Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches.

Relativistic charged-particle beams that generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam breakup instability and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat beams) have been shown to suppress the initial instability in slab-symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat beam, traversing near a planar dielectric lined structure. The measurements reveal the emergence of a previously unobserved skew-quadrupolelike interaction when the beam is canted transversely, which is not present when the flat beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two-particle theoretical model, with particle-in-cell simulations used to provide further insight for realistic particle distributions.

Fabrication of THz corrugated wakefield structure and its high power test.

We present overall process for developing terahertz (THz) corrugated structure and its beam-based measurement results. 0.2-THz corrugated structures were fabricated by die stamping method as the first step demonstration towards GW THz radiation source and GV/m THz wakefield accelerator. 150-[Formula: see text]m thick disks were produced from an OFHC (C10100) foil by stamping. Two types of disks were stacked alternately to form 46 mm structure with [Formula: see text] 170 corrugations. Custom assembly was designed to provide diffusion bonding with a high precision alignment of disks. The compliance of the fabricated structure have been verified through beam-based wakefield measurement at Argonne Wakefield Accelerator Facility. Both measured longitudinal and transverse wakefield showed good agreement with simulated wakefields. Measured peak gradients, 9.4 MV/m/nC for a long single bunch and 35.4 MV/m/nC for a four bunch trains, showed good agreement with the simulation.

Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line.

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a TR≈5 in a dielectric wakefield accelerator.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line.

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Phosphatidylinositol 3,5-bisphosphate defines a novel PI 3-kinase pathway in resting mouse fibroblasts.

PtdIns(3,5)P2 is identified as the product of an agonist-independent, wortmannin-sensitive pathway in resting mouse cells. Results are presented here to indicate that PtdIns(3,5)P2 is formed by phosphorylation of PtdIns3P at the D-5 position, and they suggest that relatively constant cellular levels of PtdIns3P and PtdIns(3, 5)P2 are maintained by the concerted action of PtdIns3P 5-kinase and PtdIns(3,5)P2 5-phosphatase. These studies imply a novel mechanism for the action of PtdIns-specific phosphoinositide 3-hydroxykinases in mammalian cells.

Platelet-derived growth factor-dependent cellular transformation requires either phospholipase Cgamma or phosphatidylinositol 3 kinase.

Although it has been well established that constitutive activation of receptor tyrosine kinases leads to cellular transformation, the signal relay pathways involved have not been systematically investigated. In this study we used a panel of platelet-derived growth factor (PDGF) beta receptor mutants (beta-PDGFR), which selectively activate various signal relay enzymes to define which signaling pathways are required for PDGF-dependent growth of cells in soft agar. The host cell line for these studies was Ph cells, a 3T3-like cell that expresses normal levels of the beta-PDGFR but no PDGF-alpha receptor (alpha-PDGFR). Hence, this cell system can be used to study signaling of mutant alphaPDGFRs or alpha/beta chimeras. We constructed chimeric receptors containing the alphaPDGFR extracellular domain and the betaPDGFR cytoplasmic domain harboring various phosphorylation site mutations. The mutants were expressed in Ph cells, and their ability to drive PDGF-dependent cellular transformation (growth in soft agar) was assayed. Cells infected with an empty expression vector failed to grow in soft agar, whereas introduction of the chimera with a wild-type beta-PDGFR cytoplasmic domain gave rise to a large number of colonies. In contrast, the N2F5 chimera, in which the binding sites for phospholipase Cgamma (PLC-gamma), RasGTPase-activating protein, phosphatidylinositol 3 kinase (PI3K), and SHP-2 were eliminated, failed to trigger proliferation. Restoring the binding sites for RasGTPase-activating protein or SHP-2 did not rescue the PDGF-dependent response. In contrast, receptors capable of associating with either PLC-gamma or PI3K relayed a growth signal that was comparable to wild-type receptors in the soft agar growth assay. These findings indicate that the PDGF receptor activates multiple signaling pathways that lead to cellular transformation, and that either PI3K or PLC-gamma are key initiators of such signal relay cascades.

Wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocks the assembly of peptide-MHC class II complexes.

Peptide-class II complexes are assembled in endocytic, lysosome-like compartments where newly synthesized class II molecules are targeted from the trans-Golgi network (TGN). Recent studies have implicated phosphatidylinositol 3-kinase (PI3-kinase) as an essential component in membrane trafficking from the TGN to lysosomes. Here, using subcellular fractionation, we show PI3-kinase activity associated with subcellular fractions which contain the class II peptide-loading compartment (IIPLC) in B cells. At concentrations required for inhibition of PI3-kinase activity in vivo, wortmannin blocked the processing and presentation of antigen by B cells to T cells. Treatment of B cells with wortmannin significantly limited the proteolytic degradation of invariant chain and the formation of peptide-class II complexes. Subcellular fractionation coupled with pulse-chase analyses showed that invariant chain and class II molecules trafficked to the IIPLC in wortmannin-treated cells. However, wortmannin prevented the maturation and correct targeting to the IIPLC of cathepsin D, a protease necessary for the degradation of invariant chain and assembly of processed antigen-class II complexes. These results suggest that li-class II complexes traffic to the IIPLC via a pathway that is relatively insensitive to wortmannin, but suggest a role for PI3-kinases in the trafficking of other components necessary for the assembly of processed antigen class II complexes to the IIPLC.

D-3 phosphoinositide metabolism in cells treated with platelet-derived growth factor.

Despite extensive analysis of phosphoinositide 3-hydroxykinases (PI 3-kinases) at the molecular level, comparatively little is known about the mechanisms by which products of these enzymes exert their expected second-messenger functions. This study examines the metabolism of D-3 phosphoinositides in mouse Ph-N2 fibroblasts lacking the platelet-derived growth factor (PDGF) alpha-receptor. Treatment of these cultures with BB PDGF, but not AA PDGF, resulted in transient activation of PI 3-kinase activity measured in vitro. Treatment of myo-[3H]inositol-labelled Ph-N2 cells with BB PDGF resulted in the rapid induction of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 and, to a smaller extent, PtdIns3P. The appearance of PtdIns(3,4,5)P3 preceded that of PtdIns(3,4)P2 and PtdIns3P after the addition of PDGF, suggesting that PtdIns(4,5)P2 is the preferred substrate of the agoniststimulated PI 3-kinase in intact cells. Treatment of both resting and PDGF-stimulated cells with the fungal metabolite wortmannin resulted in pronounced, selective effects on the levels of all D-3 phosphoinositides. Kinetic studies with this PI 3-kinase inhibitor revealed the presence of at least two independent routes for the biosynthesis of D-3 phosphoinositides in PDGF-treated cells.