F. Schwirzke

Vacuum breakdown on metal surfaces

The unipolar arc model is described. Experimental proof that unipolar arcing represents a discharge form which easily leads to explosive plasma formation is provided. Using a laser-produced plasma, it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of 0.5 MV/cm. The experimental results show that cathode spots are formed by unipolar arching. The localized buildup of plasma above an electron-emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of a unipolar arc provides explosive plasma formation. >

Correlation of stress‐wave profiles and the dynamics of the plasma produced by laser irradiation of plane solid targets

The interaction of 20‐nsec 300‐MW pulses of 1.06‐μm laser radiation with thick aluminum targets in vacuum has been studied. The time history of the target impulse has been measured with a Sandia quartz gauge. A time sequence of plasma density maps constructed from floating double‐probe data has been used with measured expansion velocities to estimate the plasma momentum. The results show that the stress wave is predominantly produced by about 10% of the evaporated target material which is ionized and expands from the surface in the form of a hot plasma during and shortly after the laser pulse. The estimated momentum of the plasma and neutral emitted particles is 5.6 g cm/sec for a typical case compared with the measured target impulse of 6.1 g cm/sec.

Onset of breakdown and formation of cathode spots

When an increasing diode voltage is applied, enhanced field emission of electrons begins from a growing number of small spots or whiskers on the cathode surface. This stimulates desorption of weakly bound adsorbates from the surface of a whisker. As the diode voltage increases, the 100-V equipotential surface moving toward the cathode is met by the desorbed neutrals moving away from the cathode, resulting in sharp risetime for the onset of ionization of desorbed neutrals by field-emitted electrons. Positive ions produced in the ionization region a few microns from the electron emitting spot are accelerated back to it. This bombardment leads to surface heating of the spot. The onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot leads to pressure and electric field distributions that ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions result in the explosive formation of cathode spot plasma. >

Background gas pressure dependence and spatial variation of spontaneously generated magnetic fields in laser−produced plasmas

A spontaneous aximuthal magnetic field generated by electron currents which flow during the interval that a high−brightness 4−nsec full width at half−maximum intensity ruby laser was incident on a metallic target has been detected, its spatial characteristics evaluated, and its dependency upon background gas pressure investigated both experimentally and theoretically. The propagation velocity of the early component of this magnetic field is greater than 108 cm/sec, more than an order of magnitude larger than the plasma convective velocity, and indicates that the electron currents responsible for this fast field must flow through the background photoionized gas. The early component is first detected at a pressure of 5×10−6 Torr, and for a fixed spatial location, reaches a peak intensity at a pressure of 2×10−3 Torr. Its intensity is typically on the order of 1−10 G, a factor 10−100 times smaller than the spontaneously generated magnetic fields which are driven by pressure gradient effects in the expanding ...

Laser Induced Unipolar Arcing

High power laser beams interact with targets by a variety of thermal, impulse and electrical effects. The laser heated plasma causes surface ablation by thermal evaporation, ion sputtering, and unipolar arcing. While the first two are purely thermal and mechanical effects, the last one, unipolar arcing, is an electrical plasma-surface interaction process which leads to crater formation, usually called laser-pitting, a process which was often observed but not well understood.

Ion formation on the surfaces of electrodes

Abstract The initial phase of the onset of electrical breakdown in a vacuum discharge is characterized by very rapid ionization of surface material which leads to a kind of “explosive” plasma formation on the electrodes. As an increasing electric field is applied between the two electrodes of a vacuum diode the ionization process is initiated by the field emission of electrons from highly localized spots on the cathode surface. Despite the fundamental importance of cathode spots for the breakdown process, the structure of cathode spots and the fast ionization rates of surface layers were not fully understood. Besides the Joule heating of the field emitting spot, the electrons also desorb contaminants and ionize some of the released neutrals. Ions produced a short distance (∼1 μm) from the spot are accelerated back towards the cathode. This ion bombardment leads to surface heating of the spot and calculations of the power deposition show that ion surface heating is initially orders of magnitude larger than Joule heating. Ion bombardment is especially important at low initial current densities since it is efficient in producing desorption and sputtering of neutrals from the surface and hence increases the neutral density which in turn increases the ionization rate. As more ions are produced, a positive space charge layer forms which enhances the electric field and thus strongly enhances the field emitted electron current. Surface heating and the buildup of positive space charge rapidly lead to further enhanced field emission and finally thermionic electron emission. The localized buildup of an ion sheath above the electron emitting spot naturally leads to pressure and electric field distributions which provide such a large electron flow and ion sputtering rates that the spot explodes into a dense plasma within a few nanoseconds.

Measurements of Spontaneous Magnetic Fields in Laser-Produced Plasmas

The creation of a dense high-temperature plasma by intense laser pulses is of prime interest to the field of thermonuclear research. Focussed onto a small deuterium-tritium fuel pellet, laser pulses can heat the matter to thermonuclear temperatures within a few nanoseconds releasing nuclear energy before the hot plasma blows apart. The present excitement about laser induced fusion is based on the results of extensive computer simulation experiments. The concept requires a sequence of time-tailored laser pulses to be focussed evenly onto a small pellet of deuterium-tritium ice. The successive ablation of outer layers of material produces a sequence of shock waves which converge space and time-wise towards the center of the sphere. These shock waves compress the core of the pellet by a factor of 103 to 104 above solid state density and heat the matter to thermonuclear temperature. In this manner a high energy state of matter can be created and investigated which otherwise exists only in the inner core of stars.

Plasma Jets Emitted by Unipolar Arcing from Spherical Laser Targets

Unipolar arcing occurs when a plasma of sufficiently high electron temperature interacts with a conducting wall. Without an external voltage applied, many electrical micro-arcs burn between the surface and the plasma driven by local variations of the sheath potential with the wall acting as both the cathode and anode. Small jets of material are ejected from cathode craters of the arcs. Spherical targets of 0.5–1 mm size were irradiated on one side with 100J/300 ps pulses from the Asterix III iodine laser at Garching. Surface damage to areas remote from the focal spot were analyzed. The occurrance of unipolar arcing and plasma jetting is consistent with the spatial energy deposition by the spreading hot-electron plasma. A model for the lateral expansion of the hot electrons around the sphere is formulated, taking into account sheath electric fields, self-generated magnetic fields and a thin dense plasma layer on the surface of the sphere produced by unipolar arcing which enables the flow of hot electrons around the sphere. Efficient ion acceleration occurs in a diode configuration formed by the magnetically insulated hot electrons on an outer spherical shell and the high positive potential of the unipolar arc plasma on the surface of the sphere.

Formation of Cathode Spots by Unipolar Arcing

Despite the fundamental importance of cathode spots for the breakdown process and the formation of a discharge, the complicated processes, the structure of the cathode spot and the source for the high current density were not fully understood. Experiments show that cathode spots are formed by unipolar arcing. The localized build-up of plasma above an electron emitting spot naturally leads to a pressure gradient and electric field distribution which causes unipolar arcing. The high current density of an arc provides explosive plasma formation of a cathode spot.

Laser Induced Breakdown and High Voltage Induced Vacuum Breakdown on Metal Surfaces

Breakdown and plasma formation on surfaces are fundamental processes in laser target interaction experiments as well as in other areas of pulsed power technology. The initial plasma formation on the surface of a laser irradiated metal target is very non-uniform. Micron-sized plasma spots form within nanoseconds. Quite similar, the initial plasma formation on the surface of a cathode of a vacuum arc, vacuum diode, and many other discharges is highly non-uniform. Micron-sized cathode spots form within nanoseconds. The concept of explosive electron emission from a cathode spot is well established in the literature. However, the details of the breakdown process were not well understood. Unipolar arcing represents a discharge form which easily leads to explosive plasma formation. Power dissipation for an unipolar arc is considerably higher than for field emitted or space charge limited current flow. Using a laser produced plasma it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have now been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of E=lMV/2.5 cm. The experimental results show that cathodes spots are formed by unipolar arcing. The localized build-up of plasma above an electron emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of an unipolar arc provides explosive plasma formation.