Heavy-Ion-Induced Electronic Desorption of Gas from Metals

Abstract

During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ioninduced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE e =dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering. DOI: 10.1103/PhysRevLett.98.064801 PACS numbers: 41.75.Ak, 34.50.Dy, 79.20.Rf Energetic ions incident on matter sputter target material and also desorb gas from the target surface. The sputter and desorption yields (number of sputtered or desorbed particles per ion impact) are known to be linked to the energy loss of the projectile inside the target. Two energy loss regimes, nuclear and electronic, have been known for decades. An example for potassium ions impacting onto stainless steel, calculated with the SRIM code [1], is shown in Fig. 1. Here for low projectile energies the nuclear energy loss dominates and for higher energies the electronic energy loss dominates the total energy loss. Sputter and desorption yield measurements from 1 m thick targets in the regime of electronic energy loss have shown that both scale with the electronic energy loss dE e =dx n to the power of n 1-3. This was observed for targets of frozen gases [2 -5], for sputtering from micron-thick coatings of protein [6,7], and for desorption of nitrogen from a conductor (carbon) by 6-13 MeV=u ions (u is the nucleon mass) [8,9]. Electronic sputtering and desorption from 1 m thick targets is found to vary with the ion impact angle from normal, , as 1=cos m to the first or higher power of m [5,10].Our research was motivated by the copious gas desorption that results from lost heavy ions striking particle accelerator vacuum chambers leading to dynamic pressure rises which limit the beam intensity in a number of heavyion accelerators [11]. Related work dates back more than 30 years, when a vacuum instability in the Intersecting Storage Rings (ISR) at CERN was identified above a critical beam current [12]. Recent requirements for orders of magnitude increase in beam intensity have motivated our search for further understanding and mitigation mechanisms. In preparation for the heavy-ion program of the Large Hadron Collider at CERN, beam-loss induced molecular desorption was intensively studied in ultra-highvacuum chambers at CERN's Heavy-Ion Accelerator (LINAC 3) [13] and the Super Proton Synchrotron (SPS) [14]. Large effective desorption yields of up to 2 10 4 molecules=Pb 53 ion (4:2 MeV=u) and 3:7 10 4 molecules=In 49 ion (158 GeV=u) were measured for ions impacting under various angles [ 0 (perpendicular), 84.8 , 89.2 at LINAC 3, and 88.3 at the SPS] onto stainless steel samples which were chemically cleaned, 950 C vacuum fired, and in situ baked at...

Critical issues for high-brightness heavy-ion beams- prioritized

“…Measurements have shown that energetic heavy ions striking walls at any angle from normal to grazing incidence will desorb copious amounts of gas [3,27,28]. We have measured the velocity of desorbed gas, and found that it corresponds to wall-temperature hydrogen, which is consistent with residual gas measurements that hydrogen is the dominant desorbed species after an ion pulse.…”

“…Electron emission decreases by factors ~20 from grazing to normal incidence, whereas desorption scales as a fractional power of 1/cos, decreasing by factors 2-3 over the same range of angles [27,28]. Both electron emission and gas desorption are minimized for impact near 0°, i.e., normal incidence and scale with the electronic component of ion energy loss in matter dE e /dx [2,3]. Gas will cause charge exchange (K + → K 0 ) and ionization (K + → K ++ or higher ionization states); ions that change charge will hit the beam tube in a long accelerator.…”

“…3. Measured and modeled the scaling of gas desorption with the electronic component of ion energy loss in matter [3,4]. 4.…”

Cosmic-ray slowing down in molecular clouds: Effects of heavy nuclei

“…For desorption processes (i.e. release of material in the gas phase) a slope α of two has been reported (Molvik et al 2007;Dartois et al 2015). Figure 8 gives the material modification rates as a function of the column density for the two different values of α for the two initial CR spectrum, E −1 and SP68.…”

Self-consistent 3D modeling of electron cloud dynamics and beam response

“…Copious electrons are generated by slamming the beam onto the end plate. We study electron effects in the magnetic section, which is heavily instrumented with diagnostic devices and suppressor electrodes dedicated to the various measurements of electron flux, electron energy spectrum, and gas analysis [26][27][28][29][30][31][32]. The primary electrons created at the end plate propagate upstream, entering only two quadrants of the fourth (last) magnet because of the sign of E × B.…”