This paper describes fabrication of polycrystalline magnesium aluminate spinel by microwave sintering and hot pressing and investigation of its dielectric properties in the terahertz frequency range. The significant interest in this material is associated with its high performance properties including the impact strength and the microhardness, the possibility to operate at high temperatures and to withstand exposure to radiation. In addition, using an appropriate fabrication method the spinel can be made optically transparent. Along with that it has been shown that MgAl2O4 ceramics obtained by microwave sintering of high-purity ultrafine powders has a sufficiently high transparency in the millimeter-wave range (the dielectric loss tangent tan δ < 6.0·10-4 at a frequency of 200 GHz) [1]. Such ceramics can be used both in microelectronics, for example, for the manufacture of microwave ceramic resonators, and in high-power electronics – to create output windows in high-power vacuum devices of the millimeter-wave range. In general, the processing route of low-loss ceramics fabrication follows the common practice of ceramic technology, viz. powder production, compaction and sintering. A special attention is paid to the purity and dispersity of the synthesized powder materials. In our case the initial powders were prepared by the sol-gel method. This method of synthesis and deep purification makes it possible to obtain high-purity, nanosized powders. We used two approaches for the ceramics consolidation in this work: microwave sintering and hot pressing of the powder compacts. The absence of heating elements in the microwave furnace ensures pure vacuum conditions, which makes it possible to obtain ceramics with low dielectric losses. It should be noted that one of the problems of spinel sintering is a considerable loss of magnesium (Mg) during long-term exposure to high temperature in vacuum. The capability of achieving high heating rates inherent in microwave processing allows retaining the stoichiometry of the spinel. As for hot pressing, this method provides additional densification of the sintered material and results in achieving high transparency in the visible and near-infrared range. However, the hot pressing method of spinel consolidation requires introduction of sintering additives, such as lithium fluoride (LiF), which can affect the dielectric characteristics of the final material. The microwave sintering of the compacts was carried out in a gyrotron-based system operating in CW mode at a frequency of 24 GHz with a variable output power up to 6 kW. Hot pressing of the compacts was carried out using a uniaxial press at a temperature of 1600 °C and a pressure of 35 MPa. To study the dielectric properties, high-density ceramic samples were produced with a diameter of 25 mm and a thickness of 2 mm. The samples fabricated by microwave sintering were either pure spinel or 1 % Y2O3-doped. The samples produced by uniaxial hot pressing with the LiF sintering additives were optically transparent. The dielectric characteristics were studied in the frequency range 0.05÷0.3 THz using a spectrometer based on an open high-quality factor Fabry –Perot resonator, and in the range 0.6 ÷3.3 THz using the method of terahertz time-domain spectroscopy. The results have shown that the refractive index of the samples varies weakly from 2.85 to 2.92 in the studied frequency range (Fig. 1). The refractive index of optically transparent ceramics is slightly higher than that of opaque samples (fabricated by microwave sintering). This fact may be associated with a higher density of optically transparent materials.

It is shown that the currently advertised 'new form of discharge' (open discharge) - photoemission with a virtually noneroding cathode and 'anomalously high' (near unity) energy efficiency of electron beam (EB) formation - cannot, in fact, be implemented. In reality, such a discharge fits well into the familiar pattern of glow discharges controlled by heavy particle emission. Thus, in known EB sources, an energy efficiency of up to approximate to 0.8 is ensured by fast atoms from ion charge exchange in strong discharge fields. However, charge multiplication and cathode-directed ion drift are both proportional to the flux of cathode electrons (including photoemitted ones), implying that the energy efficiency cannot increase through its contribution. It is this incorrect efficiency measurement methodology which gives the result of an efficiency approaching unity.

This work pursues investigations into the discharge with a cathode plasma in a cavity one wall of which is an insulating plate with a hole D in diameter (the cavity is 0.5 or 1.5 mm wide). This discharge is thoroughly analyzed in comparison with the high-voltage hollow-cathode discharge. Owing to the reduced emission of electrons from the cathode plasma, the discharge becomes more stable against transition to the low-voltage form, as a result of which an electron beam can be generated under higher gas pressures. Such a beam formed at the entrance to the cavity is used as an auxiliary one that propagates over the remaining (flat) surface of the cathode and adds to the gas ionization. Accordingly, the beam current from the main discharge to the anode rises (high-current regime). Wide-aperture (D = 22 mm) ≈1-μs-long pulsed beams with a current an order of magnitude higher than the total current of the equivalent anomalous discharge are obtained. Experiments are carried out at a helium pressure to 20 Torr and a voltage from 1 to 20 kV.

Pulsed discharge in a gas with additional ionization produced by an auxiliary electron beam has been studied. The auxiliary beam is emitted from the wall of a protrusion on the cathode and propagates along the cathode surface. The additional ionization ensures that the main wide-aperture beam current to the anode and the total discharge current exceed that of an equivalent anomalous discharge by up to an order of magnitude. The proposed modified discharge operates in a broad range of pressures (up to several dozen Torr) without other changes in the system design.

FLASH, the free electron laser at DESY in Hamburg, is the first facility of its kind providing intense, femtosecond radiation pulses in the vacuum ultraviolet and soft X-ray range for user experiments. A broad science program has been started in summer 2005 with applications in atomic and molecular physics, cluster physics, the study of warm dense matter and surface dynamics, and diffraction imaging of small structures and biological samples with nanometer resolution. A selection of first experimental results is highlighted in this paper.

A discharge with plasma filling a flat-bottom cavity of depth δ in the cathode, partly closed by a dielectric plate with a hole (determining the aperture of the discharge between the cavity bottom and the anode), has been studied. In a discharge cell of type 1 with δ = 0.5 mm and a hole diameter of 22 mm, a pulsed electron beam was obtained with a duration of t EB = 700 ns and a beam current j EB approximately 10 times greater than that (j AD) of the equivalent anomalous discharge (at fixed discharge voltage U and gas pressure p He = 3.5 Torr). An electric field with the direction opposite to the field of applied voltage appeared at the cathode that was related to a space charge formed at the cathode plasma boundary, which could not follow a rapid drop of voltage across the discharge gap. In a discharge cell of type 2 with δ = 0.5 mm and a narrow slit (S = 0.1 × 5 cm2) in the dielectric plate, a pulsed electron beam was obtained with a duration of t EB = 2 ns and a beam current of j EB = 0.7 kA/cm2 (j EB/j AD = 1.5) at U = 4.2 kV and p He = 50 Torr.

Scaling parameters that account for the efficient formation of electron beams in sources based on an anomalous glow discharge are considered, and the processes leading to the violation of scaling laws are analyzed. Estimates show that the working pressure range in such sources can be increased significantly. The results obtained can be used to optimize discharge regimes with required electron beam parameters. It is demonstrated that sources with plane-parallel continuous electrodes can operate at pressures as high as 100 Torr (which is one to two orders of magnitude higher than that in similar existing electron beam sources), provided that parasitic spark breakdowns at the insulator near the discharge cathode are suppressed. The sources designed can be used for direct excitation of laser working media or their preionization by soft X-rays.

A highly stable sparkless discharge can be obtained using preionization by an electron beam, photons, and plasma electrons formed in a barrier open discharge, for which a grid electrode ( that also serves as a cathode for the main discharge) is situated immediately on a dielectric-coated electrode surface. In the standard gas mixture for an ArF laser at a pressure of 2.5 bar, an energy deposition of 2.5 J/cm(3) in a 12-ns pulse at a specific power of 210 MW/cm(3) was achieved. The results are of interest for the development of technological excimer lasers operating at a high pulse repetition rate.

Results are presented from numerical simulations of the time evolution of open discharges in helium that are excited in the presence of an anode grid and generate electron beams over a wide range of helium pressures (up to ∼10 4 Pa). It is shown that electron emission from the cathode is almost entirely dominated by the bombardment of the cathode by heavy particles, while the contribution of photoemission is negligibly small. For conditions typical of open discharges (for a helium pressure of 4 kPa and voltage amplitude of 7.4 keV), the following percentages are obtained for the partial contributions of the main processes whereby the discharge develops: 96% for atom-electron emission, 2.3% for electron multiplication in the discharge gap, 1.7% for ion-electron emission, 2 × 10 −3 % for electron emission under the action of metastable atoms diffusing from the discharge gap toward the cathode, and 2 × 10 −4 % for photoemission from the cathode.

Previous conclusions concerning a general mechanism of electron beam formation in glow discharges of various types, including the open discharge, are confirmed. The discharge behavior is determined by ionization processes and by the bombardment of cathode with fast heavy particles, in agreement with well-established notions about the glow discharge. Neither compensation currents nor the electron beam current can account for the photoelectron mechanism of the open discharge, despite still existing opposite assumptions.

A barrier open discharge operating under extremely hindered conditions is proposed, in which the working gas (helium) pressure admitting the electron beam formation can be increased up to 0.8 atm (more than ten times as compared to the usual open discharge) without any loss in stability. The anode grid (with 0.17 mm holes) was made separately and then placed on a dielectric substrate (ε ≈ 1000), but it can also be formed using vacuum deposition or photolithographic techniques. It is suggested that, by decreasing the diameter of holes in the anode grid, it is possible to provide for a stable discharge operation under still higher gas pressures.

Methods used for the measurement of the efficiency of the electron beam formation in an open discharge are critically analyzed. It is shown that a high efficiency determined as the ratio of the beam current to the total current, η= j e / j , can serve neither as evidence for one of the open discharge mechanisms nor as a true criterion of the energy efficiency of the beam formation. Under typical open discharge conditions, when electrons in the entire discharge gap occur in the runaway regime, the η value is close to the geometric transparency of the anode grid and is independent of the coefficient of electron emission (γ) from the cathode. It is also demonstrated that recent experiments reported by P.A. Bokhan and D.E. Zakrevsky [Tech. Phys. Lett. 28 , 74 (2002) do not prove the photoelectron nature of the discharge studied but, on the contrary, confirm once again that the open discharge is not induced by photoelectrons.

An analysis of the glow discharge conditions shows that processes involved in this phenomenon should be described in terms of the electron emission coefficients γ determined in a technical (γt) rather than in an ultrahigh vacuum. The results of calculations of the electron beam formation efficiency η in a glow discharge, performed using the γt values, agree well with the experimentally measured efficiencies. Further refinement of these calculations is inexpedient because of unavoidable uncertainty of the γ values under real discharge conditions.

Estimates of the energy necessary to provide for the emission of a single photoelectron from a cathode in open discharge show that (i) the discharge cannot be maintained at the expense of photoemission and (ii) the contribution of photoelectrons to the energy efficiency of the electron beam formation is negligibly small.

The qualitative pattern of electron beam formation in discharges of various types, including hollow-anode, anomalous, and open, is considered from the most general standpoints. The considerations confirm the conclusion made by the author previously: atom-induced electron emission ensures a high efficiency of the electron beam formation, provided that sufficiently fast atoms are formed in the recharge process, which is consistent with commonly accepted notions about the glow discharge. Examples of the general character are presented which indicate that the open discharge is incompatible with the photoelectron mechanism, although publications in favor of the latter model still appear. The latest of such publications is critically analyzed.

The results of calculations reported for the first time indicate that the electron emission from a cathode, which is necessary to explain the main properties of an open discharge, is stimulated by the cathode bombardment by fast atoms rather than by photons (as was believed for a long time). The calculations are based on the results of measurements of the electric field strength in the discharge region and of the coefficient of electron emission from a cathode bombarded by helium atoms and ions. It is also demonstrated that the efficiency of the gas ionization by fast atom bombardment is significant at a voltage of several keV and becomes comparable with the electron-impact ionization in the anode plasma at a voltage equal to a few tens of keV.

The research reported in three preceding papers is summarized. The photoelectron mechanism for the formation of electron beams in an open discharge with a grid anode is revised. Revision of the discharge mechanism also requires revision of the optimal conditions for its excitation. A new method for pumping lasers by beams of fast atoms formed in an open discharge with an inverted voltage is proposed.

An attempt is made to create an electric-discharge source for pumping argon, krypton, and xenon dimer lasers. The device is based on a method proposed previously by the authors, wherein confinement of the discharge is achieved by removing the cathode spot from the main discharge region and closing the discharge to the spot along a narrow extended auxiliary plasma channel. The conditions for the formation of such a discharge are investigated. The high stability of the sparkless stage of the discharge permits the first-ever attainment of energy depositions at the level of 100 J/cm2 at pressures ∼10 atm, a level several orders of magnitude higher than is attainable by conventional methods. A discharge cell and power supply system are designed for a multisectional discharge with an active length of 200 mm, and the reliability of the entire apparatus is demonstrated in long-term use.