Preliminary proposal for SEY studies on cold surfaces

1. Preliminary proposal for SEY studies on cold surfaces AsenaKuzucan

2. The Electron-Cloud Effect • Electron cloud (EC) effects involve the interaction between high-energy beams and low-energy electrons produced in the vacuum chamber. • There are 3 different sources of EC: 1) A major contribution to the electron cloud are photoemitted electrons (PE) produced through the collision of synchrotron radiation photons and the vacuum chamber walls. The photoemitted electrons can be accelerated by a charged particle beam, acquiring sufficient energy to produce secondary electrons by getting kicked to the opposite walls of the vacuum chamber. 2) Ionization of the residual gas 3) Secondary electrons, emitted by primary electrons from the walls. This can lead to an exponential increase in the electron density during the passage of a bunch train.

3. The Secondary Electron Yield (SEY) • The electron cloud density depends on characteristics of the circulating beam (bunch length, magnetic field, charge and spacing) and the secondary electron yield (SEY) of the wall from which the electrons are generated. • If the secondary electron yield (SEY) coefficient of the wall material is greater than one, the runaway condition of beam-induced multipacting can develop. • Typically a peak SEY value is δmax ≈ 3for an as received aluminium at the room temperature. • The space charge from the cloud, if sufficiently large, can lead to beam instability and losses ultimately causing a reduction in the collider luminosity.

4. The Secondary Electron Yield (SEY) The SEY (δ) definition is determined from equation: IP is the primary current( the current leaving the electron gun and impinging on the surface of the sample) and IT is the total current measured on the sample (IT=IP-ISE). ISE is the secondary electron current leaving the target.

5. Experiment description for cold measurements • The system used to measure SEY is composed of one UHV chamber where the pressure is in the low 10-10 mbar. We use a bakeable (300°C) UHV chamber. The UHV system is pumped by a 260 l/s turbo molecular pump. A Bayard Alpert gauge and a electron gun are already installed. • The system will be equipped with a residual gas analyser too. • Two thermometers , one silicon diode and one platinum resistance thermometer, will be installed on the cold head before we mount the cold head on the system. • For the measurements the sample will be bombarded by the primary electrons with a variable energy between 50 and 3000 eV. • We are going to use an electron gun, which is able to focus a beam below 2 mm at 200 mm working distance with a beam current 10-10 to 10-9A.

6. Experimental setup Cold System Vacuum chamber The place, where the cold head will be replaced soon Electron gun Turbo pump

7. Experimental Setup • The path from electron gun to sample -y

8. Experimental Setup Cold Head • The liquid helium coming from a transfer line arrives into a phase separator. • Gas produced during the transfer feeds a heat exchanger which cools down a thermal screen at about 30K. • Liquid from the phase separator is transferred through an expansion valve to a exchanger. The helium flow and a heater on the cold source permit to regulate the temperature of the sample. • To combine good thermal contact (lowest attainable temperature) between the sample and the cold source with electrical insulation of the sample, the cold head has a indium welded copper-sapphire-copper sample holder head.

9. What is still to be done for cold measurements? • The cold head has been cleaned in the surface cleaning Lab. A temperature sensor (DT-470-SD from Lakeshore) is to be installed on the cold head. • Heater definition and installation (we need two heaters, one behind the sample and one on the cold head) • A shield around the cold head for a differential pumping is being made. • Accordingly the cold head will be mounted and the first measurements will start. • Bake out to remove the water vapor adsorbed on the surface. • We want to do measurements with gas injection to investigate the effect of the residual gas in LHC ( How does the adsorption of a gas change the SEY?) For this measurements, a gas injection table has to be designed and installed. • Electron dose effect

10. Measurements at room temperature • First Measurements for checking of the system 1

11. Measurements at room temperature • Drilled Cu Diameter of one hole=1mm, 92 holes/cm2 The microstructure can be a factor which can change the SEY. The commonly accepted hypothesis is that, for a given chemical surface, the surface with microstructures has a lower SEY than a smoother one.

12. Measurements at room temperature • Distance effect 4 stainless steel samples has been used for the distance effect measurements. All the samples were mounted on a rotating sample holder with different distances from sample to collector. The distances of each samples are like following: Sample 1=145mm, Sample 2=152mm, Sample 3=157mm, Sample 4=160mm.

13. Measurements at room temperature

14. Measurements at room temperature • Measured SEY decreases with an increasing distance. After the first measurements the results show, that our samples do not follow this thesis. The second nearest sample (sample 2) has the lowest SEY value. For the second measurements we changed the sample 1 with sample 2 to investigate, if an other effect dominates the distance effect. After the second measurements we assumed, that other effects (like the roughness of the samples) could play a role. • The samples have been given to the metrology lab for the surface examinations . They used a 3D VEECO NT3300 surface profiler for measuring step heights and surface texture. The results from the metrology lab cannot give a satisfactory answer to our problem.

15. Measurements at room temperature Ra is the arithmetic mean of the absolute departures of the roughness profile from the mean line. Rz is the maximum peak to valley height of the profile within a sampling length. The roughness parameter of the samples are quit similar so we cannot define the results just with the effect of roughness. Actually, several parameters (like chemical composition, treatment, history...) are known to affect SEY. Because of that, a full characterization of samples is important.