J. Graber

Microscopic investigation of high gradient superconducting cavities after reduction of field emission

Abstract In the previous companion paper we showed that high power RF processing (HPP) is an effective technique to reduce field emission in superconducting cavities, so higher accelerating gradients can be reached. In this work we show improved understanding of the mechanisms at work when field emitters process. Thermometry measurements of the outer wall of single-cell cavities reveal the field emission from localized sites and also the reduction in field emission by processing. Subsequent scanning electron microscope (SEM) examination of the RF surface at the emission/processed sites reveals 5–10 μm sized molten craters, micron sized molten particles of foreign elements, and sub-mm sized spots shaped like starbursts. These features indicate that processing occurs through a violent melting/vaporization phenomenon. A “model” for RF processing is presented based upon the experimental evidence, both from this study and from others.

Reduction of field emission in superconducting cavities with high power pulsed RF

Abstract A systematic study is presented of the effects of pulsed high power RF processing (HPP) as a method of reducing field emission (FE) in superconducting radio frequency (SRF) cavities to reach higher accelerating gradients for future particle accelerators. The processing apparatus was built to provide up to 150 kW peak RF power to 3 GHz cavities, for pulse lengths from 200 μs to 1 ms. Single-cell and nine-cell cavities were tested extensively. The thermal conductivity of the niobium for these cavities was made as high as possible to ensure stability against thermal breakdown of superconductivity. HPP proves to be a highly successful method of reducing FE loading in nine-cell SRF cavities. Attainable continuous wave (CW) fields increase by as much as 80% from their pre-HPP limits. The CW accelerating field achieved with nine-cell cavities improved from 8–15 MV/m with HPP to 14–20 MV/m. The benefits are stable with subsequent exposure to dust-free air. More importantly, HPP also proves effective against new field emission subsequently introduced by cold and warm vacuum “accidents” which admitted “dirty” air into the cavities. Clear correlations are obtained linking FE reduction with the maximum surface electric field attained during processing. In single cells the maximums reached were E peak = 72 MV/m and H peak = 1660 Oe. Thermal breakdown, initiated by accompanying high surface magnetic fields is the dominant limitation on the attainable fields for pulsed processing, as well as for final CW and long pulse operation. To prove that the surface magnetic field rather than the surface electric fields is the limitation to HPP effectiveness, a special two-cell cavity with a reduced magnetic to electric field ratio is successfully tested. During HPP, pulsed fields reach E peak = 113 MV/m ( H peak = 1600 Oe) and subsequent CW low power measurement reached E peak = 100 MV/m, the highest CW field ever measured in a superconducting accelerator cavity.

High peak power RF processing studies of 3 GHz niobium cavities

The effects and benefits of high peak power RF processing as a means of reducing field emission loading in 3-GHz niobium accelerator cavities are being investigated. The test apparatus includes 3-GHz klystron capable of delivering RF pulses of up to 200-kW peak power with pulse lengths up to 2.5 ms at a repetition rate of approximately 1 Hz. The test apparatus has variable coupling such that the input external Q varies between 10/sup 5/ and 10/sup 10/ without breaking the cavity vacuum. Low-power, continuous-wave (CW) tests before and after HPP show that HPP is effective in removing emissions which are unaffected by low-power RF processing. CW measurements show that field emission reduction is dependent on maximum field reached during HPP. HPP fields of E/sub peak/ = 70-72 MV/m have been attained. These tests showed FE elimination to E/sub peak/ = 40 MV/m, and maximum fields of E/sub peak/=50-55 MV/m. Temperature mapping is now available. A cavity which showed strong FE loading and had extensive temperature mapping is now being investigated in a scanning electron microscope. A nine-cell cavity has been successfully tested, and, through HPP, reached E/sub acc/=15 MV/m, with Q/sub 0/=6.0*10/sup 9/.<<ETX>>

Higher order mode RE power extraction from polarized cavities with a single output coupler

An important step in simplifying superconducting accelerator structures is to reduce the number and complexity of the couplers. The use of azimuthal shaping to polarize the higher-order transverse modes in superconducting cavities has previously been proposed to couple both polarizations through a single output. Two questions remained: would such structures multipactor? and would both polarizations be adequately damped by a single coupler? To answer these questions, single-cell and multicell polarized S-band structures have been designed, constructed, and tested. Cold tests have shown that single-cell and three-cell structures do not multipactor. The highest surface field level achieved was 31 Mv/m, limited by field emission as with nonpolarized cavities.<<ETX>>

A world record accelerating gradient in a niobium superconducting accelerator cavity

A two-cell, 3 GHz, niobium superconducting accelerator cavity has sustained a continuous wave (CW) accelerating gradient of 34.6 MV/m, with a corresponding peak surface electric field of 100 MV/m, record performances in each category for a superconducting accelerator cavity. Field emission (FE) loading of the cavity initially limited the cavity to E/sub acc/=21 MV/m (E/sub peak/=60 MV/m). The record field was achieved by reducing the FE loading through high peak power (HPP) RF processing of the cavity. Analysis of previous results of the HPP experimental program indicated that maximum fields under both pulsed and CW conditions were limited by thermal breakdown, which is related to the surface magnetic field in the cavity. The two-cell cavity shape was chosen to bypass the thermal breakdown limitations by reducing the ratio of peak surface magnetic field to peak surface electric field, from a value of H/sub peakE/sub peak/=23 Oe/(MV/m) in the previous cavity, to 14 Oe/(MV/m) in the two-cell cavity. A simple thermal model accurately simulates the pulsed breakdown.<<ETX>>

Field emission processing of superconducting RF cavities with high peak power

The main difficulty standing in the way of using superconducting RF cavities for teraelectronvolt linear colliders is that, due to field emission, accelerating gradients are generally limited to less than 20 MV/m. One technique which is commonly used to decrease field emission is the use of RF processing in the presence of helium gas. In order to push this technique to yield even higher fields, a facility is being developed to process niobium S-band structures up to 200-k peak incident power for pulse widths up to 2 ms. This system is described and preliminary results are reported. The klystron transmitter system is complete and has been operated with pulsed DC current to a peak power level of 400 kW, average power level of 2 kW, and a pulse width of 2 ms.<<ETX>>

Superconducting RF linear collider

A simulation is used to investigate the dependence of stability thresholds on the HOM (higher-order-mode) damping. The emittance growth and energy spread due to high-Q parasitic modes is computed as a function of bunch charge, loaded Q, and mode bandwidth. The HOM loading typical of a five-cell cavity with a pair of beam tube couplers is considered. It is found that a beam with bunch charge of 2*10/sup 10/ particles and bunch spacing of 100 ns accelerated to high energy in such a structure exhibits tolerable emittance growth. It is suggested that a 3-GHz superconducting linac can be incorporated very simply into a design for a high energy linear collider.<<ETX>>

An update on high peak power (HPP) RF processing of 3 GHz nine-cell niobium accelerator cavities

Two 3 GHz, nine-cell niobium accelerator structures have been fabricated and tested multiple times. An unambiguous improvement in cavity performance can be shown due to high peak power (HPP) RF processing of the cavities. The average achieved accelerating gradient prior to HPP processing was E/sub acc/=12 MV/m, (standard deviation=3 MV/m). The average maximum accelerating gradient following all HPP processing was E/sub acc/=17 MV/m, (standard deviation=2 MV/m). Gains in cavity performance can be directly correlated with magnitude of field reached during pulsed HPP processing. Durability of processing gains has been tested by exposing processed cavities to filtered air, at room temperature, and unfiltered air, under both room temperature and cryogenic conditions. Filtered air had no discernable effect on cavity performance. Unfiltered air degraded cavity performance, through increased emission, however much of the cavity performance could be regained through further RF processing.<<ETX>>

Microscopic investigation of RF surfaces of 3 GHz niobium accelerator cavities following RF processing

RF processing of superconducting accelerating cavities is achieved through a change in the electron field emission (FE) characteristics of the RF surface. We have examined the RF surfaces of several single-cell 3 GHz cavities, following RF processing, in a scanning electron microscope (SEM). The RF processing sessions included both high peak power (P/spl les/50 kW) pulsed processing, and low power (/spl les/20 W) continuous wave processing. The experimental apparatus also included a thermometer array on the cavity outer wall, allowing temperature maps to characterize the emission before and after RF processing gains. Multiple sites have been located in cavities which showed improvements in cavity behavior due to RF processing. Several SEM-located sites can be correlated with changes in thermometer signals, indicating a direct relationship between the surface site and emission reduction due to RF processing. Information gained from the SEM investigations and thermometry are used to enhance the theoretical model of RF processing.<<ETX>>

Status and outlook for high power processing of 1.3 GHz TESLA multicell cavities

In order to increase the usable accelerating gradient in Superconducting TESLA cavities, the field emission threshold barrier must be raised. As has been previously demonstrated on S-Band cavities, a way to accomplish this is with the use of high peak power RF processing. A transmitter with a peak power of 2 MWatt and 300 /spl mu/sec pulse length has been assembled and has been used to process TESLA cavities. Several five cell TESLA cavities at 1.3 GHz have been manufactured for this purpose. This transmitter and the cavities will be described and the results of the tests will be presented.<<ETX>>