Test of the Critical Theory of Electrical Breakdown in Vacuum

Wuensch

2009

224

130

A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector S c , is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity S c takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both traveling wave and standing wave, and the value of S c achieved in the experiments agrees well with analytical estimates.

Section: Pulsed Heating By Field Emission Currentssupporting

confidence: 89%

New local field quantity describing the high gradient limit of accelerating structures

Grudiev

Wuensch

2009

224

130

“…In particular, the breakdown field E b has a mean of 159 MV=m and a standard deviation of 32%, the field enhancement factor has a mean of 77 and a standard deviation of 36%, and the local breakdown field E b has a mean of 10:8 GV=m and a standard deviation of 16%. This value for the local breakdown field of copper is consistent with other experiments [28].…”

Section: A Breakdown Field and Field Enhancement Factor Measurementssupporting

confidence: 93%

“…Consequently, the breakdown field can be well predicted at any moment if the present value of the electrodes is known. This result is in agreement with previous measurements, where the local breakdown field is claimed to be only dependent on the electrode material [2,28]. The evolution of the local breakdown field during a typical conditioning experiment is given in Fig.…”

Section: A Breakdown Field and Field Enhancement Factor Measurementssupporting

confidence: 92%

Investigation of the dc vacuum breakdown mechanism

Descoeudres

Levinsen

et al. 2009

Breakdowns occurring in rf accelerating structures will limit the ultimate performance of future linear colliders such as the Compact Linear Collider (CLIC). Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN to better understand the vacuum breakdown mechanism in a simple setup. Measurements of the field enhancement factor show that the local breakdown field is constant and depends only on the electrode material. With copper electrodes, the local breakdown field is around 10:8 GV=m, independent of the gap distance. The value characterizes the electrode surface state, and the next macroscopic breakdown field can be well predicted. In breakdown rate experiments, where a constant field is applied to the electrodes, clusters of consecutive breakdowns alternate with quiet periods. The occurrence and lengths of these clusters and quiet periods depend on the evolution of. The application of a high field can even modify the electrode surface in the absence of breakdown. Measurements of time delays to breakdown show two distinct populations, immediate and delayed breakdowns, indicating that two different mechanisms could exist. The ratio of these two populations depends on the conditioning state of the electrodes and on material. Gas release during breakdown is dominated by H 2 and CO. This degassing is mainly due to electron-stimulated desorption. During the quiet periods without breakdown, gases are also released but the quantities are much smaller. All the measurements presented here emphasize the crucial role of field emission in the breakdown triggering.

“…5 are valid for the geometry of the present experiment, with the specified separation between electrodes (20 m). Modifying the interelectrode distance can lead to different values [14,23]. Moreover, the rf breakdown field is known to depend also on the pulse length, and therefore the values of E b in dc and rf can be different.…”

Section: Resultsmentioning

confidence: 99%

dc breakdown conditioning and breakdown rate of metals and metallic alloys under ultrahigh vacuum

Descoeudres

Ramsvik

et al. 2009

The rf accelerating structures of the Compact Linear Collider (CLIC) require a material capable of sustaining high electric field with a low breakdown rate and low induced damage. Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN in order to test candidate materials and surface preparations, and have a better understanding of the breakdown mechanism under ultrahigh vacuum in a simple setup. Conditioning speeds and breakdown fields of several metals and alloys have been measured. The average breakdown field after conditioning ranges from 100 MV=m for Al to 850 MV=m for stainless steel, and is around 170 MV=m for Cu which is the present base-line material for CLIC structures. The results indicate clearly that the breakdown field is limited by the cathode. The presence of a thin cuprous oxide film at the surface of copper electrodes significantly increases the breakdown field. On the other hand, the conditioning speed of Mo is improved by removing oxides at the surface with a vacuum heat treatment, typically at 875 C for 2 hours. Surface finishing treatments of Cu samples only affect the very first breakdowns. More generally, surface treatments have an effect on the conditioning process itself, but not on the average breakdown field reached after the conditioning phase. In analogy to rf, the breakdown probability has been measured in dc with Cu and Mo electrodes. The dc data show similar behavior as rf as a function of the applied electric field.