Wanming Liu

The recently built 12us long pulse RF modulator at Argonne Wakefield Accelerator (AWA) facility was having many performance problems until the thyratron used in the modulators were replaced with pulsed power solid state switches. Detail of both the problems experienced with the thyratrons and the results of successful upgrade with solid state switches are presented in this paper. The design of the solid state switches will also be discussed.

ABSTRACT In this paper, we give a complete analytical solution for wakefields generated by an azimuthally symmetric ring beam propagating in a coaxial two-channel dielectric structure. This wakefield can be used to accelerate a witness beam in the central channel. The ratio of the peak accelerating field in the center channel to the decelerating field in the ring channel (defined as transformer ratio R) is also derived. We find that, by appropriate choice of parameters, R can be much greater than 2, the limiting value for collinear wakefield accelerators.

A pulsed normal conducting flux concentrator is an optional scheme of the ILC positron source adiabatic matching device (AMD). In order to facilitate the design of flux concentrator based AMD, an equivalent circuit model was proposed. A prototype flux concentrator was built and tested to validate the model. Good agreement was obtained between the measurement results and simulations based on

ABSTRACT In the Two Beam Accelerator (TBA), wakefield power extractors which extract high power RF from a high current beam are used to power high gradient accelerating structures. A dielectric‐based Wakefield Power Extractor (DWPE) is one option in addition to the metallic structures considered previously, like the CLIC PETS (Power Extraction and Transfer Structure). 7.8 GHz and 26 GHz DWPE prototypes have been successfully built and tested at the Argonne Wakefield Accelerator (AWA) facility. We are currently designing an X‐band version for a potential application with the CLIC beam. In this article, we report on test results of the 26 GHz DWPE and the preliminary design of the X‐band structure. Future plan and possible difficulties in the development of DWPEs are also discussed.

ABSTRACT One technique to enhance the transformer ratio beyond the ordinary limit of 2 in a collinear wakefield acceleration scheme is to use a ramped bunched train (RBT). The first experimental demonstration has been reported in [1]. However, due to the mismatch between the beam bunch length and frequency of the accelerating structure, the observed transformer ratio was only marginally above 2 in the earlier experiment. We recently revisited this experiment with an optimized bunch length using the laser stacking technique at Argonne Wakefield Accelerator (AWA) facility. A transformer ratio of 3.4 has been measured using two drive bunches. Attempting to use four drive bunches met with major challenges. In this article, measurement results and data analysis from these experiments are presented in detail.

ABSTRACT We propose the conventional positron source driven by a several-GeV electron beam for ILC. Thermal load of the positron production target was a risk of the conventional positron source. To cure it, we employ a 300 Hz electron linac to create positrons with stretched pulse length. In ILC, the bunch timing structures and pulse timing structures can be diffecent in the positron source, in the DR, and in the main linac. We have some flexibility to choose timing structures in positron source and we use it for time stretching. ILC requires about 2600 bunches in a train in the main linac which pulse length is 1 ms. In the conventional source, about 130 positron bunches are created by each pulse of the 300Hz linac. Then 2600 bunches are created in 63 ms. We optimized parameters such as drive beam energy, beam size on the target, and target thickness to maximize the capture efficiency and to mitigate the target thermal load. A slow rotating tungsten disk is employed as positron production target.

By applying different symmetric boundary conditions, we found that the transverse wakefields generated by an electron bunch traveling through a partially loaded rectangular dielectric structure at an off center position can be decomposed into corresponding orthogonal longitudinal section electric (LSE) and longitudinal section magnetic (LSM) modes for guided waves as in the case of longitudinal wakefields treated previously. The wakefields are characterized using the normalized shunt impedance R/Q, a function of the geometry of the accelerating structure, for both LSE and LSM modes. A numerical example is given for an X-band waveguide structure and detailed results are given for the several leading transverse wakefield terms. The analytic results obtained are in agreement with the results from the time domain simulation tool MAFIA.

A program is under way at Argonne National Laboratory, in collaboration with the Naval Research Laboratory (NRL), to develop RF-driven dielectric-loaded accelerating (DLA) structures, with the ultimate goal of demonstrating a compact, high-gradient linear accelerator based on this technology. In this paper, we report on the most recent results from a series of high power tests that are under way at NRL's X-band Magnicon facility. The design of the latest DLA structure has been fundamentally changed from the previous generation; it now has a modular construction that separates the RF coupler from the dielectric section. In this paper we present a detailed description of the design of the new structure and of the experimental setup used during the high power tests. In addition, we will report on experimental results of high power tests carried out on an alumina-based (ε=9.4) DLA structure.

Dielectric loaded wakefield structures have potential to be used as high gradient accelerator components. Using the high current drive beam at the Argonne Wakefield Accelerator Facility, we employed cylindrical dielectric loaded wakefield structures to generate accelerating fields of up to 100 MV/m. Short electron bunches (13 ps FWHM) of up to 86 nC are used to drive these fields, either as single bunches or as bunch trains. These tested standing-wave structures have a field probe near the outer edge of the dielectric to sample the RF fields generated by the electron bunches. Monitoring of these high intensity RF fields serves to verify the absence of electric breakdown. Similar cylindrical dielectric loaded structures are used as power extractors, that is, traveling wave structures from which the RF power generated by the drive beam is efficiently coupled out.

High accelerating gradients are necessary for future electron positron colliders. To achieve high gradient without triggering the RF breakdown is of great interest and challenge. A short pulse RF will limit the RF pulse heating effect and thus might raise the bar for triggering the RF breakdown limit to a higher RF gradient. In this paper we will present one of our works towards the short pulse RF accelerator--design of the broadband RF coupling structure.

We report on the wakefield experimental results using the Argonne Wakefield Accelerator (AWA). Since the commissioning of the AWA, we have conducted numerous wakefield related experiments: plasma wakefield acceleration, dielectric collinear wakefield and step-up transformer two-beam acceleration experiments. In this paper, we summarize the experimental results and discuss the ongoing AWA upgrade to further these experimental results. Future plans for development of a 100 MeV demonstration accelerator based on wakefield method are presented.

The Argonne Wakefield Accelerator Facility is dedicated to the study of advanced accelerator concepts based on electron beam driven wakefield acceleration and RF power generation. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures as well as in metallic structures (iris loaded, photonic band gap, etc). Accelerating gradients as high as 100 MV/m have been reached in dielectric loaded structures, RF pulses have been generated with up to 44 MW at 7.8 GHz, and 20 MW at 26 GHz. In order to reach higher accelerating gradients, and also be able to generate higher RF power levels, several upgrades are underway: (a) a new RF gun with a higher quantum efficiency photocathode (Cesium Telluride) will replace the RF gun that has been used to generate the drive bunches; (b) the existing RF gun will be used to generate a witness beam to probe the wakefields; (c) three new L-band RF po...

A ceramic based power extractor operating at 21 GHz was built by DULY Research Inc. and tested at CTF2, the CERN Linear Collider (CLIC) Test Facility. The structure includes a ceramic extractor section, a 2-output-port, circular-to-rectangular waveguide coupler, and a 3-port rectangular waveguide combiner that provides for a single output waveguide. Results of cold tests and full beam tests are presented and compared with theoretical and numerical models.

A new RF photocathode electron gun and beamline have been built for the study of electron beam driven wakefield acceleration. The one and a half cell L-band gun operates with an electric field on the cathode surface of 80 MV/m, and generates electron bunches with tens of nanocoulombs of charge and rms bunch lengths of a few picoseconds. The beam diagnostics include a Cherenkov radiator and streak-camera for bunch length measurements, YAG screens for beam profile, integrating charge transformers (ICTs) for bunch charge, an energy spectrometer, and a pepper-pot plate for measurement of the transverse emittance. Measurements of the beam properties at various bunch charges are presented.

High gradient accelerating structures using dielectric‐lined circular waveguides have been developed for a number of years at Argonne National Laboratory. In this article, we first report the experimental results of high power rf testing on the quartz based Dielectric‐Loaded Accelerating (DLA) structure carried out on Feb. 2006 at the Naval Research Laboratory. The motivation for this experiment is to test the multipactor effect on different materials under high power and high vacuum condition. Up to 12 MW pulsed rf went through the tube without breakdown. Multipactor appeared during the experiment but with different features compared to other materials like alumina. Photomultiplier Tube (PMT) measurements were introduced into the experiment for the first time to observe the light emission time and intensity. In the second part of this paper, ways to achieve higher gradient for DLA structures are proposed: 1) smaller ID and longitudinal gap free DLA structures to reduce multipactor ...

High power tests are currently being conducted on RF-driven dielectric-loaded accelerating (DLA) structures to determine their viability as traveling-wave accelerators. These tests are a collaborative effort between Argonne National Laboratory (ANL) and the Naval Research Laboratory (NRL). In a previous high power test, single-surface multipactor was reported to be capable of absorbing more than half of the RF power incident on an alumina-based DLA structure. In this paper, we report on the most recent set of high power tests that are attempting to further understand multipactor and eventually suppress it. Several methods were employed to suppress multipactor including: the use of a magnetic field; a TIN surface coating; and a different dielectric material (Magnesium-Calcium-Titanate based). The effectiveness of these three methods are presented and discussed in the paper.

We describe the Argonne Wakefield Accelerator Facility (AWA), pointing out its present capabilities and goals. We present recent measurements on beam loading observed in our photocathode RF gun. Wakefield measurements in dielectric loaded structures are also reported. Our most recent wakefield structure operates at 15 GHz, and has been excited by single electron bunches and also by sets of two closely spaced bunches. When driven by 43 nC bunches, the accelerating gradient in this structure reached 23 MV/m. No signs of electric breakdown have been observed. This report ends with a brief discussion on the next activities to take place at the AWA facility.

Dielectric loaded wakefield structures have potential to be used as high gradient accelerator components. Using the high current drive beam at the Argonne Wakefield Accelerator Facility, we employed cylindrical dielectric loaded wakefield structures to generate accelerating fields of up to 86 MV/m, at 10 GHz. Short electron bunches of up to 86 nC are used to drive these fields, either as single bunches or as bunch trains. The structures consist of cylindrical ceramic tubes (cordierite) with a dielectric constant of 4.76, inserted into cylindrical copper waveguides. These standing-wave structures have a field probe near the outer diameter of the dielectric, in order to sample the RF fields generated by the electron bunches. Monitoring the field probe signal serves to verify the absence of electric breakdown in the structures. MAFIA simulations are used to calculate the amplitude of the fields generated by the traversing electrons bunches.

A 15 GHz dielectric loaded wakefield structure has been used to extract energy from a 13 MeV electron beam. Short electron bunches (13 ps FWHM) of up to 43 nC traversed the structure, generating an accelerating field of ~23 MV/m. The structure consists of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively. The 102 mm long dielectric cylinder is inserted into a cylindrical copper waveguide. Short metallic cylinders, with the same inner and outer radii as the ceramic, are inserted on both ends of the copper waveguide, making it a standing wave structure. A field probe present near the outer edge of the dielectric samples the RF field generated by the electron bunches. This signal is then sent to a mixer circuit, where the 15 GHz signal is down converted to 5 GHz and sent to an oscilloscope. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step...

Dielectric-loaded accelerating structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. However, a variety of potential problems must be studied in order to demonstrate this potential, including rf breakdown, joule heating, and gas desorption. A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate these issues, using high-power 11.424-GHz radiation from the NRL magnicon facility. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating the presence of multipactor in the tube. We will discuss a model that explains these observations, and describe means to eliminate the prob...

In its normal mode of operation, the Argonne Wakefield Accelerator Facility uses a high charge (10–100 nC), short pulse (3–5 ps) drive bunch to excite high-gradient accelerating fields in various slow-wave structures. To generate this bunch, we designed a 1.5 cell, L-band, rf photocathode gun with an emittance compensating solenoid to give optimal performance at high charge; it has recently completed commissioning. More recently, we have begun to investigate the possibility of using this gun in a high-brightness, low-charge operating mode, with charge equal to approximately 1 nC, for high-precision measurements of wakefields. Two related measurements are reported on in this paper: (1) measurements of the transverse beam envelope are compared to predictions from the beam dynamics code PARMELA; and (2) investigations into the use of a modified 3-screen emittance measurement method that uses a beam envelope model that includes both space-charge and emittance effects. Both measurements ...

We report on recent progress in a program to develop an RF-driven Dielectric-Loaded Accelerating (DLA) structure, capable of supporting high gradient acceleration. Previous high power tests revealed that the earlier DLA structures suffered from multipactor and arcing at the dielectric joint. A few new DLA structures have been designed to alleviate this limitation including the coaxial coupler based DLA structure and the clamped DLA structure. These structures were recently fabricated and high power tested at the NRL X-band Magnicon facility. Results show the multipactor can be reduced by the TiN coating on the dielectric surface. Gradient of 15 MV/m has also been tested without dielectric breakdown in the test of the clamped DLA structure. Detailed results are reported, and future plans discussed.

We report on design, fabrication, and beam test of a 7.8 GHz power extractor recently conducted at the Argonne Wakefield Accelerator (AWA) facility. The wakefields are excited using an electron beam travels through a dielectric loaded waveguide, and the generated RF power is then subsequently extracted with a properly designed RF coupler. In the experiment, 30 MW of output power is excited by a 66 nC single electron bunch, and wakefield superposition by a train consisting of four bunches is also demonstrated. Both results agree very well with theoretical predictions. Future tests include more charge transmission for higher RF power, and more bunches in a train for longer RF pulses.

In this paper, we presented an overview of the positron source for International Linear Collider (ILC). We started with description of the positron source configuration for ILC RDR baseline and SB2009 baseline followed by the status of critical components of ILC positron source R&D. We also presented some parameters of positron source for both RDR and SB2009 baseline.

The parameter set for the International Linear Collider (ILC) positron source given in the Technical Design Report (TDR) is more complicated than that presented in the ILC Reference Design Report (RDR). Studies to define and to optimize the parameters for different scenarios of center-of-mass energies have been performed at both Argonne National Lab (ANL) and Deutsches Elektronen Synchrotron (DESY)/Hamburg University. Results from both institutes agree well and are presented in this paper.

A viable positron source scheme is proposed that uses circularly polarized gamma rays generated from the main 250 GeV electron beam. The beam passes through a helical superconducting undulator with a magnetic field of â 1 Tesla and a period of 1.15 cm. The gamma-rays produced in the undulator in the energy range between â 3 MeV - 100 MeV will be directed to a titanium target and produce polarized positrons. The positrons are then captured, accelerated and transported to a Pre-Damping Ring (PDR). Detailed parameter studies of this scheme including positron yield, and undulator parameter dependence are presented. Effects on the 250 GeV CLIC main beam, including emittance growth and energy loss from the beam passing through the undulator are also discussed.

Dielectric structures promise to support high field, especially for short wakefield pulses produced by a high charged electron beam traveling in a dielectric tube. To push the gradient higher, we have tested two structures using recent upgraded Argonne wakefield accelerator facility that capable of producing up to 100 nC charge and bunch length of < 13ps (FWHM). Here we report on the experiment results that more than 80 nC beam passes through a 14 GHz dielectric loaded wakefield structure that produced an accelerating field of ~ 45 MV/m. The two structures consist of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively, and with length of 102 mm and 23 mm long. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step in these experiments, another structure, with an output coupler, has been designed and is presently being fabricated.

The generation and capture of polarized positrons at a source with a superconducting helical undulator having 4.3 cm period and 500 GeV electron drive beam have been simulated. The positron polarization has been calculated for the different undulator K values (up to K = 2.5). Without applying a photon collimator, the maximal polarization of positrons is about 25% for 231 meters active magnet length of undulator with K = 0.7. Using an undulator with K = 2.5 and a collimator with an aperture radius of 0.9 mm results in increase of positron polarization to 54%. The energy deposition, temperature rise and stress induced by high intense photon beam in the rotated titanium-alloy target have been estimated. The maximal thermal stress in the target is about 224 MPa for the source with photon collimation to achieve a positron polarization of 54%.

ABSTRACT form only given. A joint Naval Research Laboratory/Argonne National Laboratory study is under way to investigate the performance of X-band dielectric-loaded accelerating (DLA) structures using high-power 11.424-GHz radiation from the NRL Magnicon facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient RF linear accelerators. The purpose of the high-power tests is to find the RF breakdown limits of these structures and to test their ability to produce high accelerating gradients. We have recently tested DLA structures employing cylindrical ceramic liners fabricated from two different materials, high purity alumina (Al2O3), dielectric constant 9.4, and high-index magnesium calcium titanate (MgxCa1-xTiO3), dielectric constant 20. For alumina, we have seen no evidence of RF breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, strong multipactor effects were found to absorb an increasing fraction of the incident microwave power, as the power level was increased. These effects could be mitigated by use of a TiN coating on the alumina. For magnesium calcium titanate, the multipactor effects were smaller, but a problem of breakdown at dielectric joints between separate ceramic sections limited the achievable gradients. In this case, gradients of ~6 MV/m were achieved at ~1 MW drive power. Detailed experimental results will be presented

ABSTRACT In this paper, we consider the contribution of potential energy to beam dynamics as simulated by PARMELA at low energies (10-30MeV). We calculate the potential energy of the relativistic electron beam using the static coulomb potential in the rest frame (the first order approximation as used in the PARMELA code). We found that the potential energy contribution to the beam dynamics could be very significant, particularly with the high charge beams generated by an RF photocathode gun. Our results show that when the potential energy is counted correctly and added to the kinetic energy from PARMELA, the total energy is conserved. Results of potential and kinetic energy simulations for short beams (∼ 1mm) at various charges (1-100nC) generated by a high current RF photocathode gun are presented.

We propose combining a transverse-to-longitudinal emittance exchange beam line and a conventional modulator to generate high harmonics with short-wavelength free-electron lasers. The key point of this scheme is the use of a multi-slit mask to create a transverse-shaped beam. In an emittance exchange beam line, the transverse-shaped bunch will produce a longitudinal-shaped one, resulting in separated energy bands. The modulator that follows this step will generate micro-bunches containing high harmonics. A coherent synchrotron radiation effect is also considered in this paper; compared to the two-modulator scheme, the method investigated here is more flexible and can be used to relax the coherent synchrotron radiation effect.

Summary form only given. A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. The purpose of the high-power tests is to find the rf breakdown limits of these structures and to test their ability to produce high accelerating gradients. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating a strong multipactor effect in the tube. The multipactor was found to absorb an increasing fractio...

In this paper, we present updated results on power extraction testing of a 7.8 GHz dielectric loaded waveguide power extractor using both high charge single bunches and bunch trains. We have generated a 1.7 ns radio frequency (rf) pulse with 30 MW of power with a single 66 nC electron bunch. Then we have generated a pulse train of electron beam for rf generation of 10 ns and 22 ns rf pulses.

Positron beams have many applications and there are many different concepts for positron sources. In this paper, only positron source techniques for linear colliders are covered. In order to achieve high luminosity, a linear collider positron source should have a high beam current, high beam energy, small emittance and, for some applications, a high degree of beam polarization. There are several different schemes presently being developed around the globe. Both the differences between these schemes and their common technical challenges are discussed.