Slug Tuners Research Articles

Tuning an 81.25 MHz four-vane RFQ with a ramped field profile at RISP

A radio-frequency quadrupole linear accelerator (RFQ) was developed and tuned for the heavy ion accelerator facility at Rare Isotope Science Project (RISP). The RISP RFQ has an 81.25 MHz operational frequency and a four-vane structure for a continuous-wave mode operation despite the fabrication difficulties of the huge cavity generated due to the brazing technology. The cavity is inherently insensitive to perturbations due to low frequency and a short cavity length. The linearly increasing profile of inter-vane voltage was tuned for all quadrants through not only movable slug tuners but also modification of the end-plate. In this study, a low-frequency RFQ with a novel, ramped field profile was tuned and the commissioning tests were conducted with a new tuning method compatible with the modification of the end-region geometry.

Room temperature quarter wave resonator re-buncher development for a high power heavy-ion linear accelerator

In the medium energy beam transport (MEBT) line system of the RAON which consists of several quadrupole magnets, three normal-conducting re-bunchers, and several diagnostic devices, a quarter wave resonator type re-buncher was chosen for minimizing longitudinal emittance growth and manipulating a longitudinal phase ellipse into the longitudinal acceptance of the low energy linac. The re-buncher has a resonant frequency of 81.25 MHz, geometrical beta (βg) of 0.049, and physical length of 24 cm. Based on the result of numerical calculations of electromagnetic field using CST-MWS and mechanical analysis of the heat distribution and deformation, an internal structure of the re-buncher was optimized to increase the effective voltage and to reduce power losses in the wall. The criteria of the multipacting effect was estimated and it was confirmed by the experiment. The position and specification of cooling channels are designed to recover a heat load up to 15 kW with reasonable margin of 25%. The coaxial and loop type RF power coupler are positioned on the high magnetic field region and two slug tuners are installed perpendicularly to the beam axis. The frequency sensitivity as a function of the tuner depth and cooling water temperature is measured and the frequency shift is in all cases within the provided tuner range. The test with a high power of 10 kW and the continuous wave is performed and the reflection power is observed less than 450 W.

RF structure design of the China Material Irradiation Facility RFQ

The radio frequency structure design of the radio frequency quadrupole (RFQ) for the front end of China Material Irradiation Facility (CMIF), which is an accelerator based neutron irradiation facility for fusion reactor material qualification, has been completed. The RFQ is specified to accelerate 10 mA continuous deuteron beams from the energies of 20 keV/u to 1.5 MeV/u within the vane length of 5250 mm. The working frequency of the RFQ is selected to 162.5 MHz and the inter-vane voltage is set to 65 kV. Four-vane cavity type is selected and the cavity structure is designed drawing on the experience of China Initiative Accelerator Driven System (CIADS) Injector II RFQ. In order to reduce the azimuthal asymmetry of the field caused from errors in fabrication and assembly, a frequency separation between the working mode and its nearest dipole mode is reached to 17.66 MHz by utilizing 20 pairs of π-mode stabilizing loops (PISLs) distributed along the longitudinal direction with equal intervals. For the purpose of tuning, 100 slug tuners were introduced to compensate the errors caused by machining and assembly. In order to obtain a homogeneous electrical field distribution along cavity, vane cutbacks are introduced and output endplate is modified. Multi-physics study of the cavity with radio frequency power and water cooling is performed to obtain the water temperature tuning coefficients. Through comparing to the worldwide CW RFQs, it is indicated that the power density of the designed structure is moderate for operation under continuous wave (CW) mode.

Application of Extremum Seeking for Time-Varying Systems to Resonance Control of RF Cavities

A recently developed form of extremum seeking for time-varying systems is implemented in hardware for the resonance control of radio-frequency cavities without phase measurements. Normal conducting RF cavity resonance control is performed via a slug tuner, while superconducting TESLA-type cavity resonance control is performed via piezo actuators. The controller maintains resonance by minimizing reflected power by utilizing model-independent adaptive feedback. Unlike standard phase-measurement-based resonance control, the presented approach is not sensitive to arbitrary phase shifts of the RF signals due to temperature-dependent cable length or phasemeasurement hardware changes. The phase independence of this method removes common slowly varying drifts and required periodic recalibration of phase-based methods. A general overview of the adaptive controller is presented along with the proof of principle experimental results at room temperature. This method allows us to both maintain a cavity at a desired resonance frequency and also to dynamically modify its resonance frequency to track the unknown time-varying frequency of an RF source, thereby maintaining maximal cavity field strength, based only on power-level measurements.

Straightforward and accurate technique for post-coupler stabilization in drift tube linac structures

The axial electric field of Alvarez drift tube linacs (DTLs) is known to be susceptible to variations due to static and dynamic effects like manufacturing tolerances and beam loading. Post-couplers are used to stabilize the accelerating fields of DTLs against tuning errors. Tilt sensitivity and its slope have been introduced as measures for the stability right from the invention of post-couplers but since then the actual stabilization has mostly been done by tedious iteration. In the present article, the local tilt-sensitivity slope TS 0 is established as the principal measure for stabilization instead of tilt sensitivity or some visual slope, and its significance is developed on the basis of an equivalent-circuit diagram of the DTL. Experimental and 3D simulation results are used to analyze its behavior and to define a technique for stabilization that allows finding the best post-coupler settings with just four tilt-sensitivity measurements. CERN’s Linac4 DTL Tank 2 and Tank 3 have been stabilized successfully using this technique. The final tilt-sensitivity error has been reduced from � 100%=MHz down to � 3%=MHz for Tank 2 and down to � 1%/MHz for Tank 3. Finally, an accurate procedure for tuning the structure using slug tuners is discussed.

Slug tuner effect on the field stabilization of the drift tube linac

In a drift tube linac (DTL), the accelerating field is stabilized against external perturbation, through resonant coupling between each cell by using post couplers. For proper field stabilization tuning, the frequency band between the post mode and the cavity mode should be closed. In addition, the field profile along the beam axis of the highest post mode should be similar to that of the TM011 cavity mode. As a conventional method to correct the resonance frequency and to make the accelerating field flat, slug tuners are incorporated. We observed that the similarity of field profiles between the highest post mode and the TM011 cavity mode disappeared when the slug tuners were inserted too much into the DTL tank. To achieve field stabilization tuning, we limited the slug tuner insertion and used a tuning ring around each post coupler to tune the resonant frequency of the DTL tank. The details of the effect of a slug tuner on the field stabilization tuning and the solution to the resonant frequency tuning problem caused by limited slug insertion will be presented in this paper.

Design study of a re-bunching RFQ for the SPES project

An upgrade to the 2nd generation of the selective production of exotic species (SPES) to produce a radioactive ion beam (RIB) has been studied at the istituto nazionale di fisica nucleare — laboratory nazionali di Legnaro (INFN-LNL). Due to the long distance between the isotope separator online (ISOL) facility and the superconducting quarter wave resonator (QWR) cavity acceleratore lineare per ioni (ALPI), a new re-buncher cavity must be introduced to maintain the high beam quality during the beam transport. A particular radio frequency quadrupole (RFQ) structure has been suggested to meet the requirements of this project. A window-type RFQ, which has a high mode separation, less power dissipation and compact size compared to the conventional normal 4-vane RFQ, has been introduced. The RF design has been studied considering the requirements of the re-bunching machine for high figures of merit such as a proper operation frequency, a high shunt impedance, a high quality factor, a low power dissipation, etc. A sensitivity analysis of the fabrication and the misalignment error has been conducted. A micro-movement slug tuner has been introduced to compensate for the frequency variations that may occur due to the beam loading, the thermal instability, the microphonic effect, etc.

Preliminary study on the RF tuning of CSNS DTL

In the R&D of the CSNS Drift Tube Linac (DTL), the first unit tank with 28 drift tubes has been developed. The axial accelerating field is ramped from 2.2 MV/m to 3.1 MV/m in this tank. The required field flatness is less than ±2% with the standard deviation of 1% for the beam dynamics; the field stability should be less than 1% for machine stable operation. After successful alignment, RF tuning was carried out focusing on the field profile measurement. Four slug tuners and eleven post couplers were applied in this procedure. The ramped field and required stability had been achieved by fine adjustment of the slug tuners and post couplers. In this paper, the preliminary tuning results are presented and discussed.

Power-Balance Control by Slug Tuner for the 175MHz Radio-Frequency Quadrupole (RFQ) Linac in IFMIF Project

RF Power-balance control among the quadrants of IFMIF 4-vane radio-frequency quadrupole (RFQ) linac structure is a critical issue, because it may be affected by the fabrication error of the 12m-long RFQ and by the perturbation due to loop antenna installation. The power-balance controllability of slug tuners was measured by a low power test, and the distortion behavior of electric fields profiles at beam bore peripheral was calculated by an electromagnetic field simulation code. From the low power test, it was found that RF power-balance up to 80% can be controlled without overlap of modes. The calculation suggested that the design requirement of distortion limit of 1% can be attained by employing 1cm slug tuner with insertion depth of 1cm or less.

Materials Considerations for a High Power-Density Accelerator

A 100-mA 50-MeV H- accelerator is being designed at Los Alamos. The accelerating structures will operate at 425-MHz and will consist of a radio-frequency quadrupole (RFQ) to 2-MeV and a drift-tube linac (DTL) from 2 to 50-MeV. Design parameters have been specified to match the maximum operating capabilities of the rf system: 2-ms pulse length and 6% duty factor. The accelerating gradient in the DTL will be 4-MV/m; the maximum electric field will be approximately 1.2 times the Kilpatrick limit. These design parameters are substantially more ambitious than those of the accelerator test stand (ATS). The larger accelerating gradient and increased duty factor will increase the average power density in the DTL to approximately 25 times the design values for the ATS DTL. The increased duty factor will raise the average power density on the proposed RFQ to approximately 6 times the operating values of the ATS RFQ. The instantaneous power density on the drift tubes is predicted to be 60 W/sq cm, and the average power density is predicted to be 4 W/sq cm. These power densities are not excessive, but do represent design challenges in specific areas. Some components where thermal difficulties may be encountered include RFQ vanes, drift tubes, post couplers, slug tuners, rf seals, and tuning bars. That copper will be used on the rf-structure surfaces is not in question. Concern with residual activation and thermal management forces the investigation of materials other than the traditional carbon- and stainless-steel base materials used at Los Alamos.

Design of K-band slug tuners

An analysis of dual-slug line tuners is presented for two cases: distributed-transmission-line slugs and lumped-capacitance slugs. The effect of loss on tuner performance in the case of the transmission-line model is investigated and the curves obtained are used in the design of a K-band tuner over the frequency range 12–26 GHz. Good agreement is obtained between predicted and measured results.