Bosland, P

Chel, S

Devanz, G

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EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 1 -            Delivery of the 704 MHz HIPPI cavity  P. Bosland, S. Chel, and G. Devanz.  CEA/Saclay F-91191 Gif-sur-Yvette   Abstract  The so-called Cavity B of the CARE/HIPPI program is a 5-cells 704.4 MHz elliptical cavity with a beta=0.47. This cavity was fabricated by ACCEL and delivered on March 27th 2007 at Saclay. This report documents the cavity delivery and the first measurements.    EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 2 - Introduction The so-called Cavity B of the CARE/HIPPI program is a 5-cells 704.4 MHz elliptical cavity with a beta=0.47. As the coupling port should host a 100 mm diameter power coupler, the beam tube has been widen on the coupler side to a 130 mm diameter in order to reach Qex values below 106. Optimization of the geometry led to a symmetric design (130 mm diameter beam tubes ending by 80 mm flanges) which limits the number of modes excited by a modulation frequency of the Lorentz forces. Moreover, using two series of stiffening rings greatly improves the mechanical behaviour of the cavity while maintaining a wide tuning range. Their respective radii have been optimized in one hand for lowering the static Lorentz force coefficient (KL) of low frequency mechanical modes, and in the other hand for limiting the cavity stress to acceptable values (40 MPa).  This cavity was fabricated by ACCEL and delivered on March 27th 2007 at Saclay.    Figure 1: Cavity B ready for surface preparation and qualification test in vertical cryostat    First mechanical measurements   The cavity was mounted on the tuning bench for mechanical and RF measurements. The cavity has been internally inspected using an endoscope to check the quality of the electron beam welds of all irises and equators. All the welds are visually soft and of even width.  The total length, cavity unconstrained is 831.6 mm instead of 832.6 mm.  EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 3 - Cells outer diameter Only outer dimensions can be measured. Due to the presence of the weld at the equators, it is uneasy to measure the cells outer diameter. The measurement at the weld position is however giving some insight on what could be the value of the inner diameter. The measurements have been made at the center of the weld, which is its larger dimension. From now on, the cell numbering begins by the cell close to the power coupler port, which is labeled 1.   Stiffening rings Outer stiffening rings are labeled 1-2, 2-3, 3-4 and 4-5 according to their adjacent cell labels. Their outer diameter has been measured using two methods (this dimension should be 228.4 mm).   1-2 2-3 3-4 4-5 Diameter method 1 vertical measurement (mm) 227.81 228.14 227.99 228.64 Diameter method 2  vertical measurement (mm) 227.86 228.14 227.86 228.62 Diameter method 2 horizontal measurement (mm) 228.1 227.74 227.70 227.82   One obvious difference from the prescribed dimension is the diameters of the holes in the stiffening rings, 10 mm instead of 8 mm for the outer rings and 4 instead of 8 for the inner ones. This issue prevents the cavity tuning bench from being used without modifications, and delays the operation of field flatness and frequency adjustment, while parts are being reworked.   RF measurements at room temperature   The fundamental passband was measured by ACCEL and communicated to us, so was the field profile measurement of the pi mode. The main concerns are a shift of the frequencies towards lower values by approximately 4 MHz, an uneven field profile on the pi mode : a factor of  4 exists between the field amplitude in the highest field and lowest field cells  These measurements have been repeated upon arrival of the cavity at Saclay. These new measurements exhibit very similar results owing to the fact that temperature may have been different for the two different series of data. Table 1 displays the fundamental passband frequencies.    Cell # 1 2 3 4 5 Outer diameter at weld center (mm) 380.60 380.62 380.57 380.72 380.52 EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 4 - mode Measured frequency @300K @ Saclay [MHz] Measured Q0 @300K @ Saclay Measured frequency @300K @ Accel [MHz] Pi/5 691.450000 7350 691.450000 2 Pi/5 693.495312 7460 693.484 3 Pi/5 696.007813 7480 696.001 4 Pi/5 698.182812 9700 698.156 Pi 699.770313 7290 699.752 Table 1: fundamental passband mode frequencies   The corresponding theoretical frequencies are recalled in the next table. The first column corresponds to the standard operating conditions.  Table2: fundamental band mode theoretical frequencies  The coefficients needed for different temperature and preparation stages are: the thermal shrinkage of Nb from 300K to 2K which is 0.146%  the effect of changing the dielectric from vacuum to air is taken into account by multiplying the frequency by 0.9997, which produces a change by about -200 kHz the effect of the chemical etching is mode dependent, but always tends to decrease the resonant frequency no pressure effects are taken into account in this table  The main conclusion is that the pi mode frequency is 4.171 MHz below its nominal value of 703.941 MHz when measured at room temperature, in air. This discrepancy cannot be explained by the difference in cavity length, which would (for an even distribution of the pi mode) account for a difference of only 300 kHz.  The measured quality factor of the cavity on the pi mode is 7290 at room temperature (theoretical Q is 7420 for Nb = 15 10-8 .m)  Field distribution on the Pi mode A bead-pull measurement has been carried out to obtain the electric field distribution in the cavity. The setup is represented in figure 2. The perturbing object is a metallic cylinder, which is 5 mm in length, 2 mm in diameter.  mode Freq. @ 2K          after 200 m etching [MHz] Freq. @ 300K      after 200 m etching [MHz] Freq. @ 300K, air, after 200 m etching [MHz] Freq. @ 300K, air, as delivered [MHz] Pi/5 696.061 695.045 694.836 695.975 2 Pi/5 698.580 697.560 697.351 697.785 3 Pi/5 701.486 700.462 700.252 701.053 4 Pi/5 703.669 702.642 703.458 703.181 Pi 704.472 703.443 704.261 703.941 EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 5 -  Figure 2: bead-pull setup on the cavity tuning bench  The field measurement is shown in figure 3. The field distribution is uneven, the ratio between the field in the first and forth cell is 3.5.   Figure 3 : Measured electric field distribution in the cavity   EU contract number RII3-CT-2003-506395 CARE-Report-2007-027-HIPPI   - 6 - Next measurements  The tuning of the cavity will start in May 07, upon arrival of the new parts of the tuning bench compatible with the 10mm holes of the stiffening rings.  Once the field flatness and cavity frequency are correctly tuned, 150 microns of the cavity inner face will be removed by chemical polishing with FNP 1-1-2 acid mixture. Then the cavity is rinsed with ultra-pure water and heat-treated at 650 °C for 24 hours (June 07).  New RF measurements have to be done in order to insure that no degradation occurred during the previous preparation steps. After the field profile and the mode frequencies of the fundamental band have been checked, the cavity will go through the last preparation steps : 20 microns chemical polishing and high pressure rinsing.  The cavity is then ready to be equipped with RF antennas in clean room and to be tested at cryogenic temperature in vertical cryostat (summer 07).        Acknowledgements We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395) 

delivery of the 704 MHz HIPPI Cavity

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