharge in the case of the lower value of dI/dt. Both the pinch temperature and diameter depend on varying the dI/dt from 1.5 to 3 /spl times/ 10/sup 12/ A/s. The implications of plasma injection for metal vapor capillary discharges are discussed.

Ablation dynamics in wire array Z-pinches under modifications on global magnetic field topology

The dynamics of ablation streams and precursor plasma in cylindrical wire array Z-pinches under temporal variations of the global magnetic field topology is investigated through experiments and numerical simulations. The wire arrays in these experiments are modified by replacing a pair of consecutive wires with wires of a larger diameter. This modification leads to two separate effects, both of which impact the dynamics of the precursor plasma; firstly, current is unevenly distributed between the wires and secondly, the thicker wires take longer to fully ablate. The uneven distribution of current is evidenced in the experiments by the drift of the precursor off axis due to a variation in the global magnetic field topology which modifies the direction of the ablation streams tracking the precursor position. The variation of the global magnetic field due to the presence of thick wires is studied with three-dimensional magnetohydrodynamic (MHD) simulations, showing that the global field changes from the expe...

Current measurement by Faraday effect on GEPOPU.

The design of an optical current sensor to be used in a pulsed power generator is presented. The current sensor is based on the polarization rotation by the Faraday effect. GEPOPU is a pulsed power generator, 110 kA, 120 ns double transit time, 1.5 Ω coaxial geometry, and current rise time of 50 ns. Two different optical geometries surrounding the conductor were tried, using Amici roof prism and pentaprism to go around the current once, as a way to preserve the state of polarization along the optical path by means of complementary reflections within the sensing element. We believe this to be the first time that such large and rapidly varying currents have been measured with this configuration. The values obtained for both geometries agree with the values obtained with a Rogowski coil. The traces obtained are completely noise-free and no significant time lag has been observed between the current determined from the Faraday rotation and the current measured using a Rogowski coil.

Laser initiated hollow gas-embedded Z-pinch

Results of new optical method of generating a precursor plasma for a shell gas embedded Z-pinch are presented. Experiments were performed on a pulse power generator using a peak current up to 150 kA with a rise time of 70 ns, 120 ns pulse length. The optical precursor plasma was generated by using a Nd-Y AG laser, 200 ml, 8 ns at 1.06 μm. Two different optical schemes were used, one consists of a combination of lenses capable of producing a hollow beam; while the other uses an axicon to generate the hollow beam. In both cases the hollow beam was focused at the cathode surface where metallic ring plasma, of either 2 or 6 mm diameter, is created. The annular preionization is created immediately before or during the first 30 ns after the line voltage is applied. The discharge was carried out in a chamber filled with hydrogen gas at 1/3 atm. Flat electrodes were used with 10 mm separation. The anode has a 6 mm diameter central hole to allow the passage of the preionizing laser. Optical diagnostics (schlieren, shadowgraphy and interferometry) were performed using the second harmonics of the same laser used to preionize. Preionizing using an axicon results in better formed hollow discharges.

Properties of a laser-initiated pseudospark discharge

A short-pulse Nd:YAG laser is used to illuminate the back electrode surface in the hollow cathode region of a pseudospark discharge. A fast rising step voltage pulse is applied across the electrodes at various times relative to the pulsed laser illumination. The delay and jitter to the onset of gas breakdown at various filling gas pressures are measured for different laser energies and wavelengths. It is observed that the millijoule range of a green laser applied to illuminate the hollow cathode surface at up to tens of microseconds before the application of voltage would significantly reduce both delay and jitter to discharge initiation. Higher laser energy applied after voltage rise is observed to cause gas breakdown in under 100 ns. >

Plasma outflows from wire-based z-pinch experiments driven at currents of hundreds of kiloamperes.

Preliminary results on the latest experiments regarding plasma outflows from different wire-based z-pinch configurations performed in the Llampudken generator (~350kA in ~350ns) are presented. These outflows are produced from three different experiments: cylindrical, conical and nested conical arrays. Our experiments show that it is indeed possible to produce plasma outflows from moderate size pulsed power drivers with currents of some hundreds of kiloamperes. Each one of the configurations studied here can produce a dense plasma outflow characterized by its own set of dimensionless parameters; such as Reynolds number, magnetic Reynolds number, amongst others. A dense magnetized, magneto-hydrodynamically unstable plasma outflow is produced using a modified cylindrical wire array, whereas strongly collimated jets are produced from the conical configurations. Moreover, it is possible to mimic the episodic emission of plasma outflow in a collimated jet by producing temporally separated implosions from the nested conical configuration. Finally, the characteristic and dynamics of each outflow are presented and discussed.