Experimental demonstration of optical stochastic cooling.

Particle accelerators and storage rings have been transformative instruments of discovery, and, for many applications, innovations in particle-beam cooling have been a principal driver of that success1. Stochastic cooling (SC), one of the most important conceptual and technological advances in this area2-6, cools a beam through granular sampling and correction of its phase-space structure, thus bearing resemblance to a 'Maxwell's demon'. The extension of SC from the microwave regime up to optical frequencies and bandwidths has long been pursued, as it could increase the achievable cooling rates by three to four orders of magnitude and provide a powerful tool for future accelerators. First proposed nearly 30 years ago, optical stochastic cooling (OSC) replaces the conventional microwave elements of SC with optical-frequency analogues and is, in principle, compatible with any species of charged-particle beam7,8. Here we describe a demonstration of OSC in a proof-of-principle experiment at the Fermi National Accelerator Laboratory's Integrable Optics Test Accelerator9,10. The experiment used 100-MeV electrons and a non-amplified configuration of OSC with a radiation wavelength of 950 nm, and achieved strong, simultaneous cooling of the beam in all degrees of freedom. This realization of SC at optical frequencies serves as a foundation for more advanced experiments with high-gain optical amplification, and advances opportunities for future operational OSC systems with potential benefit to a broad user community in the accelerator-based sciences.

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.

Single-pass Cr:ZnSe amplifier for broadband infrared undulator radiation.

An amplifier based on a highly-doped chromium zinc-selenide (Cr:ZnSe) crystal is proposed to increase the pulse energy emitted by an electron bunch after it passes through an undulator magnet. The primary motivation is a possible use of the amplified undulator radiation emitted by a beam circulating in a particle accelerator storage ring to increase the particle beam's phase-space density-a technique dubbed optical stochastic cooling (OSC). This paper uses a simple four energy level model to estimate the single-pass gain of Cr:ZnSe and presents numerical calculations combined with wave-optics simulations of undulator radiation to estimate the expected properties of the amplified undulator wave-packet.

Passive Ballistic Microbunching of Nonultrarelativistic Electron Bunches Using Electromagnetic Wakefields in Dielectric-Lined Waveguides.

Temporally modulated electron beams have a wide array of applications ranging from the generation of coherently enhanced electromagnetic radiation to the resonant excitation of electromagnetic wakefields in advanced-accelerator concepts. Likewise producing low-energy ultrashort microbunches could be useful for ultrafast electron diffraction and new accelerator-based light-source concepts. In this Letter we propose and experimentally demonstrate a passive microbunching technique capable of forming a picosecond bunch train at ∼6 MeV. The method relies on the excitation of electromagnetic wakefields as the beam propagates through a dielectric-lined waveguide. Owing to the nonultrarelativistic nature of the beam, the induced energy modulation eventually converts into a density modulation as the beam travels in a following free-space drift. The modulated beam is further accelerated to ∼20 MeV while preserving the imparted density modulation.

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.

Intense superradiant x rays from a compact source using a nanocathode array and emittance exchange.

A novel method of producing intense short wavelength radiation from relativistic electrons is described. The electrons are periodically bunched at the wavelength of interest enabling in-phase superradiant emission that is far more intense than from unbunched electrons. The periodic bunching is achieved in steps beginning with an array of beamlets emitted from a nanoengineered field emission array. The beamlets are then manipulated and converted to a longitudinal density modulation via a transverse-to-longitudinal emittance exchange. Periodic bunching at short wavelength is shown to be possible, and the partially coherent x-ray properties produced by inverse Compton scattering from an intense laser are estimated. The proposed method increases the efficiency of x-ray production by several orders of magnitude, potentially enabling compact x-ray sources to produce brilliance and flux similar to major synchrotron facilities.

Generation and characterization of electron bunches with ramped current profiles in a dual-frequency superconducting linear accelerator.

We report on the successful experimental generation of electron bunches with ramped current profiles. The technique relies on impressing nonlinear correlations in the longitudinal phase space using a superconducing radio frequency linear accelerator operating at two frequencies and a current-enhancing dispersive section. The produced ~700-MeV bunches have peak currents of the order of a kilo-Ampère. Data taken for various accelerator settings demonstrate the versatility of the method and, in particular, its ability to produce current profiles that have a quasilinear dependency on the longitudinal (temporal) coordinate. The measured bunch parameters are shown, via numerical simulations, to produce gigavolt-per-meter peak accelerating electric fields with transformer ratios larger than 2 in dielectric-lined waveguides.

Tunable subpicosecond electron-bunch-train generation using a transverse-to-longitudinal phase-space exchange technique.

We report on the experimental generation of a train of subpicosecond electron bunches. The bunch train generation is accomplished using a beam line capable of exchanging the coordinates between the horizontal and longitudinal degrees of freedom. An initial beam consisting of a set of horizontally separated beamlets is converted into a train of bunches temporally separated with tunable bunch duration and separation. The experiment reported in this Letter unambiguously demonstrates the conversion process and its versatility.

Global AIDS epidemic: time to turn the tide.

HIV/AIDS is catastrophic both from a public health perspective and in terms of its impact on economic and social stability in many of the most severely affected nations, including virtually all of southern Africa. A public health response alone is insufficient to address this devastating epidemic. Political leadership at the highest levels is needed to mobilize a multisectoral response to the impact of HIV/AIDS on educational systems, industry, agriculture, the military, and other sectors. With a few notable exceptions, political response was slow to mobilize in the early years of the epidemic, but response has dramatically improved in the past 18 months. The Joint United Nations Programme on HIV/AIDS (UNAIDS) is involved in ongoing efforts to encourage political leaders to make a multisectoral response to the epidemic a major focus of their national plans.

AIDS in Africa: an epidemiologic paradigm.

Cases of the acquired immune deficiency syndrome (AIDS) have been reported in countries throughout the world. Initial surveillance studies in Central Africa suggest an annual incidence of AIDS of 550 to 1000 cases per million adults. The male to female ratio of cases is 1:1, with age- and sex-specific rates greater in females less than 30 years of age and greater in males over age 40. Clinically, AIDS in Africans is often characterized by a diarrhea-wasting syndrome, opportunistic infections, such as tuberculosis, cryptococcosis, and cryptosporidiosis, or disseminated Kaposi's sarcoma. From 1 to 18% of healthy blood donors and pregnant women and as many as 27 to 88% of female prostitutes have antibodies to human immunodeficiency virus (HIV). The present annual incidence of infection is approximately 0.75% among the general population of Central and East Africa. The disease is transmitted predominantly by heterosexual activity, parenteral exposure to blood transfusions and unsterilized needles, and perinatally from infected mothers to their newborns, and will continue to spread rapidly where economic and cultural factors favor these modes of transmission. Prevention and control of HIV infection through educational programs and blood bank screening should be an immediate public health priority for all African countries.

AIDS: an international perspective.

The acquired immunodeficiency syndrome (AIDS) and infection with the human immunodeficiency virus type 1 (HIV-1) constitute a worldwide public health problem. Whereas in Europe and in most of the Americas transmission of HIV-1 has occurred predominantly among homosexual men and intravenous drug abusers, in Africa a distinct epidemiologic pattern has emerged that indicates that HIV-1 infection is mainly heterosexually acquired. Heterosexual transmission appears to be increasing in some parts of Latin America and the Caribbean, and possibly in the United States. In addition to HIV-1, at least one other human retrovirus, namely HIV-2, has been implicated as a cause of AIDS in Africa and Europe. Factors that influence heterosexual transmission of HIV-1 include genital ulcerations, early or late stages of HIV-1 infection in the index case, and possibly oral contraception and immune activation. The rate of perinatal transmission is enhanced when the mother's illness is more advanced. AIDS and HIV-1 infection may have a significant impact not only on public health, but also on the demography and socioeconomic conditions of some developing countries. Programs for the prevention and control of AIDS should be an immediate priority in all countries.

Genomic heterogeneity of AIDS retroviral isolates from North America and Zaire.

In an analysis of the genomic variation of AIDS retroviral isolates from patients living in New York, Alabama, and Zaire, restriction maps were constructed by using seven enzymes, each known to cleave the proviral DNA more than once, in conjunction with Southern blot analysis. The maps of LAV, HTLV-III, and ARV-2 as deduced from their published nucleotide sequences were included in this analysis. The results demonstrated that (i) several "signature" restriction sites were common to all isolates; (ii) with the exception of LAV and HTLV-III, the North American and European isolates were all different from one another and showed no geographical specificity; (iii) the African isolates as a group were more diverse than those from North America and Europe; and (iv) the genomic variability was concentrated within the env gene.

Variability among HIV and SIV strains of African origin.

PIP: HIV-1 and HIV-2 both cause AIDS in humans. Simian immunodeficiency viruses (SIV) are non-human primate lentiviruses and the closest known relatives of the HIVs. They closely parallel HIVs in genomic organization and biologic properties. The authors discuss the known HIVs and SIVs of African origin and describe the variability which exists in the different groups. HIV-1 and HIV-2 share approximately 55-60% amino-acid homology in gag and pol, the genes most highly conserved among related retroviruses. HIV-1 is spread widely throughout the world, while HIV-2 infection appears to be concentrated in West Africa. Rare cases of HIV-2 infection have, however, been identified in Europe and America, usually in individuals connected with West Africa. The authors discuss viral genetic variation and variation of biological phenotype, and findings on HIV-1 and HIV-2 from Zaire, Uganda, Cameroon, Tanzania, Ethiopia, Congo, Ghana, the Gambia, Guinea-Bissau, and Cote d'Ivoire.^ieng

HIV-1 and immunological changes during pregnancy: a comparison between HIV-1-seropositive and HIV-1-seronegative women in Nairobi, Kenya.

OBJECTIVE: To assess changes in the proportion of CD4 and CD8 T-lymphocyte profiles during pregnancy, at delivery and postpartum, and to determine whether HIV-1 infection affects the normal profile. DESIGN AND METHODS: A total of 416 pregnant HIV-1-infected women and an age and parity-matched HIV-seronegative group of 407 pregnant women were enrolled into a prospective study on the impact of HIV-1 infection on pregnancy. Maternal blood was obtained for lymphocyte subset determination at enrollment, delivery and 6 weeks postpartum. Whole blood sample drawn in EDTA-containing tubes were used to determine T-helper/inducer (CD4) and T-suppressor/cytotoxic (CD8) cells by direct immunofluorescence using monoclonal antibodies. RESULTS: No relationship was found between gestational age and any immunological variable. The CD4 percentage was lower postpartum than antenatally, in both HIV-1-seropositive and seronegative women, but this was not true for absolute CD4 counts. CD8 absolute counts and percentages were significantly higher postpartum than antenatally. The differences between HIV-1-seropositive and seronegative women in changes over pregnancy in CD4 and CD8 cells and their ratio, were not statistically significant. CONCLUSION: Our findings do not support a short-term synergistic effect of HIV-1 and pregnancy on the immune function as determined by T-lymphocyte subsets.

PIP: The impact of HIV-1 on pregnancy was investigated in a prospective case-control study of 416 pregnant HIV-infected women and 407 age- and parity-matched pregnant HIV-seronegative women from Nairobi, Kenya. No relationship existed between gestational age (14-30 weeks) and any hematologic or immunologic variable studied. In both cases and controls, the CD4 percentage (but not absolute count) was lower postpartum than during pregnancy, while CD8 absolute counts and percentages were significantly higher in the postpartum period. The differences between HIV-positive and HIV-negative women in changes during pregnancy in CD4 and CD8 cells and their ratio were not statistically significant. These findings fail to provide support for a synergistic effect of HIV-1 and pregnancy on immune function. Further studies are needed, however, to assess the long-term effects of pregnancy in HIV-infected women, to determine the impact of pregnancy at different stages of HIV disease, and to establish normal and HIV-1-related T-lymphocyte subset profiles during the entire course of pregnancy in African women.

Rapid increase of both HIV-1 infection and syphilis among pregnant women in Nairobi, Kenya.

OBJECTIVE: To determine the prevalence of HIV-1 and syphilis antibodies in a population of pregnant women in Nairobi, Kenya, between 1989 and 1991. METHODS: As part of an ongoing prospective study on the effect of HIV-1 infection and sexually transmitted diseases, 4883 pregnant women were screened for HIV-1 and syphilis antibodies in one health-centre in Nairobi. RESULTS: HIV-1 seroprevalence increased from 6.5 to 13.0% (P < 0.001) and syphilis seroreactivity from 2.9 to 5.3% (P = 0.002), while there was no change in gonococcal infection rates. The most rapid increase in HIV-1 prevalence was observed in women aged less than 25 years. There was no evidence of demographic fluctuations in the population during this time, or of changes in sexual behaviour, except that fewer women enrolled in 1991 reported having more than one sex partner, compared with women enrolled in 1989 (39.1 versus 20.0%; P = 0.0001). HIV-1-seropositive women were more likely to be seroreactive for syphilis than HIV-1-seronegative mothers (7.7 versus 3.2%; odds ratio = 2.5; 95% confidence interval, 1.7-3.8; P < 0.001), but there was no difference between the two groups in terms of gonorrhoea prevalence. CONCLUSION: These data confirm an association between HIV-1 and syphilis infection, and indicate that both are spreading rapidly among women in Nairobi outside high-risk groups. Increased efforts to control both infections are urgently required.

PIP: Between January 1989 and December 1991, health workers took blood samples from 4883 pregnant women attending the Nairobi City Commission's Langata Clinic in Nairobi, Kenya to determine demographic factors and indicators of sexual behavior to explain the increase in HIV-1 infection and syphilis among these women of low socioeconomic status. HIV-1 seroprevalence stood at 8.8%. Syphilis seroreactivity was 3.6%. HIV-1 seropositive mothers were 2.5 times more likely to also test positive for syphilis than were HIV-1 seronegative mothers (7.7% vs. 3.2%; p.001). There was no significant association between HIV-1 seropositivity and gonococcal infection rate (7.3% vs. 8.9%), however. Women who tested HIV-1 positive tended to be from western Kenya (60.1% vs. 39.1%; p.0001). Between 1989 and 1991, annual HIV-1 seroprevalence rates increased from 6.5% to 13% (p.001) as did annual syphilis seroreactivity rates (2.9-5.3%; p=.02). The HIV-1 seroprevalence rates remained high, but did not rise significantly among syphilis seroreactive women between 1989 and 1991 (17.9-20.7%). They did rise among syphilis seronegative women (6.9-12.5%; p.0001), however. The HIV-1 infection rate increase was greater among 25-year old women (5.6-13.2%; p.001) than it was among 25-year old women (6.8-12.7%; p=.09). Indeed the annual incidence rate for 25-year old women was 3-4%. Between 1989-1991, there was a decrease in the percentage of both HIV-1 seropositive and seronegative women who had had 1 sex partner during the last 2 years (39.1% vs. 20%; p=.0001). Demographic factors remained the same throughout the study period. These results verified the link between HIV-1 infection and syphilis and their rapid rise among women in low risk groups. Thus there was a pressing need to improve HIV-1 and sexually transmitted disease prevention programs.

Advanced immunodeficiency as a risk factor for heterosexual transmission of HIV.

To define the rate of HIV infection and associated risk factors among heterosexual partners of HIV-infected individuals, 80 partners and 77 index cases were examined for clinical, behavioural and virological variables. The study population included 61 male and 16 female index cases, of whom 40 were European and 37 African. Multiple sexual partners in central Africa was the risk factor for HIV infection in 83%. The overall infection rate among the contact cases was 45%, 53% among female partners and 13% among male partners (P less than 0.001). Variables significantly associated with HIV infection in the partners included advanced clinical stage (AIDS-related complex or AIDS), a low concentration of T4 lymphocytes and African nationality of the index case, young age of the partner, and more than 50 instances of sexual contact with the index cases. In a logistic regression analysis, a low concentration of T4 lymphocytes, clinical stage ARC/AIDS and African nationality of the index case but not number of sexual contacts, remained as independent risk factors for HIV transmission. Our data indicate that there is a significant biological heterogeneity in sexual transmission of HIV and that advanced clinical stage and severe T cell depletion of the index case are major determinants of infectivity.