Studies of Heavy Ion Induced Desorption in the Energy Range MeV/u

Transcription

1 Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 425 Studies of Heavy Ion Induced Desorption in the Energy Range MeV/u EMMA HEDLUND ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2008 ISSN ISBN urn:nbn:se:uu:diva-8654

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5 List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I A. W. Molvik, H. Kollmus, E. Mahner, M. Kireeff Covo, M. C. Bellachioma, M. Bender, F. M. Bieniosek, E. Hedlund, A. Krämer, J. Kwan, O. B. Malyshev, L. Prost, P. A. Seidl, G. Westenskow, L. Westerberg Heavy-Ion-Induced Electronic Desorption of Gas from Metals Physical Review Letters, 98: (2007) II E. Hedlund, L. Westerberg, O. B. Malyshev, M. Leandersson, C- J. Fridén, E. Edqvist, H. Kollmus, M. C. Bellachioma, H. Reich- Sprenger, A. Krasnov A new test stand for heavy ion induced gas desorption measurements at TSL Nucl. Instr. and Meth. in Phys. Res. A, 586: (2008) III E. Hedlund, L. Westerberg, O. B. Malyshev, E. Edqvist, M. Leandersson, H. Kollmus, M. C. Bellachioma, M. Bender, A. Krämer, H. Reich-Sprenger, B. Zajec, A. Krasnov Ar ion induced desorption yields at the energies MeV/u In manuscript IV H. Kollmus, A. Krämer, M. Bender, M. C. Bellachioma, H. Reich-Sprenger, E. Mahner, E. Hedlund, L. Westerberg, O. B. Malyshev, M. Leandersson, E. Edqvist Energy Scaling of the Ion-Induced Desorption Yield for Perpendicular Collisions of Ar and U with Stainless Steel in the Energy Range between 5 and 100 MeV/u In manuscript V VI VII E. Hedlund, O. B. Malyshev, L. Westerberg, A. Krasnov, M. Leandersson, B. Zajec, H. Kollmus, M. C. Bellachioma, M. Bender, A. Krämer, H. Reich-Sprenger Heavy-ion induced desorption of a TiZrV coated vacuum chamber bombarded with 5 MeV/u Ar 8+ beam at grazing incidence In manuscript E. Hedlund, L. R. Pendrill Improved determination of the gas flow rate for UHV and leak metrology with laser refractometry Meas. Sci. Technol., 17: (2006) E. Hedlund, L. R. Pendrill Addendum to Improved determination of the gas flow rate for UHV and leak metrology with laser refractometry Meas. Sci. Technol., 18: (2007) 5

6 Reprints were made with permission from the publishers. 6

7 Contents 1 Introduction Theory, Simulations and Calculations Throughput Method Gas Dynamics Simulation Programs MOLFLOW SRIM and TRIM Calculations of Secondary Particle Yield Energy Loss Models Coulomb Explosion Model Thermal Spike Model Pressure-Pulse Model Experimental Setups and Measurement Procedures GSI MeV/u with Uranium Beam MeV/u with Argon Beam TSL MeV/u with Argon Beam MeV/u with Argon Beam Calibration Laser Refractometry Interferometers Free Spectral Range The Planned Setup Results and Discussion The Uranium Beam The Argon Beam Material Dependence Geometry Dependence Energy Dependence Non-Evaporable Getter (NEG) Laser Refractometry Conclusions Populärvetenskaplig Sammanfattning Acknowledgements

8 Bibliography

9 1. Introduction A particle accelerator is a device that can increase the speed of charged particles to near the speed of light. The particles range from electrons and protons up to heavy ions depending on what field of interest to study. High-energy particle beams are necessary to recreate particles that occured in the first moments of the Big Bang. Other areas could be cancer therapy, and at lower energies e. g. materials analysis. There are three basic types of accelerators: linear accelerators, circular accelerators and storage rings. There is a need to have as good vacuum as possible inside the particle accelerators to achieve the required beam life times: the lower the pressure the longer the beam life time. A typical pressure in heavy-ion particle accelerators is around mbar, which is in the ultra-high vacuum (UHV) region. The difficulties in creating a good and stable vacuum, and one of the reasons to why it is not possible to achieve an ideal vacuum, are connected with outgassing [1, 2]. The source of gas in a UHV system is the gas that desorbs from the walls of the vacuum chamber, mainly H 2, CO, CO 2 and CH 4. The gas can diffuse either from the bulk to the surface of the vacuum chamber or by permeation from the outside to the bulk. It is therefore important that materials used in particle accelerators have low outgassing among other demands. The most common materials used for vacuum chambers are stainless steel, aluminium and copper but could also be glass, ceramics and other materials. Improving the vacuum by increasing the pumping speed has very limited effects and is very costly. A better way is to apply methods to reduce the outgassing. The most common procedure is to heat up the vacuum system to up to 300 C for 24 hours [3]. This will speed up the diffusion process in the bulk. An even better method is to heat the vacuum chambers to about 950 C in a vacuum oven for 2 hours, a so called vacuum firing, and then mount the vacuum chamber and perform the usual baking which will reduce the outgassing even more [4]. Another way to reduce the outgassing is simply to add a diffusion barrier [5]. A reasonably new method is to add a non-evaporable getter (NEG) coating, usually made of TiZrV, on the vacuum chamber wall. This coating will work as a barrier like the previous coating, but it will also work as a pump, i. e. it will transform an outgassing surface to a pumping surface [6, 7, 8]. After the vacuum chambers have been baked there will be a low static outgassing and the pressure should be in the low UHV region. However, when the particle beam is present there will be additional outgassing in the form of dynamic outgassing. This is due to the fact that beam 9

10 ions change their charge state when they collide with the residual gas in the beampipe. Since the change in charge state will affect the bending radius of the particles, they will not follow the required trajectory after they have passed a bending magnet, but instead collide with the vacuum chamber wall, see Figure1.1. The corresponding gas released, so called ion induced desorption, will cause an increase of the pressure which will affect the beam lifetime, in some cases there could be a dramatic effect on the beam lifetime [9, 10]. Figure 1.1: Ion induced desorption due to collisions with residual gas. Picture from H. Kollmus and M. Bender. A new generation of anti proton and ion research facilities (FAIR) is presently under design and development at GSI (Gesellschaft für Schwerionenforschung) in Darmstadt, Germany [11]. The already existing heavy ion synchrotron, SIS18, will serve as injector for the new superconducting double ring synchrotron SIS100/300, see Figure 1.2. The SIS100/300, with a circumference of 1100 m, will provide ion beams with a gain in energy of about a factor 15 and a considerable gain in intensity of up to a factor of 1000 for primary beams and up to a factor of for secondary beams. In order to achieve these requirements, SIS18 has to be upgraded to meet the design value of U 28+ ions/s extracted at the injection energy of about 10 MeV/u. Experiments performed in 2001 at SIS18 showed that the beam lifetime of the ions in the synchrotron was decreasing with increasing number of injected particles due to vacuum instabilities caused by ion-induced desorption. Tho achieve these goals it is therefore of interest to investigate ion-induced desorption yields from different materials used for vacuum chamber components in order to choose the most suitable one. Measurements of ion induced desorption coefficients is the major subject for this thesis. Desorption yield measurements as well as a wide variety of vacuum-based experiments and industrial processes rely on accurate absolute calibration of vacuum gauges. For such calibrations it is necessary to have access to very accurate flow meters. A feasibility study on how to improve the 10

11 Figure 1.2: Layout of the already existing facility at GSI (to the left) together with the planned FAIR facility (to the right). accuracy for such a flowmeter by the use of laser refractometry is described in the later part of this thesis. 11

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13 2. Theory, Simulations and Calculations This chapter covers methods and simulation programs for deduction of desorption coefficients as well as relevant theories for description of the energy dependence of ion induced desorption coefficients. 2.1 Throughput Method There are different methods to measure desorption yield. A system often used in order to generate accurate pressures in the vacuum region (and to calibrate vacuum gauges), is the throughput system based on the continuous expansion method [12]. The throughput system consists of two vacuum chambers connected by a conductance, see Figure 2.1. The rather small conductance limits the possible pumping speed, and therefore the pressures in the two chambers could easily differ by an order of magnitude, with P1 > P2. Figure 2.1: The throughput system. The gas flow, Q, generated from the samples in chamber P1 to the chamber P2 could be described as Q = ΔP U (2.1) ΔP =(P1 dyn P1 bg ) (P2 dyn P2 bg ) (2.2) where U is the conductance and ΔP is the pressure increase calculated from the dynamic pressures when the beam is on (indexed dyn ) and the background pressures (indexed bg ) obtained in chamber P1 and chamber P2. The ion induced desorption, η [molecules/s], is defined as η = ΔP U Γk B T (2.3) 13

14 where Γ [ion/s] is the ion flux, k B [J/K]is the Boltzmann constant and T [K] is the temperature of the gas. 2.2 Gas Dynamics It is more complicated to obtain the desorption coefficients from a Non-Evaporable Getter (NEG) coated vacuum system, where the pumping capacity of the coating has to be taken into account. The gas diffusion model can be applied to convert the measured pressures into desorption yields. The equations based on the model described in [13], are derived for our experimental setup as it is shown in Figure 2.2 for two cases: for a non-pumping NEG film when the sticking probability is equal to zero (α = 0), and for a pumping NEG film when the sticking probability is>0(α > 0). Figure 2.2: The layout of the NEG tube followed by two different conductances and the test chamber for the diffusion model. Pressure P NEG is measured by a Residual Gas Analyzer (RGA) which is placed at distance x 1 from the end flange. For α = 0 we have the following expression for the pressure: P(x)= q 2u x2 +C 1 x +C 2 (2.4) where q [mbar m 2 /s] is the gas flow per unit axial length, u = A C D[m 4 /s] is the specific vacuum chamber conductance per unit axial length, A C [m 2 ]isthe vacuum chamber cross section; D [m 2 /s] is the Knudsen diffusion coefficient, x [m] is the position for the desired pressure, C 1 and C 2 are constants, U 2 = A 2 D 2 x 3 x 2 and U 3 = A 3D 3 x 4 x 3 are the vacuum chamber conductances of the connecting tubes, x i and U i positions are given in Figure 2.2. Boundary conditions give: Forx=0: P(0)=C 2 (2.5) 14 dp(0) dx = C 1 = 0 (2.6)

15 Forx=x 2 : P(x 2 )= q 2u x2 2 +C 2 (2.7) dp(x) dx The gasflow is assumed to be equal at x 2,x 3 and x 3 : with P(x 2 ) dx 2 = q u x 2 (2.8) = P(x 2) P(x 3 ) x 2 x 3 (2.9) q 2 = q 3 = q 4 (P(x 2 ) P(x 3 ))U 2 =(P(x 2 ) P TC )U ef f (2.10) dp(x) dx U ef f = U 2U 3 U 2 +U 3 (2.11) = P(x 2) P TC x 2 x 3 Inserting P 2 from 2.7 gives: C 2 = q ( u x 2 (x 2 x 3 ) U 2 x ) 2 U ef f 2 + P TC Inserting C 2 in 2.4 gives the following flow at x 1 : U ef f U 2 = q u x 2 (2.12) (2.13) u(p TC P(x 1 )) q = 1 2 (x2 1 x2 2 )+x 2(x 2 x 3 ) (U (2.14) 2+U 3 ) U 3 The desorption yield is then obtained from η = q Γk B T (2.15) where Γ [ions/(s m)] is the ion intensity per unit axial length, k B [J/K] is Boltzmann s constant and T [K] is the temperature. With P(x 1 )=P NEG we have: η = 1 Γk B T u(p TC P NEG ) 1 2 (x2 1 x2 2 )+x 2(x 2 x 3 ) (U (2.16) 2+U 3 ) U 3 For α > 0 there is a different expression for the pressure: with P(x)= q αc +C 1e ax +C 2 e ax (2.17) a = αc u (2.18) 15

16 where α is the sticking probability, C = A v/4 [m2/s] is the ideal wall pumping speed per unit axial length, v [m/s] is the mean molecular speed, A [m] is the vacuum chamber wall area per unit axial length, C 1 and C 2 are constants (different from the ones obtained above). By solving the boundary conditions and following the procedure above, the final equation for the desorption yield is: η = αc Γk B T ( ) P a NEG U 3 (x 2 x 3 )(U 2 +U 3 )tanh(ax 2 ) 1 + P TC cosh(ax 1 ) cosh(ax 2 ) a U 3 (x 2 x 3 )(U 2 +U 3 )tanh(ax 2 ) 1 + cosh(ax 1) cosh(ax 2 ) (2.19) 2.3 Simulation Programs The simulation programs listed below were used for the following purpose: MOLFLOW for making a prototype of the experimental setup and modelling of the gasflow. The SRIM program was used for calculations of the energy loss for the used ions and target materials. The TRIM program was used to dimension the target thicknesses MOLFLOW It is of great importance to have the ability to check the vacuum performance of a vacuum system without the need to make a prototype. This could be done with MOLFLOW (stands for MOLecular FLOW), which is a software program used for simulations of pressure distributions in vacuum systems [14]. The program, which is freely available, is written in Turbo Pascal by Roberto Kersevan. In the molecular flow regime, where these calculations are valid, the gas molecules only interact with the walls of the vacuum system, there are no interactions between molecules. Therefore the molecules can be treated independently. MOLFLOW consists of two linked programs: MOLFLOWE, which is an editor program and MOLFLOWR which is the simulation program, based on the well known Test Particle Monte Carlo (TPMC) statistical method. The Editor Program (MOLFLOWE) The structure to be analyzed is created in MOLFLOWE by a collection of points and facets. A point is defined by its (x i,y i,z i ) Cartesian coordinates and a facet is defined by a number of points which occupy the vertices of the chosen polygon. It is important that the points defining the facet all lie in the same plane. Each facet is given properties like e.g. sticking coefficient, reflectivity, transparency, desorption and orientation. The total number of facets to define a structure is limited to 80. If more facets are needed, one could divide the structure into different files, which later can be connected in the simulation 16

17 part. All information about the points and the facets are stored in a data base in MOLFLOWE. The editor program is divided into a screen session, used for data input, and a graphic session where the structure can be visualized, see for exampel the prototype of the experimental system at The Svedberg Laboratory in Figure 2.3. The Simulation Program (MOLFLOWR) In MOLFLOWR it is possible to combine up to 15 structures. The number of generated molecules has to be defined before the run. It could be anything from a few thousands of molecules for simple structures up to one million for more complex ones. The Monte-Carlo program begins by generating a particle from a desorbing facet, with a random generator for the directions and positions, to a surface. This molecule is traced on its way hitting a number of facets until it is pumped or enters another structure. Once the molecule is lost from the system, a new molecule will be generated randomly and so on until the specified number of molecules has been generated. The number and position of the interaction of the generated molecules with an imaginary facet will give the pressure profile in the system. The data from the simulation can be accessed from the data base in MOLFLOWE. Figure 2.3: The prototype of the experimental setup at TSL made in Molflow SRIM and TRIM The SRIM (The Stopping and Range of Ions in Matter) program simulates the stopping power and range of ions (with energy up to 2 GeV/u) penetrating into solids [15]. The stopping power, de/dx, is defined as the energy loss per unit distance x. The program is based on statistical algorithms together with an expansion of the Bethe-Bloch stopping power formula for high energy ions: 17

18 de dx = 4πr2 0 m ec 2 ( Z 2 β 2 Z 1 f (β) ln I C δ ) (2.20) Z 2 2 where r 0 is the electron radius, m e is the rest mass of the electron, c is the speed of light, Z 1 and Z 2 is the atomic number of the projectile ions and the target atoms respectively, β is v/c where v is the speed of the projectile ions, f (β) combines the relativistic terms from the original Bethe-Block formula, <I> is the mean ionization potential (i.e. the average excitation potential per electron in the target), C/Z 2 is denoted as the shell correction term and corrects for the electron velocity distribution in the target, δ/2 is a minor correction term. For more details, see [16]. The accuracy of SRIM is 6.1% for heavy ions [15]. The TRIM (The Transport of Ions in Matter) program can simulate more complex targets [15]. It is possible to choose a compound with up to eight layers where each layer could be a different material. 2.4 Calculations of Secondary Particle Yield The amount of secondary particles generated from a sample surface during the bombardment of heavy ions can be calculated as follows. The current measured from the sample is a superposition of all currents at the sample. The beam current, I b, hit the sample and cause the secondary charged particles currents: the outgoing current of positively charged particles, I +, and the outgoing current of negatively charged particles, I. The backscattered beam is believed to be much lower than the impinging one. The net current I m measured from the sample is then I m = I b I + + I (2.21) By biasing the sample negatively, the number of electrons emitted from the surface during ion impact can be a 1 times greater than without the bias depending on the surface state. The same could be valid for positively charged particles with multiplication coefficient a 2. Hence, from equation 2.21 we get the following equations: I m+ = I b a 2 I + (2.22) I m = I b + a 1 I (2.23) where I m+ and I m are the current measured with a positively and negatively biased sample. A plot of the measured currents as a function of the biasing will show a hysteresis curve where I m+ and I m both will reach a saturation level. The saturated values should be used to obtain the secondary particle yield from the following equations. 18

19 The measured current in our setup is known to show 3.3 times larger value than the beam current, hence: By combining 2.21 and 2.24 we can obtain I m0 = 3.3I b (2.24) I I + = 2.3I b (2.25) It is clear from here that I 2.3I b. Now, the upper limit for a 1 can be calculated with formula The coefficients a 1 and a 2 are coupled as following. By combining 2.21, 2.22 and 2.23 one can obtain: I m I b a 1 = I m0 + I m+ I b (2.26) a 2 I b It is clear that a 1 1 and a 2 1. Assuming that a 2 = 1, one can find the lower limit for a1 by using formula Unfortunately it was not possible to calculate an upper limit for a 2. Having I,I + and I b it is possible to estimate the secondary electron yield (SEY) and ion yield (SIY) per incoming ion as the following: where q is the charge of the ioncoming ion. SEY = I q I b (2.27) SIY = I+ q I b (2.28) 2.5 Energy Loss Models When a solid target is hit by a heavy ion, atoms or molecules are released from the surface. The desorption mechanism could be explained by the energy loss of the projectile ion in form of nuclear and electronic stopping [17]: de dx = ( de dx ) + n ( de dx ) e (2.29) Whether the energy loss is dominant by nuclear or electronic stopping depends of the velocity of the heavy ion [18]. If the velocity is below the Bohr velocity (0.22 cm/ns), the electrons in the target are moving much faster than the projectile ion. There will mainly be an elastic collision between the heavy ion and the target atom, and then the energy loss is due to nuclear scattering. The projectile nucleus can also interact inelastically with a target atom, but the main interaction will be elastic scattering. Above the Bohr velocity the energy loss is dominated by electronic stopping. Due to the high velocity of the 19

20 projectile ion the collision between the incoming ion and the target electrons will be inelastic. The transformed kinetic energy will either excite or ionize the atom. In our experiment the velocity of the heavy ions are well above the Bohr velocity, and hence the energy loss is mainly due to electronic stopping. In the electronic stopping region there are several theories on how the energy is deposited and the mechanisms that will lead to desorption. The three main models describing the process of sputtering yield are the Coulomb explosion model, the thermal spike model and the pressure-pulse model, all found in the references [19, 20]. The models predict different values of the exponent n in the energy loss dependence of the desorption yield as η (de/dx) n, where n ranges from Coulomb Explosion Model When the projectile ion hits the target it could lead to extremely high deposited energy densities [19, 21]. This could lead to ejection of δ-electrons, leaving a positive ion core formed along the ion track. Due to Coulomb repulsion, the ion core will explode and in turn generate a low energy atomic collision cascade. There are different theories on what will happen after the collision cascade. One theory is that the explosion heats up the solid and therefore desorption is due to thermal evaporation [22, 23]. In another theory it is assumed that the cascade of particles resulting from the collision will form a radially moving shockwave, and when the wave is impacting on the surface, materials will be released [22, 23]. Coulomb explosion is likely in insulators, where electrons are rare. The coulomb explosion model predicts an energy loss scaling of the yield as η (de/dx) Thermal Spike Model The projectile energy is first deposited locally, within a small volume, on the electrons of the target [19, 21, 24]. Thereafter the electron-phonon coupling transfers the energy to the atomic subsystem. How the energy is spread in the electron subsystem depends on the electron mobility. The spike in this model is an area with a high energy density along the ion track. In the spike volume almost all atoms are in motion due to the high density of the recoil cascade [21]. Since the spike is very hot, a plasma is created in the core, which in turn will lead to desorption by an evaporation process. Desorption could also be a result of a sublimation process from the heated areas around the core. The thermal spike model predicts η (de/dx) Pressure-Pulse Model In this model the projectile ion will excite the target atoms, which causes them to expand and hence there will be a pressure onto the boundaries of a 20

21 lattice cell [19, 24]. There are several different types of pressure-pulse models. One model is based on the assumption that when a molecule at the surface is expanding, it will repel itself from its neighbours and desorb into the vacuum. The pressure-pulse model is mostly used to describe processes where large molecules are sputtered from the surface. This model predicts η (de/dx) 3. 21

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23 3. Experimental Setups and Measurement Procedures Due to the wide energy range chosen for the desorption yield experiments, the measurements were performed at GSI, which covered the range MeV/u, and at The Svedberg laboratory, which covered the range MeV/u. 3.1 GSI The first two sets of heavy-ion induced desorption yield experiments have been carried out at a beam line in the HHT cave connected to the SIS18 synchrotron at GSI [25]. An outline of the GSI facility can be found in Figure MeV/u with Uranium Beam The experimental setup had three differential pumping stages in order to achieve the desired pressure in the experimental chamber of mbar, see figure 3.1. Figure 3.1: The experimental setup at GSI The materials of interest to study are the most common materials used for accelerator vacuum chambers, i. e. 316LN stainless steel, P506 (Inconel) stainless steel, Oxygen Free Electronic (OFE) copper and 6082 aluminum. 23

24 The samples were mounted on a linear and rotational feedthrough, see figure 3.2. Figure 3.2: The sample holder at GSI In order to avoid uncertainties due to beam cleaning effects, a new unused sample was selected for each measurement. The thicknesses of the samples were chosen to be deep enough for the calculated range of the projectile ions, so that the ions would be stopped in the sample. All measurements were performed at perpendicular incidence. An extractor gauge was used to measure the total pressure increase and the partial pressure distribution in the experimental chamber was measured with a residual gas analyser (RGA). The experimental setup was baked at 300 C for 48 h. The beam current was measured by using current transformers in the SIS18 and in the beamline to the experiment. The average beam flux in the experiment was U 73+ ions every 3.9 s ( U 73+ ions/s) at the beam energies 15, 40 and 100 MeV/u MeV/u with Argon Beam The second experiment was performed at the same experimental setup as before, but with some modifications with the aim to achieve a lower base pressure: The apertures of the differential pumping stages were increased and partially NEG coated and the rotational feedthrough, that the samples were mounted on, was removed due to its high outgassing during movement. Instead there was only one sample in this run, simply consisting of the 316LN stainless steel endflange of the experimental chamber. The obtained base pressure was in the low mbar range. No RGA was used, i.e. it was only possible to measure the total pressure increase by the extractor ion gauge. For this experiment an argon beam was used for the energies 40, 80 and 100 MeV/u. The average flux at these energies varied between and Ar 10+ ions/s [ref]. 24

25 3.2 TSL The following two sets of experiments were performed in the Gamma Cave at The Svedberg Laboratory (TSL) in Uppsala, Sweden [26]. A schematic picture over the laboratory can be found in figure 3.3. Figure 3.3: An overview of The Svedberg Laboratory MeV/u with Argon Beam The test stand was built at the end of the K-beamline in the Gamma cave. The layout of the new test stand is shown in Figure 3.4. The pressure at the end of the non-baked part of the K beamline was in the mid range of 10 7 mbar. To achieve the desired base pressure range of mbar, a differential pumping section consisting of a pumping unit followed by a 2 m long and 63 mm diameter TiZrV Non-Evaporable Getter (NEG) coated tube was installed. The experimental installation is based on the throughput method [12] and consists of the test chamber (TC), where the samples are installed, and the pump chamber (PC) connected to TC by a conductance (C), see Figure 3.4. In order to have sufficient sensitivity for the expected ion-induced desorption flux, it is of importance to have an accurate value of the conductance C. By estimations of the desorption flux and the known pressure range of the vacuum gauges an indication of the vacuum conductance was given to be about 25

26 Figure 3.4: The teststand at TSL. The experimental installation consists of a pumping chamber (PC), a conductance (C) and the test chamber (TC). 4 l/s for CO. This number, together with the beam size and its misalignments set the diameter to 20 mm and the length to 237 mm of the conductance tube. Since the vacuum conductance tube has a complex structure of bellows and tubes, it was also modeled with MOLFLOW. The sample holder in the test chamber is a rotatable disc with twelve positions, see figure 3.5. Ten positions are used for samples, one is reserved for a beam luminescence screen and the last one is a circular 20 mm diameter hole to let the beam through to the externally mounted Faraday cup for the heavy-ion beam current measurements, see Figure 3.4. The sample holder is however manually controlled, and therefore it is not possible to switch between the samples and the Faraday Cup in a reasonable time to measure the beam current since the beam has to be switched off in order to get access to the beamline. To solve this problem, the beam current measured in the Faraday cup was compared with the beam current measured further upstream in the K-beamline, which gives a transmission coefficient between these two locations. When the beam is bombarding the samples it is possible to obtain the beamcurrent from this transmission coefficient. Samples of two different geometries were mounted: flat and tubular samples. Flat samples are a square with dimensions 20 mm 20 mm and 1 mm thick except for Ta which was 0.1 mm thick. Tubular samples are formed from the sheet of the same material as the corresponding flat sampe to a parallelogram with opening 20 mm 20 mm and a length of 80 mm, see Figure 3.6. The reason for the different geometries was to investigate whether there would be any contribution to the desorption yield from secondary particles emitted from the surface bombarded with heavy ions. In an open geometry (flat samples) the secondary particles will bombard the test chamber wall. If they have high enough energy they will cause further desorption and the total desorption yield will therefore be a sum of ion induced desorption from the sample material and the secondary particle induced desorption from the test chamber, see Figure 3.7. In the case of tubular samples the secondary particles mainly bombard the side walls of the tube instead of hitting the test chamber wall. The total desorption will now be a sum of ion induced desorption from 26

27 Figure 3.5: The sample holder assembly consist of a sample holder (H) with flat (FS), tubular (TS) and cone (CS) samples, a rotator (R) and a Faraday cup (FC). the sample material and the secondary particle induced desorption from the sample material as well. Both flat and tubular samples are bombarded by ions at normal incident angle. There is a possibility that the tubular samples will have a beaming effect unlike the flat samples, see Figure 3.7. To investigate this, another model was built in MOLFLOW for the whole experimental installation. A detailed description of the simulation can be found in Paper II. The result was that for the same desorption flux Q from the samples the pressure increase in the TC with the flat sample is approximately 1.6 times higher than with the tubular sample as an effect of molecular beaming. The sample materials were 316LN stainless steel, OFE (Oxygen Free Electronic) copper, etched OFE copper and tantalum. All samples were cleaned with alcohol followed by a chemical cleaning with a strong detergent (Deconex 15PF, ph 11.4 for 1% dilution, from Borer Chemie) in an ultrasonic bath at 60 C for 40 minutes, followed by rinsing in cold deionized water. Finally they were dried in hot air. The samples were installed and evacuated for a few hours. The pressure was controlled by a number of gauges: cold cathode gauges (CC1, CC2 and CC3), extractor gauges (E1 and E2) and Residual Gas Analyzers (RGA1 and RGA2), see Figure 3.4. When the pressure had reached the 10 7 mbar range, the experimental setup was baked at 250 C for 20 h and the NEG film was activated at 250 C. After the bakeout and activation the pressure was mbar in the test chamber and mbar in the pump chamber. The beam energy was varied for each experimental day. Important information about the beams are summarized in Table 3.1. The samples were bombarded by the ion beam during minutes. To investigate the beam conditioning effect, two long bombardments were performed over night: one of the 27

28 Figure 3.6: The shape of the samples: A - flat, B - parallelogram and C - cone. Figure 3.7: Secondary particles emitted from a flat and a tubular sample. 28

29 316LN tubes was bombarded with an Ar 8+ beam at 5 MeV/u during 6 h and the Cu Tube sample was bombarded with an Ar 9+ beam at 9.7 MeV/u during 8.5 h. Table 3.1: Beam parameters. τ is the puls length and T is the time between the micro pulses. Energy (MeV/u) Ion τ(ns) T (ns) Ion Flux (ions/s) 5 Ar Ar Ar MeV/u with Argon Beam The experimental setup used in the previous run was slightly modified by replacing the Faraday cup with a NEG coated tube blanked by a 316LN stainless steel end flange, see figure 3.8. The length of the tube was 1256 mm with an internal diameter of 24 mm. Figure 3.8: The NEG tube placed at the end of the experimental chamber with the RGA on top. Here C is ceramic insulators, B is bellows, S is the sample (the NEG coated tube) and F is the end flange. The beam enters from left. To allow the ion beam hitting either the end flange at perpendicular incident angle or the NEG coated tube at grazing incident angle, a bellow was installed between the test chamber and the NEG coated tube. On the side wall of the tube, 506 mm from the end flange, there was a 10 mm diameter hole connecting an RGA to the NEG coated tube, see Figure

30 To limit the induced desorption inside the RGA port, a stainless steel cone was installed between the hole and the RGA in the same way as reported in reference [27], see Figure 3.9. To minimize the induced desorption from the cone, it can be biased and conditioned with electrons from a filament during the bakeout. Since the Faraday cup was removed, the current measurements could instead be performed with the NEG coated tube since the tube was isolated from the other parts of the experimental setup. Figure 3.9: Layout of the RGA port assembly with a cone: 1-a 10 mm diameter hole, 2 - Connection to an ampere meter and bias, 3 - Connection to a filament power supply, S is the sample (the NEG coated tube). The 316LN stainless steel tube was manufactured, cleaned and coated with TiZrV film by magnetron DC sputtering at Budker Institute of Nuclear Physics in Novosibirsk, Russia. The film thickness was 3-4 μm composed of Ti(30%)Zr(26%)V(44%). The same sample holder was used as in the previous run but some samples were exchanged: 316LN Flat 1, 316LN Tube 1, Etched Cu Flat 1, Etched Cu Flat 2 and Ta Tube samples were remowed. The new installed samples were Etched Cu Flat 2, Etched Cu Tube 2, a Cu Cone, sputtered Au coated Cu (flat) and galvanic Au coated Cu (flat). The copper sample folded as a cone was to study the effect of grazing incident angle, see Figure 3.6. The test chamber was pumped out for a few hours, reaching a pressure in the range of 10 7 mbar. The experimental setup bakeout and the NEG coated tube activation were performed by a procedure developed at CERN and modified at Daresbury Laboratory, see Figure More details can be found in Paper V. The cone in the RGA port was conditioned with 300 ev electrons by having a bias of +300 V and an emission current of 3 ma. The pressure after the bakeout was mbar in the test chamber and mbar in the pump chamber. The measurements with an Ar 8+ beam at 5 MeV/u had a typical beam current of 10 na with a corresponding flux of ions/s. The beam current was measured by having the beam bombarding the end flange of the NEG 30

31 Figure 3.10: The baking scheme. tube. Then the tube was shifted by 17 mrad to have the beam hitting the tube by grazing incidence angle near the RGA port, and the ion bombardment started again. To investigate whether the cone had been sufficiently conditioned, the cone was biased between -300 V and +300 V during the heavy ion bombardment. No pressure difference was observed, therefore it could be concluded that the cone was well conditioned. After the bombardment with heavy ions, first H 2 and then CO was injected into the test chamber. The pressure ratios obtained from the RGA located in the test chamber and the RGA connected to the NEG tube were recorded for each gas. These pressure ratios together with Molflow simulations could then be used to obtain the sticking probability of the NEG film, to be dicussed in the results section. Then the NEG film was saturated with continuous injection of CO at a pressure in the range of 10 6 mbar in the test chamber during 3 h. After the NEG saturation the heavy ion bombardment started again. The ion induced desorption measurements from the samples on the sample holder were performed in the same way as the previous run except for one difference. As it was mentioned before, the secondary particles generated from the surface after the ion impact might play an important role in the desorption mechanism. To study the amount of secondary particles, the electrically insulated sample holder can be biased up to ± 1500 V. During the ion bombardment each sample was biased between V and V and the corresponding currents were measured. 31

32 3.3 Calibration To calibrate the extractor gauges and Residual Gas Analyzers (RGA) that was used to measure the pressure increase, the following procedure was performed. A small amount of gas was injected into the test chamber in the experimental setup when the pressure was in the 10 9 mbar region. The gases used were H 2,CH 4, CO and CO 2. The gas was pumped through the conductance tube during the injection, and therefore the pressure had to he stabilised before the data taking. In total 11 pressure levels were recorded up to a pressure of 10 6 mbar, which is the detection limit for the RGA s. Each data point is a mean value from 10 pressure readings. The procedure can be seen in Figure The RGA s were calibrated both for the Secondary Electron Multiplier (SEM) mode and the Faraday mode. Figure 3.11: The calibration procedure. For each gas, the reading from the extractor gauge was compared to the reading of the residual pressure for the specific gas in the RGA. The RGA reading was plotted against the extractor gauge reading and the calibration coefficients could then be obtained from the slope, see Figure

33 Figure 3.12: The readings from the RGA (SEM) plotted against the readings from the extractor gauge during injection of CO. 33

34

35 4. Laser Refractometry To rely on the vacuum gauges that measure the pressure in the vacuum system, the gauges has to be properly calibrated to a primary standard. The primary standard at PTB in Germany is obtain from the throughput method. The main contribution to the total uncertainty of the total calibration comes from the pressure reading in the flowmeter [28]. In the following, which is based on Paper VI and Paper VII, the possibility to use laser refractometry in order to obtain the gas density in the flowmeter will be evaluated. Laser refractometry is a technique that measures how the speed of light is changed by the gas density of the substance, which is directly proportional to the refractive index of the substance. It is a very precise technique; the resolution of the laser refractometry measurements of gas density has been shown to be in the order of parts in 10 5 [29]. We are therefore interested to investigate whether laser refractometry can be employed instead of vacuum gauges and thereby obtain measurements of gas flow with higher accuracy. 4.1 Interferometers The working principle of an interferometer is splitting an optical wave and then allowing the waves back together. If the two waves brought together coincide with the same phase they will amplify each other - interfere constructively - while if the two waves have opposite phases they will cancel out. The basic building blocks are a monochromatic source (e. g. laser), mirrors and a detector. Two common types of refractometers are the Michelson interferometer and the Fabry-Perot interferometer [30]. In the Michelson interferometer a semitransparent mirror (so called beam splitter) divides the wave into two, where one wave will hit a fixed mirror and the other will hit a movable mirror, see Figure 4.1. When the beams are brought back together, an interference pattern results. A Fabry-Perot interferometer is made of two parallel highlyreflecting mirrors, see 4.2. A beam goes through the first mirror, and then, because of the high reflectivity of the mirrors, the beam will bounce between the mirrors many times. However, each time the light reaches the surface of the second mirror part of the light will be transmitted. This results in several beams that can interfere with each other. Due to the large number of interfering rays, the Fabry-Perot interferometer has extremely high resolution. Because of the high resolution, simplicity and compactness of the setup, the Fabry-Perot interferometer was chosen for this feasibility study. 35

36 Figure 4.1: The Michelson interferometer Figure 4.2: The Fabry-Perot interferometer 36

37 4.2 Free Spectral Range In a Fabry-Perot interferometer the distance d between the mirror plates affects the distance between the transmission peaks: the shorter the distance between the mirrors the larger the difference will be between the peaks, the so called free spectral range (FSR), according to FSR = c 2d (4.1) The value of the width at half maximum of the transmission peak, Δν/2, will scale with the value of the free spectral range as Δν 2 = FSR 2F (4.2) where F is the finesse. The finesse is commonly approximated (for reflectivity R > 0.5) by: F = π R 1 R (4.3) Since our setup only allows us to have a rather short distance between the mirrors, approximately d = 1 mm, we will have large difference between the peaks. One way to compensate for the bad FSR is to make sure to get as good finesse factor as possible. As can be seen from Figure 4.3, mirrors with high finesse will give sharper transmission peaks and lower transmission minima than mirrors with low finesse: To obtain as high finesse as possible you have to choose mirrors with as high reflectivity as possible. As can be seen from Figure 4.4, the finesse increases rapidly for mirrors with reflectivity higher than In our setup we have found mirrors with a reflectivity of R = % for a wavelength of 1064 nm. 37

38 Figure 4.3: The transmission of a mirror as a function of wavelength. A mirror with high finesse has sharper transmission peaks and lower transmission minima than a mirror with low finesse. Figure 4.4: Finesse as a function of reflectivity. Very high finesse factors require highly reflective mirrors. 38

39 4.3 The Planned Setup Our planned setup, shown in Figure 4.5 is based on heterodyne spectroscopy [31]. This method is chosen since there will be enough accuracy to determine a frequency difference of 10 khz. A laser beam, with a wavelength of 1064 nm, passes through an isolator before entering the Fabry-Perot cell. After the cell, the beam is detected in a fast photo diode (bandwidth > 1 GHz). The current from the diode is led via an amplifier back to the current driver. This way it is possible to adjust the laser to the wanted peaks. Figure 4.5: The planned setup is based on heterodyne spectroscopy in order to obtain enough accuracy to determine a frequency difference of 10 khz [31]. 39

40

41 5. Results and Discussion A summary of the results that are included in the papers in this thesis follows. 5.1 The Uranium Beam The pressure increase due to the ion bombardment was about mbar. The partial pressure increase was dominated by CO and CH 4, but H 2 was still dominating the background pressure, see Figure 5.1. Figure 5.1: The figure to the left show the pressure rise when the beam is on the target. The figure to the right show the corresponing residual pressure increase. The desorption yields from the different targets can be found in Figure 5.2. The general trend is that the highest desorption yield is from P506 stainless steel, followed by Al, 316LN stainless steel and Cu. However, for 15 MeV/u the desorption yield from P506 stainless steel is the second lowest while the other samples follow the previous ranking. 5.2 The Argon Beam The partial pressure increase during ion bombardment were mainly dominated by H 2,COandCO 2 in that order. H 2 was the dominating gas on the total pressure. This section is a summary of the results from Paper III, Paper IV and Paper V. 41

42 Figure 5.2: The desorption yield from Al, 316LN, Cu and P506 with the U 73+ beam. 42

43 5.2.1 Material Dependence The results on material dependence can be found in Paper III. The desorption yield from the flat and the tubular samples can be found in Figure 5.3. For flat samples the etched Cu has the highest desorption yield followed by 316LN stainless steel, Ta, Cu and the Au coated Cu samples. Considering the relatively large errorbars there is not a significant difference between the sputter and galvanically coated samples. For the tubular samples the order has been shifted: Highest desorption yield is from 316LN stainless steel followed by Cu, Ta and etched Cu. The 316LN stainless steel tube bombarded with Ar 8+ at 5 MeV/u show much higher desorption yield than all the other samples at all energies. The general tendency for flat samples is that the highest desorption yield is at 5 MeV/u and then the yield decreases by 2.5 times at 9.7 MeV/u. Between 9.7 and 17.7 MeV/u there is a small decrease in the desorption yield by 1.3 times. The tubular samples show similar reduction in desorption yield with the argon beam energies. The only sample that differs from this behavior is the Ta Tube at 17.7 MeV/u, which has a much higher desorption yield compared to the other tubular samples. Therefore, the sample thicknesses was compared with the ion range inside different materials. The sample thickness were 1 mm for all samples except for Ta which had a thickness of 0.1 mm. The range for the argon beams in the samples for different energies were calculated by TRIM and can be found in Table 5.1. According to the table, all energy is deposited in the samples. However, for Ta at 17.7 MeV/u the range of 83 μm is very close to the target thickness. The consequences of this is that there may be both backscattered and forward scattered particles, which might explain the high outgassing for the Ta tube at 17.7 MeV/u. Another explanation to the higher desorption yield is that the thin Ta tube did not have a very solid structure and it might therefore be possible that the tube was a bit shifted, having a minor part of the ion beam hitting the tube by grazing incidence. Table 5.1: The range (in μm)calculated by TRIM. Energy (MeV/u) 316LN Cu Ta Au For the two long conditioning experiments, the desorption yield evolution can be found in Figure 5.4. The 316LN stainless steel tubular sample was bombarded with Ar 8+ ions at 5 MeV/u during 6 hours with a total dose of ions. After conditioning, the desorption yield had decreased by a factor of 5. When comparing this well conditioned sample with a fresh 316LN tube sample, both bombarded at 17.7 MeV/u, it was found that both gave the 43

44 (a) Flat samples. (b) Tubular samples. Figure 5.3: The desorption yield from flat and tubular samples with argon beam. The flat samples have accuracy ± 25% and the tubular samples have accuracy ± 15%. 44

45 same desorption yields. This indicates that the conditioning at lower energies is not very efficient to reduce the desorption yield at higher energies. The other conditioning experiment was performed with bombardment of Ar 9+ beam at 9.7 MeV/u during 8.5 h with a total dose of ions. After the conditioning the desorption yield had decreased by a factor of 2. For the next energy at 17.7 MeV/u, the desorption yield increased compared to the conditioned sample at 9.7 MeV/u. This could be another indication to that the conditioning at lower energy is not very efficient to reduce the desorption yield at higher energy. The variation in desorption yield for the same material from sample to sample is up to 4.5 times for stainless steel and up to 3 times for etched Cu. To have better statistics more samples should be studied. In Run 1 the Etched Cu Flat1 had a significantly higher desorption yield than the Cu Flat sample. However, in Run 2 the desorption yield of the Etched Cu Flat 2 had decreased by a factor of 3 and the Cu Flat sample showed a decrease by almost a factor of 2. To investigate if the difference in the desorption yield from the etched copper samples compared to the copper sample are due to possible difference in the surface structure of the respective samples, the surfaces was studied by Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and Light Interference Microscope (Wyko). The idea behind the chemical cleening of copper is that the cleaning should smoothen sharp edges on the copper surface. The SEM images in Figure 5.5, with magnification, reveal that there are differences in the surface structure between the samples. The Cu sample has a smoother surface than the etched Cu sample where sharp islands are visible. This indicates that the chemical cleaning was not performed for a sufficient time, and therefore the purpose of the etching was not obtained. As a comparison, also pictures of 316LN Flat 1 and Ta Flat are shown. To gain some more information on the surface roughness also AFM was used. A surface area of 10 μm 10 μm was analyzed on all samples. The AFM image of the different samples can be found in Figure 5.6. The surface structure of the Cu sample is rather smooth and has a maximum height difference of approximately 200 nm. The etched Cu samples look different compared to the copper sample. The large surface structure is similar to Cu but with a lot of tiny sharp peaks. The height difference is about 500 nm for the etched copper samples. The R a (arithmetical mean deviation of the assessed profile) and R q (root mean square deviation of the assessed profile) values from AFM was confirmed with Wyko and are summarized in Table 5.2. The difference between the AFM and Wyko values is due to the fact that the Wyko analyze an area of 176 μm 232 μm, which is much larger than the area analyzed by AFM. The R a and R q values from Wyko is not that reliable though due to technical problems during the measurements. However, the relative difference between the analysed samples were the same as the ones obtained with AFM. 45

46 (a) 316LN. (b) Cu. Figure 5.4: The desorption yield from 316LN tube and Cu tube for the long dose experiments. 46

47 Figure 5.5: SEM images of Cu Flat, Etched Cu Flat 1, 316LN Flat 1 and Ta Flat. Figure 5.6: The surface structure for Cu Flat, Etched Cu Flat 1 and Etched Cu Flat 2 obtained with AFM 47

48 Table 5.2: The Ra and R q values for the different samples obtained with AFM and Wyko. AFM Wyko Sample R a R q R a R q 316LN Cu Etched Cu Ta Geometry Dependence The results from the geometry dependence are discussed in Paper III. Focussing on each material separately, the effect of the sample geometries is clearly seen, see Figure 5.7. The difference in desorption yield between a flat and a tubular sample made from the same material and bombarded by the same beam could be up to 6 times. The tubular samples all show higher desorption yields except for the etched copper which show the opposite. However, for the new etched Cu samples, the Etched Cu Tube 2 has a desorption yield 1.5 times larger than the Etched Cu Flat 2. To compare desorption yields between normal and grazing incident angle a special sample was used, the Cu cone. The incident angle for this sample was 125 mrad. The measured desorption yield of 1145 molecules/ion was three times larger than from the Cu Tube. The main idea on why to use different geometries was to investigate whether there would be any secondary particles during the ion bombardment, that could impact on the vacuum chamber wall which in turn could contribute to the total desorption yield. For flat samples, the secondary particles bombard the test chamber walls. For the tubular samples, most secondary particles will hit the side walls of the tube. From the experiments there is a clear difference in the desorption yield between the flat and the tubular samples. The potential cause to this difference could be secondary particles emitted during the bombardment, thermal heat capacity, the sample temperature and other effects. In the following sections these effects will be discussed further. Beam Currents The measured currents for the flat samples were 3.3 times larger for 5 MeV/u, 3 times larger for 9.7 MeV/u and 2.2 times larger for 17.7 MeV/u compared to the currents from the tubular samples. These figures, and therefore the beam current for the flat samples, are measured with accuracy ±15%. An explanation to the larger measured current from the flat sample could be that secondary particles are generated and leave the surface after impact of the ion beam. The accuracy of the current measurements contributes to the accuracy 48

49 Figure 5.7: A comparison of the geometry dependence on the desorption yield shown for 316LN, Ta, Cu and Etched Cu. 49

50 of the desorption yield measurements. It is ±15% for tubular samples, as described in Paper II but for flat samples it is larger because of the lower accuracy of the beam current measurements, and is estimated to ±25%. Secondary Particles The impact from secondary particles are discussed in Paper III. From the biasing of the samples, graphs of the current measurements as a function of bias for 316LN stainless steel and copper, normalized to 1 na heavy ion beam current, are shown in Figure 5.8. For the flat samples there is a significant difference between the current measured with positive and negative bias. The measured current reach a saturation level at U = ±200 V. However, at a bias of U = ±20 the current is already 80% of the saturated current level. For tubular samples the saturation level of the current was not as well pronounced as for the flat samples. The difference between the current measured with positive and negative bias was very small. This low difference between the biasing was expected since most of the secondary particles are believed to hit the sidewalls of the tube and thereby trap most of the secondaries. The current measured from the 316LN Tube sample is almost the same as the current measured with the Faraday cup. The small increase in current measured from the 316LN sample is expected since the length of this sample is only one third of the length of the Faraday Cup, hence more secondary particles may escape from the sample. The current measured from biasing of the Cu cone is larger than the current from the Cu tube, but is still smaller than the current from the flat Cu sample. From these graphs together with the equations in section 2.4 and Paper III it was possible to obtain the secondary electron yield (SEY) and the secondary ion yield (SIY). The result is a SEY around 32 electrons/ar ion and a SIY around 14 ions/ar ion, see Table 5.3. Table 5.3: The secondary electron yield and the secondary ion yield per incoming ion are shown. I SEY I max + SIY max 316LN Flat 2.3< I < < SEY < Cu Flat 2.3< I < < SEY < Etched Cu Flat 2.3< I < < SEY < Ta Flat 2.3< I < < SEY < From the bias measurements it was found that the energy of the secondary electrons was 20 ev. It is reported in [32, 33, 34] that electron stimulated desorption for electrons at E = 20 ev could be in the range of molecule/electron. That implies in our case with a SEY of 32 electrons/ion that there may be up to 32 molecules desorbed per ion, which is about 5-10% of the total desorption yield. 50

51 (a) 316LN. (b) Cu. Figure 5.8: The measured currents from 316LN and Cu during bias. 51

52 The energy and nature of the secondary ions could not be extracted from the bias measurements. Therefore, SRIM simulations were performed, which show that the ionization energy for Cu is 3.5 ev. For energies of the projectile ion of up to 20 ev, only 13 ions out of 4000 will be sputtered, i.e. in principle no ions will be sputtered from the surface. It was also found that there were no backscattered ions. The conclusion is that the impact from secondary particles on the desorption yield is small, and could not explain the difference between the flat and the tubular samples. The power dissipation inside the sample due to the intensive beam bombardment will cause local and overall temperature heating effect, which is different for different energies, sample materials and geometries. This increase in temperature may cause higher desorption, which is not included in our analysis and in the errorbars. There is a possibility that the beam may strike some of the walls of the tubular samples, and thereby the ion induced desorption would be from grazing incidence angle instead of perpendicular incidence. But since the entrance hole is smaller than the tube size, except for in the middle of each sidewall, this effect should also be negligible Energy Dependence The desorption yields from the 316LN flat samples with uranium and argon beams measured at the setups at GSI and TSL were plotted in the same graph together with the energy loss, see Paper IV. It is clear that the desorption yield for the uranium beam is higher compared to the argon beams. It was found that the desorption yields scale with (de/dx) 2 for both the uranium beam and the argon beams, however different scaling factors were introduced in front of the equation to merge the calculations with the experimental data. The quadratic scaling of the energy loss corresponds to the thermal spike model, discussed in section The large errorbars are due to three different kinds of errors: absolute, relative and systematic errors. The absolute errors are the errors obtained from the desorption yield measurements on the samples. The relative error is due to the variation from sample to sample, and the systematic errors are due to comparison between the different setups. In total, this gives errorbars of up to ±50%. From the energy loss function, the highest energy loss is around the Bragg peak, which is around 1 MeV/u for Ar and 10 MeV/u for U. Since the desorption yield scales with the energy loss, this implies that the highest desorption yields would be at the Bragg peak. For the FAIR project, the injection energy of the U 28+ ions will be at 10 MeV/u. To minimize the initial desorption yield it would be preferable to increase the injection energy in order to pass the Bragg peak, if feasible. This would also imply that the desorption yield would decrease during an acceleration cycle in SIS18 since the desorption yield is 52

53 Figure 5.9: Desorption yield versus projectile impact energy for Ar and U hitting 316LN stainless steel. The lines represent the calculated electronic energy loss for Ar and U to the power of two. decreasing with increasing energy. In addition the ionization cross section for U 28+ is decreasing with beam energy Non-Evaporable Getter (NEG) This section covers the results from Paper V. The dynamic pressure increase during the ion bombardment were mainly from CH 4 followed by H 2, CO, C 2 H 6 and CO 2. The reason for the high pressure level for the hydrocarbons is that they are not pumped by the NEG film and they have very limited conductance through the narrow chamber. The ratios of the pressure recorded in the NEG tube to the pressure recorded in the test chamber were calculated and the sticking probabilities for H 2 and CO for the activated NEG film could then be obtained from Figure The sticking probabilities were α(h2) = and α(co) = 0.03 for the activated NEG film. During CO injection the CO 2 pressure in the TC also increased which allowed to estimate α(co 2 ) = The measured sticking probability for CO is an order of magnitude less than expected for a new coating, which could be explained by the age of the coating and the intensive use of it in previous experiments. These initial sticking probabilities were used to calculate the desorption yields of the activated NEG coating. The heavy ion bombardment of the saturated NEG film showed a dynamic pressure (ΔP) increase of 30 times for H 2, 8 times for C 2 H 6, 20 times for CO and 2 times for CO 2, but CH 4 remained the same. 53

54 Figure 5.10: Sticking probability of the NEG coated tube vs the pressure ratio calculated with Test Particles Monte-Carlo (TPMC) and Angular Coefficient (AC) methods. For CO, the ratio and sticking probability were reduced to R(CO) = 120 and α(co) = This means that the NEG coating was not fully saturated and thereby the sticking probability will not be uniformly distributed along the tube: the tube will be more saturated near the test chamber and less saturated near the RGA in the NEG tube. The sticking probability could therefore not be estimated correctly since the Molflow code does not allow studying a non-uniform saturation process. It is possible to conclude the following: For the ratio measured at the RGA in the NEG coated tube, the corresponding α would be the smallest sticking probability in the NEG coated tube according to the discussion above and therefore one obtain that α(co) > In the same way the sticking probability for CO 2 was estimated to α(co 2 ) = The calculated desorption yields from the activated and saturated NEG film are shown in Table 5.4. The total desorption yield from the activated NEG coating was found to be 2600 molecules/ion, which is approximately 4-13 times higher yields compared to the previous measurements on materials at normal incident angle. This significant difference could be explained by increased desorption yield at grazing incident angles, which was the case for the previous experiments with copper at normal and grazing incidence beam. It was found that the desorption yields for H 2,CO 2 and CH 4 remain almost on the same level before and after saturation, desorption yields for CO, C 2 H 6 and H 2 O increased a few times. The CO sticking probability near the RGA on the NEG coated tube went down approximately less than 3-6 times after CO injection. Therefore, the 20 54