114310946
DESIGN AND MODEL CAVITY TEST OF THE DEMOUNTABLE DAMPED CAVITY
Demountable Damped Cavity (DDC) has been designed in an ILC R&D. DDC has two design features. One is to use coaxial waveguide for strongly coupled out HOMs and apply choke filter to reflect accelerating mode. The axial symmetric RF design can reduce the beam kick effect. The other feature is demountable structure, which makes end group cleaning easy to suppress the Q-slope problem at a high field, and also could bring cost reduction in the cavity fabrication. INTRODUCTION It is important both strong HOM damping and high acceleration in the superconducting RF cavity for the ILC main linac. In multi-bunch operation, the long-range wakefields are excited by the forward bunches passing in the structure. If the wakefields are keeping existence, they deflect the coming bunches, and result in the multi-bunch beam breakup (or cumulative BBU) instability. The strong HOM damping is essential to suppress such effects. To obtain enough damping, the Q value of HOMs is to be lower than 1x10 for ILC machine [1]. OVERVIEW OF HOM DAMPERS Various types of HOM dampers are used according to the purpose of the accelerator. The coaxial antenna HOM coupler based on TESLA [2] is now adapted as ILC baseline design. The coaxial transmission line does not have the cutoff frequency and easily takes out HOMs. It can be designed compactly. Another merit is less cryogenic load, which is a cost driver in SRF machines, because the HOM power is taken out outside of the cryostat. In the high-current accelerator like KEKB [3], HOM damping becomes much more serious. The RF absorber is mounted on the end of beam pipe (this method will be called “Beam pipe method”). As the beampipe is a cylindrical waveguide, it has the cutoff frequency. To damp HOM enough, a larger beam pipe is used for trapped HOMs to leak out the cavity and dissipate the power in the absorber. However, such a large beam pipe decreases the R/Q of the accelerating mode, which is unsuitable to the high-energy accelerator. Cornell University has developed the special fluted beam pipe method instead of such a large beampipe for CESR-B [4]. It lets the trapped modes propagate out the cavity through the fluted waveguide, which decreases the Qext of the HOMs without decreasing R/Q of the accelerating mode. The concept of the choke mode cavity (e.g. SCSS [5]) has two devices: improved design in the transit line to take out HOMs easily like the special fluted beam pipe method, and choke cavity to reflect the accelerating mode at the transmit line, which makes the waveguide port imaginary short on the accelerating mode. Table 1 compares the structure, the R/Q of accelerating mode, the Qext of HOMs, the fabrication cost, and the space factor. RF DESIGN OF DDC Fig.1 shows the DDC design on the Ichiro single end cell cavity. The RF design of the DDC is based on the upgraded choke mode cavity by Dr. Kageyama [6]. Keys of the RF design are followings. One is the coaxial waveguide, which takes out HOMs, especially trapped modes, at the end of the beam pipe, then damps in the absorber mounted there. On the other hand, TM010 mode (accelerating mode) propagats in the coaxial waveguide if no device is put. It is reflected back the cell using a choke cavity. The coupling constant between the cell and the coaxial waveguide can be adjusted by the length of the inner conductor. However a part of the accelerating mode is converted to TE01 mode at the joint of the choke structure and propagates out through the waveguide. This problem is reasonably solved using the cutoff frequency. The distance between the choke and the absorber is lengthened a little for this problem but the whole end Table 1: Comparison by the Shape of Dampers. ___________________________________________ *Work supported by KEK konomi@post.kek.jp WEPE014 Proceedings of IPAC’10, Kyoto, Japan 3374 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A03 Linear Colliders group length is still same as ILC base line design. Another key is the demountable end group. It has aimed to overcome a problem of high field Q slope observed in the TESLA type coupler [7]. Separating the end group from the cavity cell makes easy cleaning in such complicated end group. The separating point must be at a location where RF field is weak. We choose the point at outside of the choke cavity where the RF H-field is small about one sixth of that in the cell. The liquid helium baseplate is used as a part of the choke cavity to save the space. The demountable end group will be connected at the base-plate flange using MO seal. We will establish the elemental technology in the single cell at first, then demonstrate on 9-cell cavity. Fig. 1: Design of the DDC. RF SIMULATION We use Superfish and MW-Studio for calculating the RF characteristics of the DDC. Table 2 shows the comparison of Ichiro regular cell (both beampipe diameter are 60mm), Ichiro end cell (beampipe diameters are 60mm and 80mm), and the DDC. We can see the RF characteristics are even improved from Ichiro end cell because the choke cavity reflects the accelerator mode back to the cell. Table 3 shows the Qext of HOMs which should be seriously damped both on the DDC (Fig.2) and the TESLA type HOM coupler (Fig.3 [8]). Superfish RF calculation is limited only for monopole modes, while MW studio can calculate all HOMs. For the DDC, we calculated the Qext of HOMs with MW studio. In this calculation, ferrite is supposed as the absorber material. We referred the TESLA like HOM coupler (KEK-STF baseline) from Dr. K. Watanabe’s doctor thesis [8]. The Qext of the TESLA like type HOM coupler is higher than the DDC by ten times or more. In the TESLA type HOM coupler, TM110 (trapped mode) which has to be damped seriously, degenerates in two frequencies with different Qexts, while the DDC does not degenerate due to the axial symmetric configuration. The DDC can remarkably damp the trapped mode in an ILC cavity. Table 3: Qext of HOMs. Fig. 2: DDC RF simulation model. Fig. 3: TESLA type HOM coupler model [8]. MODEL CAVITY TEST The purpose of the RF model cavity is to confirm that accelerating mode does not propagate to absorber while HOMs propagate out enough at absorber location. We measured the 2 type cavities: the DDC without absorber (Fig.4 left) and the simple Ichiro end cell cavity with the coaxial inner conductor inserted in the φ80 beampipe (Fig.4 right), which is referred as “Simple coaxial cavity”. For both cavities, an input coupler is inserted in φ60 beam pipe and a pick up antenna locates between φ80 beam pipe and the inner conductor. Fig. 4: DC (left), Simple coaxial cavity (right). Fig.5 shows the results of network-analyzer measurement (S21) in the Simple coaxial cavity and the DDC without absorber. We can see that the TM010 mode (fundamental mode 1300MHz) is rejected in the DDC (red solid line in Fig.5) while it propagates to the end of Table 2: Comparison of the RF Parameters. Proceedings of IPAC’10, Kyoto, Japan WEPE014 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A03 Linear Colliders 3375 beampipe in the Simple coaxial cavity (blue dot line in Fig.5). A lot of HOMs appears in Fig.5 because both cavities do not equip absorber. The mode assignment was done with the frequency by RF simulation. We labeled the HOMs as cavity modes, choke modes and coaxial modes, which show where the fields are mainly exited. From the simulation results, we have confirmed that both choke and coaxial modes propagate to the absorber and damped enough with the DDC, for instance their Qexts are between100 and 1000.We will measure the DDC with absorber soon. -100 -80 -60 -40 -20
55249742
Activities on Superconducting Cavities at TOSHIBA
Abstructs R&D activities on superconducting cavities at TOSHIBA are presented. Two L-band single-cell niobium cavities and an L-band three-cell structure were fabricated in our company and measured at KEK. The maximum field gradient of Eacc,max>30MV/m in both single-cell cavities and Eacc,max =18MV/m in the three-cell structure were attained respectively. An optimization study on the annealing temperature for the hydrogen Q-disease is reported. R&D of hydroforming to fabricate seamless cavities is also presented.
137182571
A New Material Evaluation Method on Niobium by Magnetization Measurement
As a new evaluation method for the material defects or surface contamination of niobium, we have tried magnetization measurement using very small niobium samples. We could see the hyst~resis curve varied depending on the sample's history. This method has a high sensitivity. Th~ m.f1uence ?y surface treatments was observed on the hysteresis curve; annealing, chemical p~hshmg, Q-dlsease due to hydrogen etc .. In this paper we will report several study results by this method.
94312812
High Field Studies on L-band Superconducting Cavities at KEK
Several 1.3 GHz single-cell and nine-cell niobium cavities were fabricated to attain a higher maximum accelerating gradient and to understand the phenomena limiting it. Vertical cold tests at {approx}1.8 K for the 4 single-cell cavities prepared by various surface treatments have been conducted 15 times. In eight of the tests, maximum accelerating gradients (E{sub acc}, max) of more than 14 MV/m were obtained with no field emission. Furthermore, E{sub acc}, max values of higher than 20 MV/m with a high Q{sub 0} value of {approx}10{sup 10} were achieved repeatedly after heat treatment at 1400 degrees C. Once quenching was observed at E{sub acc}, max (14 {approx} 20 MV/m), however, a common phenomenon resulting in Q{sub 0}-deterioration occurred in every test. The cavity performances and the phenomenon at high fields are reported in this paper.
125432193
State-of-the-art and future prospects in rf superconductivity
6846765
HIGHER HARMONICS BEAM LOADING COMPENSATION FOR A BROAD BAND MA-LOADED RF CAVITY
A broad band RF cavity loaded with magnetic alloy cores has been developed for the Japanese Hadron Facility synchrotrons. Its Q-value can be chosen at between 0.5 and 5. To reduce transient beam loading effects, a small Qvalue (< 3) is preferable to shorten the transient period. However, the circulating beam current of up to 7 A will induce a spurious voltage at the accelerating gap, which deforms the gap voltage due to higher harmonics components, if the Q-value is less than 3. The resulting RFbucket distortion may cause the beam loss. Therefore we have designed a beam loading compensation system to suppress the higher harmonics components.
17047743
Charge Symmetry Violation in Nuclear Physics
The study of charge symmetry violation in nuclear physics is a potentially enormous subject. Through a few topical examples we aim to show that it is not a subject of peripheral interest but rather goes to the heart of our understanding of hadronic systems.
56351344
RF SYSTEMS FOR JHF SYNCHROTRONS
Based on the measurements of the magnetic cores and R&D works, MA(Magnetic Alloy)-loaded cavity will be adopted for the RF cavity of JHF 50 GeV Main Ring and 3 GeV Booster. Because of high stability at large RF amplitude, high gradient of acceleration voltage (>50 kV/m) is available. By a test cavity, 2.5 kV/core has been already achieved in the CW operation using one MA core of 2.5 cm thickness. For the pulse operation, 5.5 kV/core has been obtained. Another advantage is that the system does not require the tuning system because of the cavity band width is wide enough to cover the RF frequency. A stability analysis shows that the RF system without the tuning loop has wider stable region than high Q cavity system with the loop. A multi-particle simulation shows that the RF system is also stable for the periodic transient and the RF bucket distortion. However, the distortion should be corrected when the Q value of the cavity is low (Q<2).
14603783
A Measurement System Using Gp-Ib For A Magnetic Core Test Cavity
In the design of the RF system of the JHF (Japan Hadron Facilit y) proton synchrotrons, it is very important to find a magnetic core to which satisfies the required condition of the JHF accelerators[1]. Ferrite cores are usually used for the magnetic core of the proton synchrotron, however there are some problems. Generally, some ferrite cores have non-linear characteristics which depend on the RF magnetic field. And, there is a size-dependence of the characteristics. Thus, it is required to measure the characteristics of large size magnetic cores. To measure characteristics of the magnetic cores, we have developed a test cavity which is capable of measuring an actual size one, and measurement system using software ‘HP-VEE’[ 2,3] which is possible to control the measurement devices by using a GP-IB command directly. In this measuring system, the acquisition of data and calculation of the characteristics of the magnetic cores are automatically processed.
55610926
EVALUATION OF MAGNETIC ALLOYS FOR JHF RF CAVITY
A new type of rf accelerating cavity using a high permeability magnetic alloy (MA) has been developed for the high intensity JHF proton synchrotrons. Compared with ordinary ferrites, the MA cores are very stable at high rf magnetic field and thus very high accelerating gradient (=accelerating voltage per cavity length) is available. We have measured several magnetic alloys in order to find a proper material for the cavity. In this paper, the test results are summarized and the evaluation of these materials is presented.
123001860
NEW APPROACH FOR FEW-BODY SYSTEMS BASED ON JACOBI-COORDINATE BASIS ANTI-SYMMETRIZED MOLECULAR DYNAMICS FROM ATOMS TO QUARK-SYSTEMS
Proposed a new method which is suitable for systematic calculations in few-body systems, we call it the Jacobi-coordinate-basis antisymmetrized molecular dynamics (JAMD). We show good abilities of the method by demonstrating some examples in atoms, molecules, nuclei, and baryons systems.