Improvement of the operational performance of SRF cavities via in situ helium processing and waveguide vacuum processing

E., Reece, C.; Drury, M.; Rao, M. G.; Nguyen-Tuong, V.

doi:10.1109/pac.1997.753123

Cited by 8 publications

(6 citation statements)

References 2 publications

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“…It is believed the krypton becomes ionized in the presence of field emission and then attracted to the field emitters, changing their geometrical shape from sharp to blunt via sputtering [29]. Helium inert gas processing is a common technique to eliminate field emission inside SRF cavities [30], but is not very effective at eliminating field emission inside DC high voltage photoguns [28]. It is believed the heavier krypton gas imparts more kinetic energy to the field emitter.…”

Section: Krypton Processingmentioning

confidence: 99%

Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

BastaniNejad

Mohamed

Elmustafa

et al. 2012

Phys. Rev. ST Accel. Beams

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The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirrorlike finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission (< 10 pA) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of 18:7 MV=m.

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Section: Krypton Processingmentioning

confidence: 99%

Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

BastaniNejad

Mohamed

Elmustafa

et al. 2012

Phys. Rev. ST Accel. Beams

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show abstract

“…Helium processing is a well-developed technique for SRF cavities over decades [13], and successful demonstrations of cavity performance improvement have been approved at various facilities [14,15].…”

Section: Helium Processingmentioning

confidence: 99%

Operation of the 56 MHz superconducting rf cavity in RHIC with higher order mode damper

Belomestnykh

Ben‐Zvi

et al. 2019

Phys. Rev. Accel. Beams

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A 56 MHz superconducting RF cavity was designed and installed in the Relativistic Heavy Ion Collider (RHIC). It is the first superconducting quarter wave resonator (QWR) operating in a highenergy storage ring. We discuss herein the cavity operation with Au+Au collisions, and with asymmetrical Au+He3 collisions. The cavity is a storage cavity, meaning that it becomes active only at the energy of experiment, after the acceleration cycle is completed. With the cavity at 300 kV, an improvement in luminosity was detected from direct measurements, and the bunch length has been reduced. The uniqueness of the QWR demands an innovative design of the higher order mode dampers with high-pass filters, and a distinctive fundamental mode damper that enables the cavity to be bypassed during the acceleration stage.

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“…The basic concept for plasma cleaning of surface contaminants is shown in Figure 4. The newly developed plasma processing technique for SRF cavities is very different than the Helium processing technique [3,41]. Plasma processing is done at room temperature using a chemically reactive plasma, with gas pressure around 100 mTorr and electric field level in the order of 0.01 MV/m.…”

Section: Insert Figure (Height~35" Width~33")mentioning

confidence: 99%

In-situ plasma processing to increase the accelerating gradients of superconducting radio-frequency cavities

Doléans

Tyagi

Afanador

et al. 2016

Nuclear Instruments and Methods in Physics Research Section A:

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A new in-situ plasma processing technique is being developed at the Spallation Neutron Source (SNS) to improve the performance of the cavities in operation. The technique utilizes a low-density reactive oxygen plasma at room-temperature to remove top-surface hydrocarbons. The plasma processing technique increases the work function of the cavity surface and reduces the overall amount of vacuum and electron activity during cavity operation; in particular it increases the field-emission onset, which enables cavity operation at higher accelerating gradients. Experimental evidence also suggests that the SEY of the Nb surface decreases after plasma processing which helps mitigating multipacting issues. In this article, the main developments and results from the plasma processing R&D are presented and experimental results for in-situ plasma processing of dressed cavities in the SNS horizontal test apparatus are discussed. 2. FIELD EMISSION AND END-GROUP THERMAL INSTABILITY LIMITING THE ACCELERATING GRADIENTS IN THE SNS LINAC Field emission in superconducting radio-frequency (SRF) cavities is a well-known limiting factor for operation at high accelerating gradients [1-3]. Beyond certain electric field thresholds, the electrons from the metal surface of the cavity have a non-negligible probability of tunneling out. The field emitted electrons are accelerated by the stored electromagnetic fields in the cavity and subsequently deposit their energy by collision with the cavity radio-frequency (RF) surface leading to vacuum activity, increase of the surface temperature and Bremsstrahlung radiation. If the deposited energy-density is larger than the cooling capacity it can also lead to thermal breakdown of the superconductivity.

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Improvement of the operational performance of SRF cavities via in situ helium processing and waveguide vacuum processing

Cited by 8 publications

References 2 publications

Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

Operation of the 56 MHz superconducting rf cavity in RHIC with higher order mode damper

In-situ plasma processing to increase the accelerating gradients of superconducting radio-frequency cavities

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