Secondary electron yield on cryogenic surfaces as a function of physisorbed gases

Abstract

In LHC the electron cloud induced by photoelectrons, gas ionization and secondary electrons emitted from the beam pipe walls could be a limitation of the performance. The electron cloud induce heat load on the cryogenic system, cause pressure rise, emittance growth and beam instabilities, which in the end will limit the beam"s lifetime. Beam-induced multipacting, which can arise through oscillatory motion of photoelectrons and low-energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive passage of proton bunches, represent therefore a potential problem for the machine.The secondary electron yield (SEY) is one of the key parameters for the electron cloud build up and multipacting phenomenon. An electron cloud occurs if the metal surface secondary electron yield is high enough for electron multiplication. This parameter has been extensively studied on room temperature samples but uncertainties remain for samples at cryogenic temperature. Indeed, at low surface temperature SEY is strongly dependent on the nature of the physisorbed gases and on the surface coverage.In this work the secondary electron yield (SEY) at cryogenic temperatures has been measured and the results are presented. Of particular interest is the variation of the SEY with the gas coverage as most gases especially CO, CO 2 , and CH 4 in case of LHC condense on the cryogenic parts of the machine. In addition to these gases measurements have been performed with N 2, because of its importance as calibration gas for most of the pressure gauges and pumps. Also measurements with Kr have been done, to compare the results with existing works. In order to acquire a better understanding about the behaviour of SEY as a function of different gas coverage, measurements of work function have been made. The measurements of the SEY at cryogenic surfaces require a UHV system with a sample holder which can be cooled down to cryogenic temperatures, a source of primary electrons and detection of sample current and secondary electron current. In addition a gas injection system is necessary to produce variable coverage with different gases. In the present system the sample (Cu, Al, electro-polished Cu, Nb and C) was cooled down to 4.7K, irradiated with an electron gun and currents were measured on the sample and on a collector electrode. Work function was measured with a Kelvin Probe. KURZFASSUNG

Optimization of the secondary electron yield of laser-structured copper surfaces at room and cryogenic temperature

“…After irradiation of the samples with doses of 6.1 × 10 −6 C mm −2 and up to 1.8 × 10 −5 C mm −2 (corresponding to 5 and 10 min of constant irradiation, respectively, similar to what done on the CH-patterned samples), a progressive reduction of the SEY values is observed which remains stable upon further measurements. This conditioning process might be ascribed to desorption of surface adsorbates [14]. The behavior of the sample treated in air is entirely different but similar to that already reported for CHpatterned samples.…”

“…As discussed in Ref. [14], this dose is well below the level of damage of the adsorbates and cannot produce any substantial surface modification. Stability of the SEY under electron bombardment ("conditioning") is investigated by continuous irradiation at an electron energy of 250 eV, the beam current being approximately doubled and the beam size increased from 7 to 11 mm 2 , in order to cover an area larger than the measurement spot.…”

“…SEY measurements at cryogenic temperatures were carried out in a separate custom-made system ("cold-SEY") described in detail in Ref. [14]. This system is an all-metal UHV system with an operating base pressure of 1 × 10 −9 mbar.…”

Plasma sputtering robotic device for in-situ thick coatings of long, small diameter vacuum tubes

“…Obtained results coupled with those of literature 10,[13][14][15][16] indicate that baked and scrubbed copper coating can achieve a SEY max of 1. For completeness, it should be pointed out that recently, it has been shown that well-scrubbed stainless steel, by electrons and positrons, 19 can have its SEY max reduced to 1.3.…”

Electron cloud in accelerators

“…27, where we report SEY as measured on a copper surface held at 4.7 K as a function of increasing CO coverage. 140 Already a single CO ML physisorbed on the Cu cold surface significantly modifies its SEY and additional CO deposition results in a completely different SEY value of about 1.2. This result is not only very important to confirm the extreme surface sensitivity of SEY but suggests that, when discussing SEY of cold surfaces (like for the LHC dipoles) great care must be taken in considering any eventual surface modification induced by gas physisorption.…”