Vertical Electro-Polishing at CEA Saclay: Commissioning of ... · VEP) to be more appropriate. For this reasons, a VEP set-up has been designed at Saclay for the electro-polishing

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VERTICAL ELECTRO-POLISHING AT CEA SACLAY: COMMISSIONING OF A NEW SET-UP AND MODELING OF THE PROCESS APPLIED TO

DIFFERENT CAVITIES

F. Éozénou , S. Chel , Z. Wang , Y. Gasser , J-P. Poupeau , C. Servouin#a a ab a a a

aCEA-Saclay, DSM/Irfu/SACM-91191 Gif-Sur-Yvette – France bENSAM ParisTech – 75013 Paris – France

Abstract

Reproducible operation at high performances of sc cavities is required for superconducting linacs, at least in their high energy section in the case of accelerators for proton beams. High beta elliptical cavities are thus of concern, and, to achieve required performances for such cavities, surface preparation including electro-polishing (EP) is recommended.

Moreover, for large dimension cavities we consider the EP treatment in vertical configuration (abbreviated as VEP) to be more appropriate. For this reasons, a VEP set-up has been designed at Saclay for the electro-polishing of muticells elliptical cavities.

Chosen equipment will make it possible to use a wide range of parameters (voltage, flowrate, temperature, nitrogen inerting) with R&D purpose in mind. Optimization will be done using modeling with COMSOL software for different cavities. As examples, we present some results for the 704 MHz high-beta SPL cavity and the 1300 MHz ILC cavity and show the influence of cathode shape on both acid flow and electric field distribution during the process. At last, importance of the size of the cavity will be commented.

INTRODUCTION Electro-polishing (abbreviated as EP) is believed to be

the most desirable treatment for SRF cavities [1]. EP is an anodic electrochemical treatment carried out in concentrated hydrofluoric - sulphuric (HF-H2SO4) acids. Generally, the cavity is electro-polished following a process developed by KEK [1]: A voltage is applied between the cavity and a cylindrical aluminium cathode (set in its centre) while it is placed in horizontal position, rotating, and half-filled with the circulating acid. This process makes it possible to reach high gradients on 9cell Tesla shape cavities and has been chosen for the surface treatment of cavities for the XFEL linac. However, it induces some drawbacks: the rotating seals are more easily prone to leaks, the cavity must be switched full of acid for draining and the footprint of the set-up is rather large. Furthermore, the removal rate in the cavity depends

on the location of the cell [2]. To overcome these drawbacks, some laboratories are investigating an alternate process where the cavity is electro-polished in vertical position [3]. Cornell University has proved that VEP with static acid could make it possible to reach high gradients [4], and as VEP is suitable to the treatment of large elliptical cavities, we have developed at CEA-Saclay a VEP set-up sized for surface treatment of the largest high beta and high gradient cavity we have designed, that is to say a 704 MHz cavity for the Superconducting Proton Linac. All other “smaller” cavities, as ILC cavities, fit into the set-up too.

In this paper, this VEP set-up is described as well as results concerning the modeling of VEP for both high beta SPL [5] and ILC cavities. The acid flow and electrical field distribution expected during VEP will be discussed.

CHARACTERISTICS OF THE SET-UP

Generalities This set-up is designed to electro-polish a cavity in

presence of circulating acid electrolyte. The expected flowrate range is between 5L/min and 40L/min. A constant voltage EP process has been chosen for the treatment of the cavities. The available electric power for the electro-polishing is 30kW (20V – 1500A). The set-up is divided into two distinct ventilated cabinets: The acid storage area and the main treatment cabinet (Figure 1). Insertion of the cathode into the cavity and assembly of connecting pieces are done on a dedicated table close to the set up. The cavity is then transferred in the main cabinet with a specific handling tool.

The acid storage tank is located in a pit connected to the liquid exhaust storage area of the laboratory. The storage capacity is 300L. This capacity has been calculated in order to maintain a niobium concentration in the electrolyte below 10g/L after a bulk EP for SPL cavities.

The cavity is filled up from the bottom by a membrane pump located in the pit. Once the acid reaches the connection tube on the top of the cavity, the acid flows to

Proceedings of SRF2011, Chicago, IL USA TUPO062

07 Cavity preparation and production 549

the tank by gravity. The circulation of acid is uninterrupted when voltage is on. Temperature of the acid bath is stabilized thanks to circulation of cold water through a Teflon heat exchanger inserted inside the acid tank, as shown on the flowsheet of the process on Figure 2.

At the end of an electro-polishing sequence, the acid is drained back to the tank and the cavity is rinsed with Ultra-Pure water. A predetermined sequence of filling (from the bottom)/draining of the cavities is followed by a continuous circulating sequence. Rinsing stops automatically as soon as water conductivity is low enough.

All the process is automated and run through a touchscreen (See Figure 2).

Figure 1: VEP set-up at CEA Saclay. 1: Main cabinet for cavity treatment. 2: Acid tank located in a pit. 3: Table for assembly prior to VEP.

Figure 2: Flowsheet of the process displayed on the touchscreen.

Safety Safety aspects have been carefully taken into

consideration to design the set-up. Nitrogen is blown on

the top of the cavity and on the top of the acid tank to dilute the hydrogen generated during the process and prevent any risk of explosion. Furthermore, the program of the automat includes specific procedures if sensors (temperature, pressure, hydrogen, flows, acid level) should detect any failure:

- high temperature detected: the process is stopped and acid is drained,

- insufficient nitrogen flowrate: the current is stopped,

- extraction failure: The nitrogen flowrate is stopped as well as the generator,

- high level of acid detected on the top of the cavity: the pump stops.

Fundamental parameters (temperature, flowrate, voltage) evolution is monitored on the touchscreen of the automat, as well as failure occurrence. A USB port allows an easy exportation of data.

Materials The acid tank is coated with Teflon and piping/valves

are made of PFA. PVDF has been chosen for some pieces requiring higher mechanical resistance (connections for the cavity, inserts for sensors).

MODELING OF VERTICAL EP

2D Axi-symmetry Model with COMSOL Multiphysics

The described set-up makes it possible to use a wide range for operating parameters. (acid flowrate between 5 and 40L/min, voltage between 0 and 20V, nitrogen flowrate between 0 and 50L/min, etc.). It was designed with R&D purpose in mind, with the view of tracking the optimal parameters for VEP process. A complementary approach consists in modeling the process using, COMSOL Multiphysics software which allows evaluating some parameters not easily accessible by experiments. Previously, studies (fluids, electric field, concentrations, thermal properties) have been carried out with different softwares. [6,7,8] In this paper, fluid distribution and electric field have been modeled for SPL and ILC cavities with a cylindrical cathode (30 mm diameter) shape. In a second step, alternative cathode shapes have been numerically investigated for possible improvement of the process.

Contrarily to horizontal EP, VEP is a symmetrical process. Gravity force, represented by Fz on Figure 3. does not depend on radial component. It is thus possible to create a 2D model with symmetry according to the cavity z-axis (see Figure 3) which makes the calculation easier compared to a 3D model.

Fluid Dynamics Modeling For the considered set-up, acid is introduced in the cavity through eight holes equidistant from the cathode. To

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TUPO062 Proceedings of SRF2011, Chicago, IL USA

550 07 Cavity preparation and production

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Figure 9: Potena cylindrical canode, V=15V

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cathode) a

Table 1: EILC cavity

ShapeABC D E A BCD E

Similar

5cell caviTested cat

10: Alternacally tested fo

V, at the cathoe are in red (A,

s (Voltage of are summed up

Electric field ay for different

Insulation NoNoNo No No Yes YesYesYes Yes

r investigation ity with a cylithodes are show

ative insulateor the Tesla caode. V=0. Ins, B, C) and in l

15V between p in the recapit

at the iris and acathode configEi (V/m) Ee

825 816 913

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has been carrindrical cathodwn below (Fig

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the cavity antulative Table1

at the equator gurations e (V/m) Ei/E

20 4118 4533 2743 2635 1750 4.15 6.30 7.35 3.32 7.

ried out for thede and the shagure 11.).

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Table 2: ElectrSPL cavity for

Shape Insu

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For both IL

make it possibthe cells. Howinsulated cathremoval rate.

OUTLThe Electro

adapted for lahas been comachieved withprocess. Howechosen in oModeling wappropriate catake into accocathode and telectro-polishefor full underst

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Community-RFP7 programmThis work hasof the “ConseiASTRE progra

Alternative tested for thethe cathode. in red.

d results are pr

ric field at ther different catho

ulation Ei (VNo 292No 409Yes 80Yes 111

LC and SPL ble to improvewever, we havhode will be

LOOK AND-Polishing in Varge elliptical mmissioned ath circulating ever, proper prder to achi

with COMSOathodes designount parameterthe cavity sured, results wiltanding of the

CKNOWLEwledge the

Research Infrasme (EuCARD been carried oil General de lam.

insulated cae SPL cavity. V=0. Insulate

resented in Tab

iris and at theode configurat

V/m) Ee (V/m2.7 8.1 9.5 21.7 0 5.7 1.7 19.0

cavities, desie electric fieldve to keep inresponsible fo

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t Saclay. The acid through arameters migieve satisfactoOL allows

ns. The studiedrs such as gas face. Once thl be comparedprocess.

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out with the finl’Essone” in th

athode shapeAt the cavity

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ble2 below.

e equator of thtions.

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USION guration is weledicated set-up

VEP will ban automated

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investigatingd cases did noforming at th

he first cavitied with models

NTS the Europeanvity under thmber 227579)nancial supporhe frame of th

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[1] K. Sa217 1

[2] E.KaWork2007

[3] R.L. InternIthac

[4] Z.A.InternBerlin

[5] J. Plobeta=confe

[6] M. BInternBeijin

[7] N. StInternBeijin

[8] C.E.WorkGerm

[9] A. WWorkGerm

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Geng, et alnational Worka, NY, 2005, TConway et

national Workn, Germany, 2ouin et al., “O=1 cavity forerence. Bruchon et anational Workng, China, 200teinhau-Kuehlnational Workng, China, 200Reece in Proc

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Documents

Vertical Electro-Polishing at CEA Saclay: Commissioning of ... · VEP) to be more appropriate. For this reasons, a VEP set-up has been designed at Saclay for the electro-polishing