2: Historical Limitations on SRF Cavities and Their Solutions
SRF cavities have been in operation for over twenty-five years, and through that time many limitations have been encountered and overcome. We will mention several of the more important limitations here, along with the method in which they were overcome.
2.1: Multipacting
Multipacting, or Resonant Field Emission, was an early limitation on SRF cavities. In multipacting, electrons emitted from the RF surface into the cavity follow a trajectory such that they impact back at the surface of the cavity an integral number of RF cycles after emission. The impacting electron then frees further electrons which repeat the cycle causing an avalanche effect, until all available power goes into this process.
Multipacting was overcome by changing the cavity cross section from a rectangular to a spherical or elliptical shape, as was shown in
Figure 1. In the spherical shape, the fundamental mode has no electrical field at the equator region of the cavity. The key to elimination of multipacting as a limit, however, is that the fields are such that emitted electrons will drift towards the equator, eventually ending up in a region with zero surface electric field, thus stopping the avalanche effect, as is shown in Figure 3.
/www/pub/Home/Research/SRF/SrfCavitiesAPrimerTwo/SRFBas3.GIF
Figure 3 . Comparison of multipacting trajectories in rectangular and elliptically shaped RF cavities. In the rectangular cavity, the electrons return to essentially the same point from which they were emitted, where they can cause secondary emission, eventually cascading such that all available RF energy is absorbed. In the elliptical cavity, the emitted electrons drift towards the cavity equator, where the electric field is not strong enough for secondary emission to recur.
2.2: Thermal Breakdown
Thermal breakdown, or quench, is a phenomenon where the temperature of part or all of the RF surface exceeds the critical temperature, thereby becoming normal conducting and rapidly dissipating all stored energy in the cavity fields. Thermal breakdown is most often a localized effect, where a small "defect" in the RF surface dissipates power more rapidly than the surrounding superconducting walls. Breakdown occurs when the power dissipation overwhelms the ability of the surrounding metal to conduct away the heat. The field at which breakdown occurs is dependent upon multiple factors, including thermal conductivity of the bulk niobium, heat transfer from the niobium to liquid helium bath, and size and resistance of the defect.
The primary method of bypassing the thermal breakdown limitation has been inproving the thermal conductivity of the niobium. Improved thermal conductivity comes from improved purity of the metal. Material purity, and thus thermal conductivity, are described via the Residual Resistivity Ratio (
RRR), which is the ratio of the resistivity at room temperature to the normal conducting resistivity at 4.2 K. A more complete description of
RRR and its measurement can be found in Appendix D of the Ph.D. Dissertation of J. Graber. Bulk material purity has improved greatly (from
RRR*of 10-30 to *RRR ³ 250) in the last twenty years through improved purification methods, e.g. high vacuum electron beam melting.
[26]
Niobium can be further purified of interstitial oxygen by solid state gettering.
[27]-[29] In gettering the niobium is baked to 1400 degrees C, with exposure to either yttrium or titanium vapor. The vapor adsorbs to the surface of the niobium; the higher affinity of Y or Ti (compared to Nb) to oxygen effectively removes the oxygen from the bulk niobium. Following baking, the oxide layer at the outer surface is removed, leaving purified bulk niobium. Typically,
RRR = 250 material can be purified to
RRR = 500 through yttrification.
The earliest niobium SRF cavities were made of reactor grade material,
RRR Ã… 25, and experienced thermal breakdown with suface magnetic fields in the range
Hsurface = 200-400 Oe (corresponding to
Epeak = 10-20 MV/m in a typical SRF cavity). Improvement to
RRR = 250 raised breakdown fields to 800-900 Oe (
Epeak = 35-40 MV/m). Gettering improvements to
RRR = 500 have raised breakdown fields as high as 1200-1300 Oe (
Epeak = 50-60 MV/m), as we will show below.
2.3: Q Virus
The "
Q virus
[30][31]" is a recently discovered phenomenon, in which excessive hydrogen in high purity niobium can condense onto the RF surface of the cavity, forming a niobium hydride with poor superconducting characteristics. The
Q virus is characterized by an anomalously low cavity
Q (high surface resistance) at low electric field, followed by a rapid
Q decrease with increasing fields. Once this behavior has been identified, a vacuum bake to 900 degrees C is sufficient to remove the hydrogen from the niobium, while not damaging the cavity.
The hydrogen contamination can be avoided completely, however, by controlling the acid etch which is used to prepare a sufficiently clean surface for RF testing. Investigation of the
Q virus indicated that hydrogen contamination was a danger when the temperature of the niobium/acid region rises above 20-25 degrees C. This condition is avoided by controlling the rate of reaction, through acid temperature, volume of acid, and time of exposure. In extreme geometries, such as 3 GHz nine-cell cavities with a relatively high surface to volume ratio and small beam tube radius, this is not always sufficient, and the 900 degrees C bake is standard procedure.
