Large-Volume Resonant Microwave Discharge for Plasma Cleaning of a CEBAF 5Cell SRF Cavity (original) (raw)

Cleaning of inner vacuum surfaces in the Uragan-3M facility by radio-frequency discharges

Plasma Physics Reports, 2013

A method for cleaning vacuum surfaces by a low temperature (T e ~ 10 eV) relatively dense (n e ≈ 10 12 cm-3) plasma of an RF discharge was developed and successfully applied at the Uragan 3M torsatron. The convenience of the method is that it can be implemented with the same antenna system and RF genera tors that are used to produce and heat the plasma in the operating mode and does not require retuning the frequencies of the antennas and RF generators. The RF discharge has a high efficiency from the standpoint of cleaning vacuum surfaces. After performing a series of cleanings by the low temperature RF discharge plasma (about 20000 pulses), (i) the intensity of the CIII impurity line was substantially reduced, (ii) a quasi steady operating mode with a duration of up to 50 ms, a plasma density of n e ≈ 10 12 cm-3 , and an electron temperature of up to T e ~ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.

ECR plasma cleaning: an in-situ processing technique for RF cavities

2008

A condition for Electron Cyclotron Resonance (ECR) can be established inside a fully assembled RF cavity without the need for removing high-power couplers. As such, plasma generated by this process can be used as a final cleaning step, or as an alternative cleaning step in place of other techniques. We will describe the current effort to study plasma cleaning by ECR in a 3.9GHz cavity.

Design and characterization of an RF plasma cleaner

Brazilian Journal of Physics, 2010

The design and characterization of an innovative device for plasma cleaning, based upon a non-conventional radio-frequency discharge, is described. The RF fields are produced by an antenna placed inside the metallic vacuum chamber. Theoretical models are described to calculate both the electromagnetic field structure and the spatial impurity distribution, due to erosion of the antenna. The electron energy distribution function is determined with a standard RF-filtered electrostatic probe; it is found that the plasma is well described by a Druyvesteyn energy distribution function.

Plasma treatment of bulk niobium surface for superconducting rf cavities: Optimization of the experimental conditions on flat samples

Physical Review Special Topics - Accelerators and Beams, 2010

Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate nonsuperconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk niobium (Nb) presents an alternative surface preparation method to the commonly used buffered chemical polishing and electropolishing methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on flat samples. We have achieved an etching rate of 1:7 m= min using only 3% chlorine in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the sample surface to the environment. We intend to apply the optimized experimental conditions to the preparation of single cell cavities, pursuing the improvement of their rf performance.

OPTIMIZATION OF PLASMA PARAMETERS FOR ETCHING OF SUPERCONDUCTING RADIO FREQUENCY CAVITY SURFACE IN Ar/Cl 2 PLASMA

We are pursuing the development of low cost environmentally friendly dry etching of superconducting radio frequency (SRF) cavities in Ar/Cl 2 discharges. It has been proven with flat samples that the bulk Niobium (Nb) removal rate and the surface roughness after plasma etchings are equal to or better than wet etching processes. The plasma properties inside the single cell SRF cavity depend on frequency, pressure and power. To understand the plasma properties and chemical kinetics of the plasma etching process inside the single cell cavity, we are using a single cell cavity with 20 sample holders symmetrically distributed over the cell. These holders are being used for niobium coupon etching as well as diagnostic ports for optical measurements. Multiple optical probes with optical fibers have been utilized for optical emission spectroscopy measurements. A power supply in the radio frequency regime (100 MHz) and another power supply in the microwave frequency regime (2.45 GHz) are use...

Apparatus and method for plasma processing of SRF cavities

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2016

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring-and disk-type samples and the etch rate of these samples was measured.

UWAVS first mirror plasma cleaning technology using 30–60 MHz RF discharges

Fusion Engineering and Design, 2018

In ITER, first mirrors for the Upper Wide Angle Viewing System (UWAVS) are expected to experience contamination with beryllium and tungsten. Contamination levels of 10 nm can degrade mirror performance. In the UWAVS, a first mirror cleaning system will use radio-frequency (RF) gas discharge. A mockup in representative geometry was developed. The tests focused on power delivery, hardware survivability, cleaning uniformity, and removal of contaminants. Using RF discharges without introducing surface damage was confirmed. Sputtering was investigated for 10-20 nm thick Al-coatings used as proxies for Be due to health and safety issues. Helium was chosen for many tests due to its effectiveness and suitability for ITER operations. The most effective cleaning was obtained in Helium at 5 Pa, 36-43 MHz, and 50-150 W. The sputtering conditions were determined through ion energy measurements. Ion energies of 100-150 eV and currents of 0.5-0.8 A/m 2 were found to be optimum. The measured cleaning rate in He for Al-and Al-oxide was 1.5-3 nm/hour. Aluminum was used as a proxy material for Be. The setup was operated for 500 h. No significant chamber contamination was identified.

Plasma treatment of bulk niobium surface for SRF cavities

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2006

Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate non-superconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk Nb presents an alternative surface preparation method to the commonly used BCP and EP methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples. We have achieved etching rate of 1.7 μm/min using only 3% Cl 2 in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the fresh sample surface to the environment. We will apply the optimized experimental conditions to the preparation of some single cell cavities, in pursuing improvement of their RF performance.

Cryogenic rf test of the first plasma etched SRF cavity

arXiv (Cornell University), 2016

Plasma etching has a potential to be an alternative processing technology for superconducting radio frequency (SRF) cavities. An apparatus and a method are developed for plasma etching of the inner surfaces of SRF cavities. To test the effect of the plasma etching on the cavity rf performance, a 1497 MHz single cell SRF cavity is used. The single cell cavity is mechanically polished, buffer chemically etched afterwards and rf tested at cryogenic temperatures for a baseline test. This cavity is then plasma processed. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise manner to establish segmented plasma processing. The cavity is rf tested afterwards at cryogenic temperatures. The rf test and surface condition results are presented.

Decontamination Process Using CF 4 -O 2 Microwave Discharge Plasma at Atmospheric Pressure

Journal of Nuclear Science and Technology, 2000

Application to decontamination is proposed of microwave discharge plasma in carbon tetrafluoride and oxygen mixture at atmospheric pressure. Preliminary cold experiments were conducted using cobalt oxide deposited on stainless steel plate. Fluorine atoms proved to be generated in high concentration in the plasma, which fluorinated the deposited metal oxide into volatile gaseous form. Upon plasma irradiation during about 200 s, the cobalt oxide deposit was removed to 100% from plate surface. Thermodynamic equilibrium calculations on CF 4-02 systems containing cobalt served to elucidate the mechanism of metal oxide removal by microwave discharge plasma, and similar calculations on systems containing actinides (U, Pu etc.) and lanthanides (La, Gd etc.) indicated that the process should be applicable also to the removal of these radioactive elements. It is concluded from the experimental and calculated results that decontamination is basically feasible with the proposed method of using microwave discharge plasma in a CF4-02 system at atmospheric pressure.