Dielectric loaded resonators, by virtue of their high sensitivity, are a de facto standard for the measurement of the microwave surface impedance Z of good conductors, including superconductors. However, new perspective applications of superconductors request measurements in increasingly extreme conditions: cryogenic and magnetic environments, often combined together. Careful evaluations of the dielectric resonator (DR) design are necessary to operate in these conditions. Moreover, the evaluation of the uncertainties and the issues related to an appropriate calibration become problematic. We present here a performance analysis of DRs designed for cryogenic temperature and high magnetic field operation and some sample experimental results on Z measurements on the most technologically interesting superconductors.

Challenging microwave resonant measurement techniques for conducting material characterization

Particle physics and radio-frequency (RF) superconductivity have driven each other on since the 1970s. The unique properties of superconductors (SC) have been the enabling keys for the realization of accelerators with always increased performances thanks to the realization of all-superconducting cavities. The use of increasingly pure superconducting coatings for accelerating cavities, with lower and lower RF losses, determined such high-quality factors [1] that the need to operate at low temperatures (below the superconducting transition temperature $T_{c}$ ) was well paid for. Recently, with respect to the path followed by the high frequency superconductivity [2], a new field opened since SCs are being considered for GHz operation in high dc magnetic fields, and measurements (and optimization) of totally different quantities are needed. The possibility of successfully using SCs in high magnetic fields for these purposes is far from obvious and it depends on the outcome of accurate measurements of usually overlooked quantities.

Surface Impedance Measurements in Superconductors in DC Magnetic Fields: Challenges and Relevance to Particle Physics Experiments

Abstract As an alternative to research nuclear reactors, a compact accelerator-driven neutron generator that uses a lithium beam driver could be a promising candidate since it produces almost no undesired radiation. However, providing an intense lithium-ion beam has been difficult, and it has been thought that the practical application of such a device would be impossible. The most critical problem of insufficient ion fluxes has been solved by applying a direct plasma injection scheme. In this scheme, a pulsed high-density plasma from a metallic lithium foil generated by laser ablation is efficiently injected and accelerated by a radio-frequency quadrupole linear accelerator (RFQ linac). We have obtained a peak beam current of 35 mA accelerated to 1.43 MeV, which is two orders of magnitude higher than a conventional injector and accelerator system can deliver. 

/pdf/demonstration-of-an-intense-lithium-beam-for-forward-1inl3gla.pdf

Demonstration of an intense lithium beam for forward-directed pulsed neutron generation

Novel deuteron RFQ design with trapezoidal electrodes and double dipole four-vane structure

A transportable accelerator-driven neutron source based on the Radio Frequency Quadrupole (RFQ) linac is developing at Xi’an Jiaotong University (X-TANS). It will be a promising tool for the non-destructive inspection of immovable objects. The neutron yield above 10 11 n/s can be obtained via 7 Li(p,n) 7 Be reaction with a proton beam energy of 2.5 MeV. To exactly guarantee the transportability of the neutron source, it is essential to achieve the light, compact and low-power consumption of RFQ linac. The operation frequency of 325 MHz is chosen considering the balance among cavity size, cavity consumption, and electrode aperture. In addition, compact dynamics scheme is adopted to achieve short RFQ by sacrificing a beam bunching efficiency. The beam dynamics design and optimization have been performed based on Parmteqm. Beam tracking results show that RFQ can accelerate the proton beam to 2.5 MeV with an electrode length of 2.6 m and an effective transmission efficiency of 93.8%. To enlarge the redundancy of the input beam mismatch and make the beam match easier from LEBT to RFQ, the electrode aperture is designed to shrink gradually in the RFQ entrance. In addition, we also performed the RF electromagnetic design and optimization based on the full-length 3D RFQ model. Multiphysics simulations of the cavity in different working conditions are investigated at last. 

Design of a compact RFQ linac for the transportable neutron source

The proposed Compact Intense Fast NEutron Facility (CIFNEF) will have a wide range of applications, including the investigation of the exotic structure of neutron-rich nuclei, data for the nuclear fission , and the simulation of the neutron field in a star. To satisfy aims of producing high-intensity fast neutrons and forward neutrons with very low environmental background, the CIFNEF linac should have the capacity of accelerating continuous wave (CW) deuteron (D + ), hydrogen ( H 2 + ) and lithium (7Li 3 + ) beams to 2.5 MeV/u with maximum beam currents of 10 mA, 5 mA and 10 uA, respectively. Based on the above requirements, we proposed a novel compact linac using a combination of RFQ and DTL structures. The dynamics of RFQ and DTL are completed to meet all requirements and start-to-end simulation results show that the three ion species can be accelerated to the final energy with transmission efficiency above 99% as well as good beam quality with lower emittance growth. In addition, we performed error sensitivity analysis and combined error study to evaluate the error tolerance limits of the obtained design.

Design of a CW linac for the Compact Intense Fast NEutron Facility

For a high-current Radio Frequency Quadrupole (RFQ) accelerator, space charge will strongly affect the stability of beams, which brings challenges and difficulties to the beam dynamics design. For an in-depth study of this space charge effect, a project has been proposed by Peking University and the Institute of Modern Physics of the Chinese Academy of Sciences. This project requires a 162.5 MHz RFQ to accelerate a 100-mA continuous-wave (CW) proton beam to 3 MeV. The beam dynamics design of this RFQ is based on the four-section procedure with improved radial focusing strength along the RFQ. We have investigated the relationship between limiting current and beam transmission to analyze the reasons for emittance growth and beam loss. We have also carried out an irradiation experiment based on this beam dynamics design to study the irradiation damage of the electrode surface, as this can be one main reason for reduced transmission or unstable operation that most existing high-current RFQs encounter after long-time operation. The results show that low-energy beam losses need to be kept as low as possible to prolong the life of the RFQ electrodes. The final simulated transmission efficiency reached 99.8%, as confirmed by the code toutatis .

Beam dynamics design and irradiation experiment of beam loss for a CW 100-mA proton RFQ

https://iopscience.iop.org/article/10.1088/1748-0221/13/08/T08002/pdf

CW RFQ design and investigation for multi-charge-state acceleration of radioactive beams from BISOL

A 10 mm diameter 60 mm long plasma window has been designed and managed to generate arc discharge with argon gas experimentally in Peking University. Based on the previous experiments and simulations, we have measured the electron temperature and density of the plasma via argon spectral diagnostics, and analysed the conditions to satisfy the criterion of local thermal equilibrium (L.T.E). The electron temperature is in the range of 12000 K to 16000 K. The electron density is in the range of . × 6 − to . × 6 − , increasing with discharge current and gas flow rate. The results indicate that our argon plasma is in the L.T.E status. The sealing pressure characteristics of the plasma window is mentioned as well. INTRODUCTION Plasma window has been designed to use plasma discharge to separate vacuum from high pressure. As a windowless vacuum seal device, plasma window has many advantages over conventional solid windows which have bad effects on the beam qualities. It can provide lossless transmission of high purity and mono-energetic beams due to its lower equivalent thickness. Small aperture plasma windows have been experimentally realized to separate 2.85 bar from 0.6 mbar with a 2.36 mm diameter 40mm long arc [1], and up to 9 atm from 2 mbar with 2mm channel in diameter [2]. To exploring the application of plasma window on high current heavy ion beams, a 10 mm diameter plasma window has been built and studied in Peking University [3]. The structure of the plasma window is shown in Fig.1. It mainly consists of a cascade of 9 mm thick copper plates that are electrically insulated from each other. Electrical insulation and vacuum sealing are accomplished by 1mm thick boron-nitride (BN) spacer plates inside O-rings, mechanically stabilized by PEEK material washers outside the O-rings. Three cathodes, made of thoriated tungsten, are inserted at an angle of 45°into cathode chamber and axial symmetrically located. Cathodes and anode are connected to a constant-current power supply, which can deliver current up to 80 A. Argon gas is fed to Plasma window through gas inlet tube. At the end of Plasma window, the lower pressure is sustained by differential pumping system. In this paper, the experimental data will be shown and the influence of the operating conditions (gas flow rate and discharge current) on the electron density, temperature and sealing pressure capability will be discussed. Figure 1: Diagram of Plasma window. EXPERIMENT SETUP AND METHODOLOGY Our spectroscopy setup (see Fig.2) consists of a spectrometer (Horiba IHR 550) along with a CCD camera (synapse) and a computer for running the spectroscopy program (SynerJY). Spectrum signal passing through quartz window is collected by spectrometer with optical fiber and the outcome data is stored by the computer. The spectrometer has three different gratings (300 gr/mm, 1200 gr/mm and 2400 gr/mm) to satisfy the requirements of different spectral coverage and resolution. Figure 2: Diagram of spectroscopy setup. The electron excitation temperature can be determined by atomic Boltzmann plot method [4] from the following equation: Iλ ���� ⁄ = C − ��/ � (1) where I is spectrum intensity, λ is the wavelength of the line and is Boltzmann constant. ��, �� and �� are transition probability, excitation energy and statistical weight of atomic state u, respectively. These values can be found in literatures. According to equation (1), we can get a straight line by linear fitting method. The inverse of line slop presents the electron temperature, assuming that the ___________________________________________ † Corresponding author: zhukun@pku.edu.cn WEPIK025 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 2978 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 07 Accelerator Technology T14 Vacuum Technology population of the emitting levels follows the Boltzmann’s distribution. Emission spectra of Ar II lines in the spectral range of 450-520 nm are shown in Fig.3. Seven resolved and distinct Ar II lines were chosen and corresponding ��, �� and �� were taken from literature [5]. Iλ ���� ⁄ vs �� were plotted (see Fig.4), and the slope was obtained from the linear fitting. The correlation coefficient was more than 0.95 and the standard error of the slope was 9%. The electron temperature is about 11900 K at argon gas flow rate of 1.2 SLM and discharge current of 40 A. Figure 3: Emission spectra of Ar II lines in the spectral range of 450-520 nm. Figure 4: Typical Boltzmann plot for seven selected Ar II emission lines. The Stark broadening of a certain spectral line emitted from plasma carries the electron density information. In our case, the Ar I line 430 nm is often used to calculate the electron density, as it is separated from other lines (see Figure 5), which guarantees a good resolution and accuracy. The empirical formula is given [6]: �� − = . + . �∆� / (2) where ∆� / is the half width of Ar I line. Figure 5: Emission spectra of Ar 430 nm line in the spectral range of 424-436 nm. RESULTS AND DISCUSSION The measured electron temperature as a function of gas flow rate is shown in Fig.6. We can see that the temperature decreases with the gas flow rate. It can be explained as that electron temperature decreases with increasing the pressure in the discharge channel [7] which can be caused by increasing gas flow rate. The simulated temperature distribution is shown in Fig.7. In simulation, the inlet pressure and outlet pressure are 6.8 kPa and 74 Pa, respectively. Electric potential of the cathode is -42 V, while the anode is 0 V. Figure 6: Temperature of argon plasma as a function of gas flow rate. Proceedings of IPAC2017, Copenhagen, Denmark WEPIK025 07 Accelerator Technology T14 Vacuum Technology ISBN 978-3-95450-182-3 2979 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs Figure 7: Simulated temperature distribution. From poiseuille’s equation, we can derive the equation (3) for argon plasma channel, under the condition that the outlet pressure is much smaller than the inlet pressure. ��� = �√� �� 2 (3) where � is a constant, � is the dynamic viscosity coefficent, and are the diameter and length of the channel, is the Boltzmann constant, T is the temperature and � is the gas flow rate. In our case, the diameter and length of plasma channel is fixed and the dynamic viscosity is nearly a constant at temperature between 12000 K and 16000 K. So the inlet pressure is proportional to the square root of temperature and gas low rate. Figure 8 shows the proportional relation between the inlet pressure and the square root of discharge current and gas low rate. For all the measurements, the gas flow rate and discharge current are in the range of 1.9 SLM to 9.8 SLM and 40 A to 80 A, respectively. Correlation coefficient is better than 0.99. Indirectly, we can see that the temperature of the plasma is proportional to the discharge current. Figure 8: Inlet pressure as a function of square root of discharge current and gas flow rate. For all the measurements, the gas flow rate and discharge current are in the range of 1.9 SLM to 9.8 SLM and 40 A to 80 A, respectively. Correlation coefficient is better than 0.99. The measured electron density at different discharge current and gas flow rate are shown in Fig.9 and 10. It can be seen that the electron density increases with the increasing discharge current or gas flow rate. According to Grime’s theoretical criteria [8] for local thermal equilibrium (L.T.E) which has considered radiation absorption and resonance absorption, the critical electron density is about . × 6 − for argon plasma electron temperature of 14500 K. The measured electron densities are more than × 6 − . So the L.T.E has been established in the plasma window. But it is only in the axial region because of the high gradient temperature and velocity distribution from the plasma centre to water cooled wall. Figure 9: Electron density as a function of discharge cur-

http://ir.ihep.ac.cn/bitstream/311005/262454/1/WEPIK025.pdf

Spectral Diagnostics of Argon Plasma in a 10mm Aperture Plasma Window

For studying new physics beyond the Standard Model, Supper proton-proton Collider (SPPC) with a circumfer-ence of 100 km and a centre mass energy of 100 TeV is proposed and under study in China. Due to the high particle energies and 16 T high magnet field, the synchrotron radiation power emitted from the proton beams reaches 48.5 W/m in the bending magnets, two orders of magnitude higher than that of LHC. A novel beam screen is anticipated to screen cold chamber walls from the massive synchrotron radiation power and transfer the heat load to cryogenic cooling fluid. For drastically reducing resistive wall impedance and saving refrigerator power, we have studied high temperature superconductor (HTS) coated beam screen operating in liquid nitrogen temperature area. Singly from the point of temperature, the feasibility of HTS-coated beam screen is demonstrated by steady-state thermal analysis. Two kinds of potential HTS material are also discussed in this paper.

HTS-Coated Beam Screen for SPPC Bending Magnets

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