IMP, Lanzhou, People's Republic of China
UMD, College Park, Maryland, USA
It is known that newly fabricated accelerating structures have almost ideally smooth surface. However, ‘post mortem’ examination of these structures reveals that their surface can be significantly modified after high-gradient operation. This surface modification can be caused by the appearance of microscopic protrusions*. One of the factors leading to heating, melting and evaporation of these protrusions (factors resulting in the RF breakdown) is ion bombardment**. In our study we analyze ion motion in the vicinity of microprotrusions both analytically and numerically. First, we study the ion motion in the RF electric field magnified by the protrusion in the absence of electron field emitted current and show that most of the ions do not reach the structure surface. Then we add into consideration the interaction of ions with Fowler-Nordheim current emitted from the tip of protrusion (dark current). First, we develop a model describing this interaction and then we supplement it with numerical results using PIC code WARP***. We show that the ions move towards the area occupied by the dark current, but this does not increase the bombardment of micro-protrusions.
* R.B. Palmer,et al, Phys. Rev ST Accel. Beams 12, 031002 (2009).
** P. Wilson, AIP Conf. Proc., 877, Melville, New York, 2006, p. 27.
***J.-L. Vay, et al, Physics of Plasmas, 11, 2928 (2004).
ODU, Norfolk, Virginia, USA
JLAB, Newport News, Virginia, USA
National University of Defense Technology, Changsha, Kaifu District, People's Republic of China
The influences of the inner-conductor on microwave characteristics in an L-band relativistic backward-wave oscillator (RBWO) are investigated theoretically and experimentally. The numerical results show that the resonance frequency decreases obviously with the increase in the inner-conductor radius. To verify the above conclusions, an L-band coaxial RBWO is investigated in detail with particle-in-cell (PIC) code. It is shown that the frequency is lowered from 1.63 GHz to 1.51 GHz when the inner-conductor radius increases from 0.5 cm to 2.5 cm. And the efficiency varies in the range of 35.4-27.7%. Furthermore, experiments are carried out at the Torch-01 accelerator. When the diode voltage is 887.6 kV and the current is 7.65 kA, the radiated microwave with frequency of 1.61 GHz, power of 2.13 GW and efficiency of 31.3% is generated. It is found that the frequency decreases from 1.64 GHz to 1.58 GHz when the inner-conductor radius increases from 0.5 cm to 1.5 cm. And the efficiency varies in the range of 31.3-29.8%.
MIT, Cambridge, Massachusetts, USA
IF-UFRGS, Porto Alegre, Brazil
A theory is presented that describes steady-state one-dimensional Child-Langmuir flow at a self-consistent finite temperature distribution. In particular, warm-fluid equations and adiabatic equation of state are used to derive the self-consistent Poisson equation. The profiles of the charged-particle density, the velocity, the electrostatic potential, the pressure and the temperature are computed. Results are compared with self-consistent simulations.
UMD, College Park, Maryland, USA
Multipactor (MP) manifests itself as a rapid growth of the number of secondary electrons emitted from a solid surface in the presence of the RF field under vacuum conditions. The secondary electrons appear as the result of surface impacts of energetic primary electrons accelerated by the RF field. MP occurs in various microwave and RF systems and usually severely degrades their performance. Therefore, theoretical and experimental studies of MP are of great interest to researchers working in related areas of physics and engineering. In this paper we study MP in dielectric-loaded accelerating (DLA) structures. We started our work with the development of a self-consistent time-dependent 2D model of MP in such structures*. To benchmark that model, we compared its results with available experimental data**. The comparison showed good agreement between theory and experiment for DLA structures of larger diameter, however for structures of smaller diameter a significant discrepancy was observed. Therefore, we decided to develop a new quasi-3D model of MP that would allow us to take into account the effects ignored in our 2D studies. Results of our 3D analysis are presented in this paper.
* O. V. Sinitsyn, G. S. Nusinovich and T. M. Antonsen, Jr., Phys. Plasmas, 16, 073102 (2009).
** O. V. Sinitsyn, G. S. Nusinovich and T. M. Antonsen, Jr., AIP Conf. Proc., 1299, 302 (2010).
CLASSE, Ithaca, New York, USA
CMU, Pittsburgh, Pennsylvania, USA
LBNL, Berkeley, California, USA
Harvard University, Cambridge, Massachusetts, USA
The Cornell Electron Storage Ring has been reconfigured as a test accelerator (CesrTA). Measurements of electron cloud densities have been made at CesrTA using the TE Wave transmission technique. However, interpretation of the data based on single pass transmission is problematic because of the reflections and standing waves produced by discontinuities in the beam pipe - from pumps, bellows, etc. that are normally present in an accelerator vacuum chamber. An alternative model is that of a resonant cavity, formed by the beampipe and its discontinuities. The theory for the measurement of plasma densities in cavities is well established. This paper will apply this theory to electron cloud measurements, present some simplified measurements on waveguide, and apply this model to the interpretation of some of the data taken at CesrTA.
CEA, St Paul Lez Durance, France
CERN, Geneva, Switzerland
Graduate School of the Chinese Academy of Sciences, Beijing, People's Republic of China
IMP, Lanzhou, People's Republic of China
RRCAT, Indore (M.P.), India
SLAC, Menlo Park, California, USA
JAI, Oxford, United Kingdom
FACET (Facility for Advanced Accelerator and Experimental Tests) is a new User Facility at SLAC National Accelerator Laboratory. Its high power electron and positron beams make it a unique facility, ideal for beam-driven Plasma Wakefield Acceleration studies. The first 2 km of the SLAC linac produce 23 GeV, 3.2 nC electron and positron beams with short bunch lengths of 20 um. A final focusing system can produce beam spots 10um wide. User-aided Commissioning took place in summer 2011 and FACET will formally come online in early 2012. We present the User Facility, the current features, planned upgrades and the opportunities for further experiments.
USTRAT/SUPA, Glasgow, United Kingdom
University of Dundee, Nethergate, Dundee, Scotland, United Kingdom
UAD, Dundee, United Kingdom
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laser-plasma accelerators for the production of ultra-short electron beams as drivers of incoherent and coherent radiation sources from plasma and magnetic undulators. Here we report on the latest laser wakefield accelerator experiments on the University of Strathclyde ALPHA-X accelerator beam line looking at high quality electron beams. ALPHA-X uses a 26 TW Ti:sapphire laser (energy 900 mJ, duration 35 fs) focused into a helium gas jet (nozzle length 2 mm) to generate high quality monoenergetic electron beams with central energy in the range 80-180 MeV. The beam is fully characterized in terms of the charge, bunch length, energy spread and transverse emittance. The energy spectrum (with less than 1% measured energy spread) is obtained using a high resolution magnetic dipole imaging spectrometer while pepper-pot mask measurements show that the normalized transverse emittance is as low as 1.1 pi mm mrad (resolution limited). The conditions needed to obtain this high quality are discussed.
ITEP, Moscow, Russia
* A. Drozdovskiy et al., IPAC'10, Kioto, Japan, http://cern.ch/AccelConf/IPAC10 /MOPE040.
** A.Drozdovskiy et al., RUPAC’10, Protvino, Russia, http://cern.ch/AccelConf/RUPAC10 /THCHA01.
TEMF, TU Darmstadt, Darmstadt, Germany
CERN, Geneva, Switzerland
The aim of our study is the numerical computation of the wakefield, impedance and energy loss for an energetic, short (< 10 ns) ion bunch penetrating an electron cloud plasma residing in the beam pipe. We use a 3-D self-consistent and higher order PIC code based on the full-wave solution of the Maxwell equations in the time domain. In our simulations we observe the induced density oscillations in the electron cloud in the longitudinal as well as in the transverse directions. A special numerical procedure is applied to compute the longitudinal wake potential and the broadband coupling impedance due to the beam-electron cloud interaction. The code is applied to the case of the CERN SPS and the projected SIS-100 at GSI. The effects of the beam pipe, electron density, bunch intensity and external magnetic dipole fields are studied. The results are compared to analytical and numerical models of reduced complexity.
ESS-Bilbao, Zamudio, Spain
University of the Basque Country, Faculty of Science and Technology, Bilbao, Spain
ESS Bilbao, Derio, Spain
ISHN project consist on a Penning ion source which will deliver up to 65 mA of H− beam pulsed at 50 Hz with a diagnostics vessel for beam testing purposes. The present work summarizes the control system of this research facility, and presents its future developments. ISHN consist of several power supplies for plasma generation and beam extraction, including auxiliary equipment and several diagnostics elements. The control system implemented with LabVIEW is based on PXI systems from National Instruments, using two PXI chassis connected through a dedicated fiber optic link between HV platform and ground. Source operation is managed by a real time processor, while additional tasks are performed by means of an FPGA. In addition, the control system uses a MySQL database for data logging, by means of a LabVIEW application connected to such DB. The integration of EPICS into the control system by deploying a Channel Access Server is the ongoing work, several alternatives are being tested. Finally, a high resolution synchronization system has been designed, for generating timing for triggers of plasma generation and extraction as well as data acquisition for beam diagnostics.
Penn State University, University Park, Pennsylvania, USA
Moderate-energy, high-repetition-rate electron beams are needed in a variety of applications such as those in security and medicine, while requiring that the acceleration be realized in a compact and relatively inexpensive package. Laser wakefield acceleration is an attractive technology which meets most of those requirements, but it requires the use of relatively high peak power lasers which do not scale readily to high repetition rates. We are developing the theoretical and experimental basis for advancing the science and technology of direct laser acceleration (DLA) of charged particles using the axial component of the electric field of a radially polarized intense laser pulse. DLA is an acceleration method which exhibits no threshold and is thus compatible with the use of lower peak power, but much higher repetition rate lasers. We are currently numerically investigating the conditions for quasi-phase-matched DLA of electrons in plasma waveguides and experimentally implementing the quasi-phase-matched waveguide structure in laser-produced plasmas.
Saint-Petersburg State University, Saint-Petersburg, Russia
Investigation of a field of a particle bunch in a waveguide loaded with a dielectric is important for the wakefield acceleration (WFA) technique and other problems in the accelerator physics. One of subjects of investigation in this area consists in analysis of transition radiation generated by the bunch flying into (out of) the dielectric structure. This radiation can be both destructive (for WFA) and useful (for diagnostics of bunch or material). We investigate the total field of small bunch crossing a boundary between two dielectrics in the waveguide. It includes a “forced” field and a “free” one. The “forced” field is the field of the charge in the unbounded waveguide (it can contain the wakefield). The “free” field is connected with influence of the boundary (it includes transition radiation). Two cases are analyzed in detail: the bunch flies from vacuum into dielectric and from dielectric into vacuum. The behavior of the field depending on distance and time is explored analytically and numerically. Some interesting physical effects are noted. As well, we make a comparison with the case of intersection between vacuum and cold plasma.
IF-UFRGS, Porto Alegre, Brazil
SLAC, Menlo Park, California, USA
University of Oslo, Oslo, Norway
UCLA, Los Angeles, California, USA
USC, Los Angeles, California, USA
We report initial results of the Plasma Wakefield Acceleration (PWFA) Experiments performed at FACET - Facility for Advanced aCcelertor Experimental Tests at SLAC National Accelerator Laboratory. At FACET a 23 GeV electron beam with 1.8x1010 electrons is compressed to 20 microns longitudinally and focused down to 10 microns x 10 microns transverse spot size for user driven experiments. Construction of the FACET facility completed in May 2011 with a first run of user assisted commissioning throughout the summer. The first PWFA experiments will use single electron bunches combined with a high density lithium plasma to produce accelerating gradients >10GeV/m benchmarking the FACET beam and the newly installed experimental hardware. Future plans for further study of plasma wakefield acceleration will be reviewed.
MPI-P, München, Germany
MPI, Muenchen, Germany
BARC, Mumbai, India
Lucknow University, Lucknow, India
CBS, Mumbai, India
University of Oslo, Oslo, Norway
UCLA, Los Angeles, California, USA
SLAC, Menlo Park, California, USA
In the FACET plasma-wakefield acceleration experiments a dense 23 GeV electron beam will interact with lithium and cesium plasmas, leading to plasma ion-channel formation. The interaction between the electron beam and the plasma sheath-electrons may lead to a fast growing electron hose instability. By using optics dispersion knobs to induce a controlled z-x tilt along the beam entering the plasma, we investigate the transverse behavior of the beam in the plasma as function of the tilt. We seek to quantify limits on the instability in order to further explore potential limitations on future plasma wakefield accelerators due to the electron hose instability.
APRI-GIST, Gwangju, Republic of Korea
CERN, Geneva, Switzerland
MPI-P, München, Germany
MPI, Muenchen, Germany
CERN, Geneva, Switzerland
KIT, Karlsruhe, Germany
HHUD, Dusseldorf, Germany
UCLA, Los Angeles, California, USA
Tech-X, Boulder, Colorado, USA
* Umstadter, D., et al, Phys. Rev. Lett. 72, 1224
** Umstadter, D., et al, Phys. Rev. E 51, 3484
USC, Los Angeles, California, USA
CERN, Geneva, Switzerland
KIT, Karlsruhe, Germany
MPI, Muenchen, Germany
Recent plasma wakefield accelerator (PWFA) experiments showed that an accelerating gradient as high as 50GV/m can be driven and sustained over a meter-long plasma*. Based on this result, a strawman design for a future, multi-stage, PWFA-based electron/positron collider with an energy gain of ~25GeV/stage has been generated**. However, the choice of plasma density remains open. On one hand, high density means large accelerating gradients and possibly a shorter collider. On the other it means that the accelerating structure dimensions become very small, on the order of the plasma wavelength (<100 microns in each dimension?). Operating at high gradient and with such small structure imposes very strong constraints on the particle bunches: small dimensions and spacing, large current or limited charge, etc. These constraints result in challenges in producing bunches (compression, shaping for optimum loading, etc.) and could limit the achievable collider luminosity (beam-beam effects, etc.). We explore the global implications of operating at a lower accelerating gradient with the goal of relaxing the beam and plasma parameters while meeting the requirements of the collider.
* P. Muggli and M.J. Hogan, Comptes Rendus Physique, 10(2-3), 116 (2009).
** A. Seryi, M.J. Hogan, T. Raubenheimer, private communication.
National University of Defense Technology, Changsha, Kaifu District, People's Republic of China
As high power microwave (HPM) technologies gradually matured, the technologies for enhancing the output capacity of HPMs are becoming more and more attractive. However, limited by physics and technology, the approaches for enhancing the output capacity with a single HPM source have encountered difficulties. An alternative method for enhancing the output capacity of HPM sources is the coupling output of dual channel HPM sources. However, if the microwave sources have some coupling with each other, they maybe inter modulate with each other, and the phase-locking of the HPM sources may occur. In order to make sure that the beat waves are generating on the right way, a waveguide diplexer is introduced. Each channel has disjoint pass frequency band, and dual-channel HPM sources are isolated. As the dual-channel electron beams are driven by one accelerator, the HPM sources are expected to have a better match with the accelerator, and even higher microwave power is possible. In the high power experiments, the radiated powers of the beat waves are measured to be about 4.3 GW, 40 ns, the frequencies are about 9.41 GHz and 9.59 GHz.
Correspondence: Guolin Li, College of Optoelectric Science and Engineering, National University of Defense Technology, Changsha, 410073, P. R. China. Email: nudt-liguolin@hotmail.com
NSC/KIPT, Kharkov, Ukraine
* A.V.Gaponov et al., Izv. VUZov(USSR), Radiofizika 10(9), 10, 1967.
** V.Khoruzhiy et al., Phys. J.of Ukraine 49(2), 126, 2004.
*** V.Khoruzhiy et al., Phys. J.of Ukraine 50(11), 1230, 2005.
Graduate University, Chinese Academy of Sciences, Beijing, People's Republic of China
PKU/IHIP, Beijing, People's Republic of China
With the development of accelerator technology, to obtain an ion beam with high intensity and low emittance is becoming one of the main goals of research for ion sources. At Peking University we have developed several 2.45 GHz electron cyclotron resonance (ECR) ion sources for different projects and we paid close attention to the beam intensity increasing as well as the beam emittance reduction. Methods are adopted to improve beam intensity by increasing the density of plasma inside the discharge chamber, optimizing the geometry pinch effect and the perveance at the extraction aperture. To suppress the emmitance increasing of an extracted beam, the shape of the electrodes as well as the voltage of suppression electrode are carefully selected With these efforts, a 120 mA total proton beam has been extracted from the permanent magnet ECR ion source at 50 kV, and the measured normalized rms emittance is less than 0.2 pi.mm.mrad. The beam current density at the extraction aperture is about 420 mA/cm2.
ESS-Bilbao, Zamudio, Spain
University of the Basque Country, Faculty of Science and Technology, Bilbao, Spain
The Ion Source responsible intended to generate a high current and low emittance proton beam for the ESS-Bilbao is currently under construction. The plasma in the source is generated by coupling the 2.72 GHz power input from a Klystron through a magnetic field with an intensity close to the electron cyclotron resonance (ECR) field at the input RF frequency. The electrical behavior of the plasma strongly depends on different plasma characteristics which, at the same time, also depend on the microwave absorption. Thus, in order to maximize the RF power transferred to the plasma, a waveguide automatic tuner unit is employed to match the generator output to the electric impedance of the plasma. This device is generally adjusted manually. In this paper, the design, the 1D and 3D simulation, and measurements are presented which allows us to propose an automatic and real time control of the device. In a first approximation, with the aim of testing the proper operation of the automatic tuner unit, an in-house variable phase shifter and attenuator has been designed and manufactured to simulate the electric behavior of the plasma.
CERN, Geneva, Switzerland
CERN has 2 operational primary beam ion sources, that are presently used for the production of beam for LHC as well as several other facilities. Protons are produced by a duoplasmatron source, and ions from the GTS-LHC ECR ion source. In addition, new sources are required for a new 160MeV H− Linac, and development has been made on a high power RF plasma generator which could serve for a future high power Linac. In this report, the present status will be given, along with operational statistics and experience for the operation sources, and the development programme reported for the future sources.
Muons, Inc, Batavia, USA
ORNL, Oak Ridge, Tennessee, USA
In this project is developed a prototype RF H− surface plasma source (SPS) with saddle (SA) RF antenna which will provide better power efficiency for high pulsed and average current, higher brightness with longer lifetime and higher reliability. Several versions of new plasma generators with a small AlN test chamber and different antennas and magnetic field configurations were tested in the SNS ion source Test Stand. A prototype SA SPS was installed in the Test Stand with a larger, normal-sized SNS AlN chamber that achieved unanalyzed peak currents of up to 67 mA with an apparent efficiency of 1.6 mA/kW. Control experiments with H− beam produced by SNS SPS with internal and external antennas were conducted. A new version of the RF triggering plasma source (TPS) has been designed. A Saddle antenna SPS with water cooling is being fabricated for high duty factor testing.
Muons, Inc, Batavia, USA
Features of cesiation* in different modifications of H-/D- source designs have been considered. New sources under development include advanced versions of Compact Surface Plasma Sources (CSPS) which will efficiently generate brighter beam in noiseless discharge, deliver significantly increased (up to 20 mA) average current with better electrode cooling using new materials, have significantly extended lifetime and reduced cesium consumption. Related ion sources that use cesium are described and an improved cesiation procedure for reproducible production of high efficiency H− ion generation is considered.
* V. Dudnikov, SU Author Certificate, C1.H01 3/04, No. 411542, 10 March, 1972.
IAP, Frankfurt am Main, Germany
Space charge lenses using a stable electron cloud for focusing low energy heavy ion beams are an alternative concept to conventional ion optics. Due to external fields electrons are confined inside the lens’ volume. In case of a homogeneously distributed electron cloud the linear electric space charge field enables beam focusing free of aberration. Since the mapping quality of the lens is related to the confinement, non-destructive diagnostics has been developed to determine the plasma parameters and to characterize the collective behavior of the confined nonneutral plasma. Moreover, a scaled up space charge lens was constructed for a detailed investigation of the nonneutral plasma properties as well as beam interactions with a stable confined electron cloud. Experimental results will be presented in comparison with numerical simulations.
GSI, Darmstadt, Germany
Universidad de Castilla-La Mancha, Ciudad Real, Spain
IPCP, Chernogolovka, Moscow region, Russia
BINP SB RAS, Novosibirsk, Russia
* N.A. Tahir et al., PRL 95 (2005) 135004.
** N.A. Tahir et al., New J. Phys. 12 (2010) 073022.
*** N.A. Tahir et al., Phys. Plasmas 18 (2011) 032704.