High Shunt Impedance Research Articles

Design evolution and properties of superconducting parallel-bar rf-dipole deflecting and crabbing cavities

Deflecting/crabbing cavities serve a variety of purposes in different accelerator applications, primarily in separating a single beam into multiple beams and in rotating bunches for head-on collisions at the interaction point in particle colliders. Deflecting/crabbing cavities are also used for transverse and longitudinal emittance exchange in beams, x-ray pulse compression, and for beam diagnostics. Compact superconducting deflecting/crabbing cavities are under development due to strict dimensional constraints and requirements for higher field gradients with low surface losses. The TEM-like superconducting parallel-bar cavity supports low operating frequencies, thus making the design favorable for many of the deflecting/crabbing cavity applications. The design of the parallel-bar cavity based on cylindrical straight loading elements and rectangular outer conductors has evolved and been adapted to improve the design properties by modifying the design geometry. The improved design with trapezoidal-shaped loading elements and cylindrical outer conductor has attractive properties such as low and well-balanced peak surface fields and high transverse shunt impedance. Additionally, the wide separation of modes in the higher-order mode spectrum and the absence of lower-order mode are advantageous in high current applications. The evolution of the parallel-bar geometry into an rf-dipole geometry is presented with a detailed analysis of the properties for each design.

Design of hybrid electron linac with standing wave buncher and traveling wave structure

A disk-loaded waveguide (DLW) is the most common structure for compact linear accelerators working in a traveling wave (TW) regime. Among its advantages are high shunt impedance and manufacturing simplicity. The other popular structure is an on-axis coupled bi-periodical accelerating structure (BAS) that works in standing wave (SW) regime. Both the standing and the traveling wave regimes have their own advantages and disadvantages. The design of the hybrid accelerator with SW buncher and TW accelerating section presented in this paper unites the advantages of both regimes. For example, the buncher in the hybrid accelerator is shorter than in a pure TW accelerator, and it requires no solenoid; this structure is more technologically convenient as it does not require a circulator.The other way to combine the advantages of DLW and BAS is to design a magnetic coupled disk-loaded waveguide (DLW-M). This paper also presents the results of a survey study that analyzed the electrodynamical parameters of such a structure and compared them with those of DLW. The experimental data is also presented. Higher order modes, multipacting discharge and thermal simulations show that DLW-M structure is more preferable to classical DLW.

An untuned MA-loaded cavity for HIRFL-CSR

In order to realize high energy density physics and plasma physics research at HIRFL-CSR, a magnetic alloy (MA)-loaded cavity has been studied. According to the theoretical calculation and simulation for the MA-loaded cavity, we achieved a better result. The MA-loaded cavity had a higher μQf value, with a higher shunt impedance and a higher accelerating gradient. The accelerating gradient was about 95 kV/m at 1.8003 MHz, 130 kV/m at 0.9000 MHz. Compared with the ferrite-loaded cavities that are used at HIRFL-CSR, with about 10 kV/m accelerating gradient, the MA-loaded cavity obviously has an advantage. The results of the theoretical calculation and the simulation, which meet the design requirements are in good agreement.

805 MHz and 201 MHz RF cavity development for MUCOOL

A muon cooling channel calls for very high accelerating gradient RF structures to restore the energy lost by muons in the absorbers. The RF structures have to be operated in a strong magnetic field and thus the use of superconducting RF cavities is excluded. To achieve a high shunt impedance while maintaining a large enough aperture to accommodate a large transverse emittance muon beam, the cavity design adopted is a pillbox-like geometry with thin Be foils to terminate the electromagnetic field at the cavity iris. The possibility of using grids of thin-walled metallic tubes for the termination is also being explored. Many of the RF-related issues for muon cooling channels are being studied both theoretically and experimentally using an 805 MHz cavity that has a pillbox-like geometry with thin Be windows to terminate the cavity aperture. The design and performance of this cavity are reported here. High-power RF tests of the 805 MHz cavity are in progress at Lab G in Fermilab. The cavity has exceeded its design gradient of 30 MV m−1, reaching 34 MV m−1 without external magnetic field. No surface damage was observed at this gradient. The cavity is currently under conditioning at Lab G with an external magnetic field of 2.5 T. We also present here a 201 MHz cavity design for muon cooling channels. The proposed cavity design is also suitable for use in a proof-of-principle muon ionization cooling experiment.

Design of 57.5 MHz cw RFQ for medium energy heavy ion superconducting linac

The nuclear science community considers the construction of the Rare Isotope Accelerator (RIA) facility as a top priority. The RIA includes a 1.4 GV superconducting linac for production of 400 kW cw heavy ion beams. The initial acceleration of heavy ions delivered from an electron cyclotron resonance ion source can be effectively performed by a 57.5 MHz 4-m long room temperature RFQ. The principal specifications of the RFQ are (i) formation of extremely low longitudinal emittance, (ii) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation, and (iii) simultaneous acceleration of two-charge states of uranium ions. cw operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode, and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses the beam dynamics and electrodynamics design of the RFQ cavity, as well as some aspects of the mechanical design of the low-frequency cw RFQ.

Travelling wave accelerating structure design for TESLA positron injector linac

A modified cup-like TW accelerating structure for TESLA Positron Pre-Accelerator (PPA) is designed by optimizing the structure geometry and by changing the iris thickness cell by cell in a section . This structure has high shunt-impedance and a large iris radius to meet with the requirements of high gradient and large transverse acceptance. The beam dynamics in the structure with the optimum solenoid focus field are studied. A satisfactory positron beam transmission and the beam performance at the PPA output have been obtained. In this paper the accelerating structure design is described in detail and the results are presented.

Longitudinal feedback system kicker for the PLS storage ring

The higher order modes (HOMs) of RF cavities at the Pohang Light Source (PLS) storage ring cause longitudinal coupled bunch mode instabilities. A longitudinal feedback system (LFS) is introduced to cure these instabilities. As a key component of the LFS, there is a single-ridged waveguide-overloaded cavity for a longitudinal bunch-by-bunch LFS kicker. Since the RF frequency is 500 MHz, the bandwidth of this kicker should be wider than 250 MHz to damp any coupled bunch mode. Also, a higher shunt impedance of the kicker is selected to use a lower power amplifier. One aluminum kicker is fabricated, and the bandwidth, HOMs, and the shunt impedance of the kicker are measured with a network analyzer. This kicker has several different features from the LFS kicker for DA/spl Phi/NE. First of all, there are four input/output ports to obtain a wider bandwidth. Second, there is a nose cone to obtain a higher shunt impedance. Third, the symmetric frequency response of the shunt impedance around the central frequency is provided. Finally, low beam coupling power (4 W per port at 400 mA) is obtained. According to the simulation result by the high frequency structure simulator (HFSS) code, we obtained the high shunt impedance of 621.3 /spl Omega/ (transit time factor considered value) and the wide bandwidth of 255 MHz. These are compared with the measured result.

Development of high gradient IH linac

We have studied Interdigital-H (IH)-type structures for application in heavy-ion inertial fusion (HIF) and other projects, at the Tokyo Institute of Technology (TIT). We have developed an IH linac with a high acceleration rate. It makes use of the fact that IH linacs have 5–10 times as high shunt impedance as Alvarez and RFQs in low and middle regions of energy [1–5]. We calculated trajectories of particles and made experiments by model cavities. Now an IH linac that we call high gradient IH linac is under construction. The calculated result of particle dynamics is that the transverse acceptance and an acceleration rate of this linac are 113π mm mrad and 5.5 MV/m, respectively. This linac is able to accelerate particles with a charge to mass ratio ( q/ A) greater than 1/16 from 219 keV/u up to 600 keV/u. The cavity length of this linac is 1120 mm. The frequency is 81 MHz.

InSb working standard radiometers

InSb radiometers have been developed and characterized for use as transfer standards at the National Institute of Standards and Technology (NIST). The InSb radiometers will hold a portion of the recently realized infrared spectral response scale of the NIST and will serve as working standards for the calibration of test detectors. These radiometers will measure radiant power or irradiance from 2 µm to 5.4 µm. They have a 17° field-of-view. The InSb detectors were selected for high shunt impedance to achieve high sensitivity. The transimpedance-gain, voltage-gain, and loop-gain of the current-to-voltage converters were optimized for optical radiation chopped at 40 Hz. At the peak response, the noise-equivalent-power is 0.6 pW in an electrical bandwidth of 0.3 Hz. The ratio of the dc ambient-background current to the ac noise-equivalent current is 107. Spatial and angular responsivities, linearity, stability, and flashing were also characterized.

Efficiency of the broadband RF cavity longitudinal kicker in DAΦNE

Multibunch instabilities will be damped in DAΦNE main rings by a bunch-by-bunch feedback system using a longitudinal kicker to give the proper energy correction to each bunch. Stripline based kickers now in use have shown several HOMs with quite high shunt impedances that contribute to the total machine broad band impedance, and may cause significant heating of the electrodes. In this paper we investigate the impact on the beam dynamics of a new broad band RF cavity kicker loaded with waveguides that broaden the fundamental-mode bandwidth and damp the cavity HOMs.

Conceptual design of compact heavy-ion inertial fusion driver with an r.f. LINAC with high acceleration rate

The interdigital-H-type (IH) linear accelerator (LINAC) is well known for its high shunt impedance at low and medium particle velocities. Therefore, it can be used to operate efficiently with a high acceleration gradient. The IH LINAC cavity is able to generate 10 MV m −1 (average acceleration gradient) with focusing of the particles by a superconducting solenoid and quadrupole. The LINAC can accelerate particles with a charge to mass ratio ( q/ A) greater than 1/250 from 0.3 MeV a.m.u. −. In a compact heavy-ion inertial fusion driver design, the total effective length of the IH LINAC cavities is about 1250 m.

Large improvement in HgCdTe photovoltaic detector performances at LETI

The standard infrared photovoltaic technology developed for HgCdTe by LETI and industrialized by SOFRADIR is based on the very simple approach of planar ion-implanted n-on-p homojunctions. Both the growth by liquid-phase epitaxy of excellent-quality epitaxial layers and the simplicity of the planar ion-implanted process enables state-of-the-art detectors to be achieved with a high technological yield. These detectors present high shunt impedance, good quantum efficiency, and a low l/f noise level. The diodes are diffusion-limited down to temperatures much lower than 77K. Their saturation current is limited by the minority-carrier lifetime in the p-side material. R0A values around 30 ohm-cm2 are routinely obtained for 10.0 μm cutoff wavelength detectors at 77K. In this paper, we show that with a new process we can increase the diode R0A by more than one order of magnitude. This effect is obtained as a result of an increase of minority-carrier lifetime in the n-on-p homojunction configuration. The maximum R0A value obtained was 655 ohm-cm2 on a 10.0 μm cutoff wavelength detector at 77K. Furthermore, other figures of merit much as quantum efficiency or shunt impedance are slightly improved, and l/f noise is not affected. The data presented in the 40–200K temperature range and 9–13 μm cutoff wavelength range show that this decrease of dark current is kept throughout these temperature wavelength ranges. Therefore, we show that a simple planar ion-implanted homojunction can lead to very large R0A, close to theoretical limits and comparable to data published for p-on-n heterojunctions.

Development of seven-gap resonators for the Heidelberg high current injector

A high current injector for the heavy ion storage ring TSR in Heidelberg is under construction. As a part of the injector eight seven-gap resonators with high shunt impedance are being developed. These resonators ( ƒ 0 = 108.48 MHz ) are designed for the synchronous velocities of β s = 3.7, 4.5, 5.1 and 5.7%. Low power models with scaling factors of 1:2.5 were built in order to study the characteristics of these new resonators. Following low level measurements to optimize the voltage distribution and eigenfrequency, a first power resonator was built and successfully tested at 80 kW (duty cycle of 25%). At this power, the resonator generated a maximum voltage (summation of all gap amplitude voltages) of 1.75 MV. This paper describes the design of the resonators and gives some details of the measurements.

Traveling wave acceleration of protons

Protons can be accelerated efficiently by an H-coupled traveling wave linac in L- or S-band. Different modes of operation are possible with high shunt impedance values and stable beam dynamics.

High-lifetime strained Si1-xGex films grown by rapid thermal chemical vapor deposition

Abstract The electrical and optical properties of strained epitaxial Si 1-x Ge x films on silicon have been examined to determine the potential application of the material for integrated infrared sensors on silicon substrates. The generation lifetime in films grown by rapid thermal chemical vapor deposition was found to be in excess of 1 μs, which is important for the fabrication of sensors with high shunt impedances. By comparison of the well-resolved photoluminescence of these films with absorption data, it is also concluded that the initial absorption mechanism in these alloys proceeds by a no-phonon process, despite the fact that the films have an indirect bandgap.

Heavy-ion postaccelerator at the University of Tsukuba

A linear rf accelerator of the interdigital-H type has been constructed as a heavy-ion postaccelerator for the 12 UD Pelletron tandem accelerator. The high effective shunt impedance of the accelerating cavity resulted in the generation of a total accelerating voltage of 1.77 MV with an rf power of only 8 kW at 100 MHz. By means of this postaccelerator, the energy of 35Cl 15+ ions delivered by the tandem accelerator at 110 MeV increased up to a final energy of 136.5 MeV. A new method of the determination of drift-tube lengths has also been developed. In this method, the gaps between the drift tubes are adjusted by using an empirical formula in order to obtain a uniform distribution of accelerating fields as well as to satisfy the synchronous condition for ions to be accelerated. The present postaccelerator has been successfully used for nuclear spectroscopy as a simple, low-cost and powerful apparatus.

The upgraded Munich linear heavy ion postaccelerator

The linear postaccelerator at the Munich tandem laboratory was extended by a second resonator cavity and a de-/rebuncher, which both have twice the frequency of the first tank. The simple construction principle and the high shunt impedance of the first cavity, an IH structure, can be transferred onto the sections of a long linac by using a special kind of beam dynamics. In addition to the postacceleration of heavy ions, this method also provides an economical acceleration of low energy beams with small q/ A values. The second Munich postaccelerator section has an effective shunt impedance of 167 MΩ/ m for β = 0.1 , its total length is 3 m. The de-/rebuncher (5–7 gaps) shows that also short IH structures are very effective.

Critical Test of Coupled-Bunch Instability Theory

Longitudinal and transverse coupled-bunch instabilities were observed in the Photon Factory storage ring. They were all caused by the high shunt impedances of the RF accelerating cavities and their properties were studied in detail. The frequency dependences of threshold currents of these instabilities were measured, and well described in terms of the frequency dependent structures of the cavity impedances. Absolute values of the thresholds are discussed in a comparison with perturbation theory.

A 3-λ/4 posi coupler for drift-tube linacs

New permanent magnets for quadrupole focusing make possible smaller drift tubes in drift-tube linacs (DTLs), resulting in higher shunt impedance. However, ordinary post couplers cannot stabilize a DTL whose drift-tube-to-wall spacing exceeds one-quarter wavelength (λ/4) by more than a few per cent for the accelerating mode frequency. We have built and tested post couplers that operate in the 3λ/4 mode. These 3λX/4 post couplers, when substituted for λ/4 couplers, had similar stabilization properties. In addition, the coupling between post and drift tube, and the post's resonant frequency can be independently adjusted.

Electron Linac Optimisation for Short RF and Beam Pulse Lengths

New applications of electron linacs require better use of the space available and the success of the SLED RF compression method renews the interest for the conventionnal E-coupled TW accelerating structure which has the required short filling time. We propose to use the less conventionnal backward-wave H-coupled TW accelerating structure which allows an higher shunt impedance. A unit of acceleration could include a 37 MW TH 2094 klystron, with SLED feeding through a 3 db hybrid coupler including a load in the fourth arm, two 6 meter long backward TW structures with thickened irises near the acceleration axis. Output loads could be avoided. The choice of the H-coupled 4 ?/5 mode and the cell shapes and properties are analyzed. Conclusion stresses the advantages as simplicity and also the limits of this optimization.