We report on a deep, multiwavelength study of the galaxy cluster MACS J1931.8−2634 using Chandra X-ray, Subaru optical and Very Large Array 1.4-GHz radio data. This cluster (z = 0.352) harbours one of the most X-ray luminous cool cores yet discovered, with an equivalent mass, cooling rate within the central 50 h −1 70 kpc is ∼700 M yr −1 . Unique features observed in the central core of MACS J1931.8−2634 hint to a wealth of past activity that has greatly disrupted the original cool core. The X-ray and optical data suggest oscillatory motion of the cool core along a roughly north-south direction. We also observe a spiral of relatively cool, dense, X-ray emitting gas connected to the cool core, as well as highly elongated intracluster light (ICL) surrounding the cD galaxy. For a cluster with such a high-nominal cooling rate, this cluster is missing the central metallicity peak almost always seen in the cool-core clusters, which suggest bulk transport of cool gas out to large distances from the centre. Extended radio emission is observed surrounding the central active galactic nucleus (AGN), elongated in the east-west direction, spatially coincident with X-ray cavities. The power input required to inflate these 'bubbles' is estimated from both the X-ray and radio emission to reside between P jet ∼ 4-14 × 10 45 erg s −1 , putting it among the most powerful jets ever observed. This combination of a powerful AGN outburst and bulk motion of the cool core has resulted in two X-ray bright ridges to form to the north and south of the central AGN at a distance of approximately 25 kpc. The northern ridge has spectral characteristics typical of cool cores: it contains low-temperature high-density metal-rich gas and is consistent with being a remnant of the cool core after it was disrupted by the AGN and bulk motions. It is also the site of Hα filaments and young stars. The X-ray spectroscopic cooling rate associated with this ridge is ∼165 M yr −1 , which agrees with the estimate of the star formation rate from broad-band optical imaging (∼170 M yr −1 ). MACS J1931.8−2634 appears to harbour one of the most profoundly disrupted low-entropy cores observed in a cluster, and offers new insights into the survivability of cool cores in the context of hierarchical structure formation.

In the course of developing new high peak power klystrons at SLAC, high electric fields in several regions of these devices have become an important source of vacuum breakdown phenomena. In addition, a. renewed interest in breakdown phenomena for nanosecond pulse, multi-megavoll per centimeter fields has been sparked by recent R&D work in the area of gigawalt RF sourcesThe most important regions of electrical breakdown are in the output c*vjiy gap area, the RF ceramic windows, and the gun ceramic insulator.The details of the observed breakdown in these regions, ex periments performed to understand the phenomena arid solu tions found to alleviate the problems will be discussed.Recently experiments have been performed on a new proto type R&D klystron. Peak electric fields across the output cavity gaps of this klystron exceed 2 MV/cm. The effect of peak field duration (i,e> pulse width) on the onset of breakdown havt-been measured. The pulse widths varied from tens of nanoseconds to microseconds. Results from these experiments will be presented.

The paper describes the proposed design for the 476 MHz accelerating cavity for the SLAC/LBL/LLNL B-Factory. This machine will require a high power throughput to the beam because of the large synchrotron radiation losses, and very low impedances for the higher order modes because of the high current proposed. Use of conventional construction in copper means that careful consideration has to be paid to the problem of cooling. The need for a high shunt impedance for the accelerating mode dictated the use of a re-entrant shape. This maximized the impedance of the fundamental mode with respect to the troublesome longitudinal and deflecting higher order modes, when compared to open or "bell shaped" designs. A specialized damping scheme was employed to reduce the higher order mode impedances while sacrificing as little of the fundamental mode power as possible. This was required to suppress the growth of coupled bunch beam instabilities and minimize the workload of the feedback system needed to control them. A window design capable of handling the high power was also required.

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