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Performance of MSU Niobium Single Cell Cavities ( Proton Driver, Half Re-entrant) after Post-Purification Heat Treatment at Jlab

Jlab-TN Performance of MSU Niobium Single Cell Cavities ( Proton Driver, Half Re-entrant) after Post-Purification Heat Treatment at Jlab Introduction P. Kneisel The SRF group at MSU had been collaborating
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Jlab-TN Performance of MSU Niobium Single Cell Cavities ( Proton Driver, Half Re-entrant) after Post-Purification Heat Treatment at Jlab Introduction P. Kneisel The SRF group at MSU had been collaborating with FermiLab on the development of cavities suitable for a proton driver [1]. During this collaboration, two single cell cavities from fine grain niobium (sheets supplied by Jlab, originating from CBMM with a specific Ta contents [2]) and two single cell cavities from large grain material were fabricated at MSU. The fine grain cavities were tested at MSU, whereas the cavities from large grain material were treated and tested at Jlab [3]. After the initial tests at MSU, Jlab was asked to evaluate the cavity performance after post-purification heat treatment at 1250 C. In addition, the SRF group at MSU had developed a half-reentrant cavity [4] with an optimized ratio of H peak /E acc and after testing at MSU the interest in evaluating the performance of this cavity after post purification arose and Jlab was asked to carry out these evaluation testing. This tech. note reports about the results from these experiments Cavity Treatment Prior to post-purification of the cavities at 1250 C for 3 hrs in a Titanium box they were ultrasonically degreased in a water/detergent solution for app. 30 min. : Performance of Proton Driver cavities The treatment after the post-purification and the cavity performances are summarized in Table1: Cavity#/Test# BCP HPR Q (2K,E max ) E acc,max [MV/m] B peak [mt] 1/1 ~ 40 µm 1 hr 2.7x (Q, FE) 112 1/2 ~ 10 µm 1 hr 6.5x (Q,QD) 125 2/1 ~ 40 µm 1 hr 4.2x ( Q,FE) 119 2/2 ~ 10 µm 1 hr 6.2x (Q,QD) 131 Table 1: Performance of Proton Driver Cavities after Post-Purification: Q=Quench, FE = field emission, QD = Q-drop Figure 1 shows Q vs E acc for the best performance of each cavity and in figure 2 the Q- value is plotted vs the maximum surface magnetic field. Fine Grain PD Single Cell Cavities after Post-Purification 1.0E+11 Cavity #1 Cavity #2 Qo 1.0E+10 No FE, Q-drop Quench at 23.8 MV/m Quench at 24.9 MV/m 1.0E Eacc [MV/m] Figure 1: Q vs E acc for both Proton Driver Cavities Fine Grain PD Single Cell Cavities after Post-Purification 1.0E+11 Cavity #1 Cavity #2 Q0 1.0E+10 Quench 1.0E Hpeak [mt] Figure 2: Q vs H peak Due to the post-purification heat treatment some improvement in cavity performance could be achieved: cavity #1 reached prior to the heat treatment a gradient of E acc ~ 18.5 MV/m and cavity # 2 was limited to E acc ~ 16 MV/m [1]. In both cases the Q-values were degraded by field emission. This could be avoided in the Jlab tests and the decrease Q- value above H peak ~ 110 mt was solely due to the Q-drop phenomenon. The quench fields of 125 mt and 131 mt (see table1) are slightly lower than the performances measured on the large grain proton driver cavities [3], namely 135 mt and 148 mt. In the second test of cavity #2 the temperature dependence of the surface resistance was measured and fitted with the BCS theory; the following material parameters came out of this analysis, consistent with typical parameters for unbaked fine grain material (see below, half re-entrant cavity). Critical temperature 9.25K [fixed] London Penetration depth : 32 nm [fixed] Coherence length: 62 nm [fixed] Residual resistance: ( ) nω [fitted] Mean free path: ( ) nm [fitted] / k T c : ( ) [fitted] Performance of Half Re-Entrant Cavity In a recent test of a ILC Low Loss cavity at Jlab, the cavity deformed significantly in the cold test after a 1250C post-purification heat treatment had been applied. Therefore, the half-reentrant cavity was constrained in an arrangement as shown in figure 3, consisting of two stainless steel spiders mounted on the top and bottom of the beam tube flanges and connected to three titanium rods. This was done in the first 2 tests (see table2) despite the fact that stability calculations had indicated that the cavity would be stable even after the heat treatment [5]. The assembly of the cavity with the stiffeners was quite difficult, e.g. the input-coupler had to be assembled last, and as the first two tests showed, a contamination-free assembly could not be accomplished. Therefore, in test #3 the cavity was tested unconstrained. Test # BCP Q(2K,E max ) E acc,max B peak [mt] Comment [MV/m] 1(C ) ~ 40 µm 6.2e FE, aborted 2 (C ) ~ 10 µm 4.2e FE, aborted 3 (FH ) ~ 10 µm 1e Quench,no FE Table 2: Summary of Tests with the Half Re-entrant Cavity ( C = constrained, FH = free hanging) Ti- Rod Figure 3: Constrained Cavity SS Spider In test #1 the temperature dependence of the surface resistance was measured between 4.2k and 2K at low power and the data were fitted to the BCS theory, resulting in the following material parameters: Critical temperature 9.25K [fixed] London Penetration depth : 32 nm [fixed] Coherence length: 62 nm [fixed] Residual resistance: ( ) nω [fitted] Mean free path: ( ) nm [fitted] / k T c : ( ) [fitted] Figure 4 shows the T-dependence of the surface resistance R(T) - Test #1 9.01E E E E-07 R [ Ohm] 5.01E E E E E E /T [1/K] Figure 4: Temperature Dependence of the Surface Resistance The pressure sensitivity of the free hanging cavity is shown in figure 5 and the Lorentzforce detuning is shown in figure 6. The frequency shift due to He pressure changes is ~ 318 Hz/ mbar and the Lorentz-force detuning coefficient was 6.07 Hz/(MV/m)^2 Half Re-entrant cavity, unconstraint y = x Frequency [MHz] He - Pressure[mbar] Figure 5: Pressure Sensitivity MSU - half Re-entrant Cavity Lorentz Force Detuning delta f [Hz] y = x Eacc^2 Figure 6: Lorentz Force Detuning of Half Re-entrant Cavity Figure 7 displays the dependence of the Q-value on accelerating gradient. The cavity was free of field-emission and quenched at an accelerating gradient of E acc ~ 35.3 MV/m, corresponding to a peak magnetic field of B peak ~ 125 mt. The corresponding peak surface electric field is E peak ~ 85 MV/m. The slight degradation of the Q-value above ~ 32 MV/m is most likely the onset of the high field Q-drop MSU - Half Re -entrant Cavity 1.0E+11 Qo 1.0E+10 Quench at Eacc = 35.3 MV/m Hpeak = 125 mt 1.0E Eacc [MV/m] Figure 7: Performance of the Half- Re-Entrant Cavity in Test #3 Summary All three cavities showed an improved performance of app. 30 % after the postpurification heat treatment and a removal of µm of material from the surface by buffered chemical polishing. In all cases the cavities were limited by quench and no field emission was observed in the final performance tests. All cavities reached consistently peak surface magnetic fields of B peak ~ mt. These values are slightly lower than the quench fields measured with the proton driver cavities made from large grain material [3]. References [1] W.Hartung et al.; Linac 2006, paper THP076 [2] P. Kneisel et al; PAC 2005, paper TPPT 075 [3] P. Kneisel, G.Ciovati; Jlab Technical Note TN [4] M. Meidlinger et al.; MSU Report March 26, 2007 [5] W. Hartung, private communication
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