In situ SRF cavity processing using optical ionization of gases

Abstract

A system and method for the in situ processing of internal SRF cavity surfaces to reduce field emission and improve maximum gradient. An electromagnetic radiation source is introduced in the bore of a superconducting cavity to enhance ionization or dissociation of gases which then remove contaminants from the surface of the cavity, either through direct surface bombardment, chemical reaction or through the production of radiation which interacts with the contaminants. An RF or low frequency electromagnetic field may be established in the cavity which further enhances the ionization or dissociation process and may cause the ions to bombard sites with enhanced electric fields. The invention removes the requirement that the RF field be sufficient by itself to ionize gas in the cavity.

Claims

1. A method for processing a superconducting radio-frequency (SRF) cavity structure to reduce field emission and improve maximum gradient, comprising: introducing gas into the structure; operating a vacuum pump to pull the gas through the structure; controlling the pressure of the gas in the structure to 10 to 1000 milliTorr; introducing radiation into the structure to ionize the gas, said radiation having a power density between 10 mW/cm.sup.2 and 1000 W/cm.sup.2; and establishing a radio frequency (RF) or low frequency electromagnetic field in the structure to enhance the ionization of the gas.

2. The method of claim 1 comprising reflecting the radiation back through the structure to further enhance ionization and dissociation of the gas.

3. The method of claim 1 comprising the radiation is selected from the group consisting of ultraviolet photon radiation and visible photon radiation.

4. The method of claim 1 comprising the radiation includes a wavelength less than 400 nm.

5. The method of claim 1 comprising the radiation includes a wavelength of 157 nm.

6. The method of claim 1 comprising exhausting a portion of the gas from the structure.

7. The method of claim 6 comprising measuring the concentration of carbon in the exhaust gas to monitor the efficacy of the ionization and ionization and dissociation process.

8. The method of claim 1 comprising the gas is a mixture of a higher weight noble gas and a reactive gas.

9. The method of claim 8 wherein the higher atomic weight noble gas is selected from the group consisting of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og).

10. The method of claim 8 wherein the reactive gas is selected from the group consisting of oxygen, argon fluoride, and argon chloride.

11. The method of claim 8 comprising the reactive gas is 0.2% to 99.9% of the gas mixture.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) Reference is made herein to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

(2) FIG. 1 is a schematic depicting in situ SRF cavity processing using optical ionization of gases according to the current invention.

DETAILED DESCRIPTION

(3) The present invention is a system and method to allow SRF cavities to have internal cavity surfaces processed in situ to reduce field emission and improve maximum gradient.

(4) The invention allows for SRF cavities to be processed at room temperature after assembly without disassembly of the cavity vacuum space. The invention involves the use of two or more flanges with gas inlets, pump ports to flow gas through the structure, and optical windows mounted outboard of the in-process structure's upstream and downstream valves. One of the flanges has a window, such as MgF2, LiF, quartz, or sapphire, transparent at the wavelength of the radiation used to ionize the gas, while the other flange has either a steerable mirror which allows the radiation to be retro reflected through the cavity, a radiation beam dump for the exiting radiation, or is transparent to the incident radiation, in which case an external beam dump may be necessary. In addition, the optics used may allow the radiation to be focused, which allows the radiation beam to be large at the optical window but go through a waist in the specific region of the structure being processed.

(5) As shown in FIG. 1, a system to achieve in situ SRF cavity processing using optical ionization of gases includes a structure 20 having an inlet 22 for gas, which may be filtered. The system preferably includes a throttling valve 25, an optical port 26, and potentially a second optical port 28. A radiation source 30 includes a high power density and a wavelength short enough to ionize the gas. The term high power density as used herein means power densities between 10 mW/cm.sup.2 to 1000 W/cm.sup.2. The term wavelength short enough to ionize the gas as used herein means wavelengths below 400 nm. The radiation source 30 may be an excimer pulsed laser using a fluorine system at 157 nm, which would be compact, but other gases and radiation sources would also work. The optical port 28 may include a mirror 34 which reflects the electromagnetic radiation and provides a means for monitoring the progress of the in situ process. The gas flow exits the structure through the pump out port 24. A vacuum pump 36 may include a valve 38 on its inlet to enable further throttling of the gas flow rate through the structure in order to control the pressure in the structure 20 during the cavity processing. Additionally, radio frequency or low frequency electromagnetic fields may be applied inside the cavity through one or more ports 40 to enhance ionization and dissociation of gases or the cavity cleaning process.

(6) The in situ system of the present invention allows the structure to remain semiconductor grade clean by placing a set of clean optical elements outside the structure gate valves and then pumping those out before the structure valves are opened. All hardware used in the cleaning process is external to the structure gate valves.

(7) As an example, a structure having a 10m length is subjected to in situ refurbishing according to the invention. Vacuum tees are installed on the structure being processed according to ISO 5 standards. One of the tees is attached to a clean vacuum pump to allow gas to be pumped through the structure. The gas used to process the cavity is a mixture of a higher atomic weight noble gas, such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), or oganesson (Og), with a small percentage of a potentially reactive gas such as O.sub.2 a fraction of which is dissociated in the plasma forming reactive atomic and ionic oxygen atoms. The gas is preferably filtered and introduced into the module. Flow is controlled using a mass flow controller or other variable valve assembly. The vacuum pump has a valve on its inlet to allow the gas flow rate through the structure to be throttled and in order to control the pressure in the structure during the process. The radiation source is attenuated and the optical path of the radiation source may be adjustable so that, for example, it is kept on the centerline of the structure. In this example the pressure in the cavity will be maintained between 10 to 1000 milliTorr (mT). The noble gas may include a reactive gas such as O.sub.2, ArF.sub.2, and ArCl. As an example in which the reactive gas is O.sub.2, the reactive O.sub.2 preferably comprises 0.2% to 99.9% of the noble gas/reactive gas mixture.

(8) The photoionization cross section for O.sub.2 is about 110.sup.8 at 6.3 eV. At 100 mT pressure and room temperature, and assuming the gas used behaves according to the ideal gas law, the number of moles in the 6 by 3 mm path of the optical radiation projected down the 10 meter length of the structure, can be calculated by n=PV/RT, or about 110.sup.3 moles, where P is the pressure, V is the volume, R is Avogadro's number and a T is the absolute temperature.

(9) Multiplying by Avogadro's number, 6.0210.sup.23, to get the number of molecules, about 610.sup.20 molecules are in the interaction volume. A 10 watt source at 150 nm would produce about 110.sup.19 photons. Multiplying this by the cross section, the number of ions produced is about 610.sup.11 per second. This number of ions is sufficient to couple RF coming through the coupler into the cavity. It also will allow the ions produced to back-bombard cavity surface imperfections which have enhanced electric field. In addition the oxygen ions scavenge carbon and hydrocarbons from the Nb surfaces which has the effect of increasing the surface work function. This is only one example of a possible combination of gas and radiation source, but represents many other possible combinations described by the invention. The efficacy of the process is monitored by measuring the concentration of carbon or other species in the exhaust gas either spectroscopically or by a mass spectrometer, such as a residual gas analyzer (RGA).

(10) The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.