We report the results of observations of a new regime of ion instabilities at the Advanced Light Source (ALS). With artificially increased pressure and gaps in the bunch train large enough to avoid multiturn ion trapping, we observed a factor of 2 -3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. The observations are qualitatively consistent with the "fast beam-ion instability" [T. O. Raubenheimer and F. Zimmermann, Phys. Rev. E 52, 5487 (1995) Ion trapping has long been recognized as a potential limitation in electron storage rings. The ions, generated by beam-gas collisions, become trapped in the negative potential of the beam and accumulate over multiple beam passages. The trapped ions are then observed to cause a number of deleterious effects such as an increasing beam phase space, a broadening and shifting of the beam transverse oscillation frequencies (tunes), collective beam instabilities, and beam lifetime reductions [1][2][3]. All of these effects are of concern for the next generation of accelerators, such as the B factories or damping rings for future linear colliders, which will store high beam currents with closely spaced bunches and ultralow beam emittances.One of the standard solutions used to prevent ion trapping is to include a gap in the bunch train which is long compared to the bunch spacing. In this case, the ions are first strongly focused by the passing electron bunches and then over focused in the gap. With a sufficiently large gap, the ions can be driven to large amplitudes, where they form a diffuse halo and do not affect the beam.In this paper, we describe experiments studying a new regime of transient ion instabilities that was predicted to arise in future electron storage rings [4,5] and linacs with bunch trains. These future rings and linacs, which will be operated with higher beam currents, small transverse beam emittances, and long bunch trains, will use ion clearing gaps to prevent conventional ion trapping. But, while the ion clearing gap may suppress the conventional ion instabilities, it will not suppress a transient beamion instability, where ions generated and trapped during the passage of a single train lead to a fast instability. While both conventional and transient ion instabilities have the same origin, namely, ions produced by the beam, they have different manifestations and, more importantly, the new transient instability can arise even after the conventional ion instability is cured.In many future rings, this transient instability is predicted to have very fast growth rates, much faster than the damping rates of existing and proposed transverse feedback systems, and thus is a potential limitation. To study this instability at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, we intentionally added helium gas to the storage-ring vacuum system until the residual gas pressure was increased from the nominal value of 0.25 nTorr by about 2 orders of magnitude. This brought the predicted...

Magnetically aligned samples of sintered YBa&Cu307 z were used to test the effects of oxygendeficiency 5 (with 5 0.2) on the superconductive magnetization M, critical current density J" irreversibility field B;",upper critical field H, t, coherence length g, condensation energy F"penetration depth A, , and related properties as a function of temperature T and applied field H~~c. In selected cases, studies were also made with H~~ab. The 0 content was monitored in situ by thermogravimetric analysis. The open porosity and granularity of the material allowed rapid and homogeneous oxygenation. We found no significant enhancement of intragrain J, with chain-site 0 defects. With few exceptions, maximum J, occurred at full oxygenation. This implies that chain-site 0 defects are not strong or effective pinning centers over most of the field-temperature regime investigated. Except for T"which was practically independent of 5 within the interval 0 & 5~0. 11 (so called "90-K T, plateau" ), most properties such as J" F"H,z, B;", (T), A, , and g were strongly and continuously influenced by the 0 deficiency. The observed abnormal magnetization with H~~c was weak at low T but became more pronounced as T and fi increased. No abnormal magnetization was detected with H~~ab. As oxygen was removed, B;"(T)and H,~(T) separated, and both lines shifted to lower T and lower B. Moreover, B;,"was strongly correlated with J, at low temperature. Determination of the thermodynamic critical field H, yielded condensation energies F,(5) that were well correlated with J,(5). The results indicate that 0 defects weaken the energy barrier of the existing pinning sites and have a negative overall effect on the capacity of YBazCu307 to carry loss-free currents.

Large enhancements in the critical current density Jc were produced in single crystals of the high-temperature superconductor Bi2Sr2Ca1Cu2O8 by irradiation with high energy Sn ions. In addition, the irreversibility line was moved to considerably higher magnetic fields. In contrast with analogous studies on Y1Ba2Cu3O7, there was little, if any, selective pinning when the magnetizing field was applied parallel to the linear, ion-damage-produced tracks.

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