The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of molybdenum and tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with short pulses (16 ns) in the CLIC Test Facility at CERN. At 30 GHz these structures reached gradients of 150 MV/m for tungsten and 195 MV/m for molybdenum. These experiments were designed to provide data on the dependence of rf breakdown on pulse length and frequency. This paper reports in particular on the high-gradient test of the tungsten-iris structure. At the shortest possible pulse length of 22 ns a gradient of 125 MV/m was reached at X-band, 20 % lower than the 150 MV/m measured at 30 GHz in the CLIC Test Facility. The pulse length dependence and the dependence of the break down rate as a function of gradient were measured in detail. The results are compared to data obtained from the molybdenum-iris experiment at X-band which took place earlier as well as to 30 GHz data

Adolphsen, C

Döbert, Steffen

Grudiev, A

Heikkinen, Samuli Tapio

Laurent, L

Rodríguez, José Alberto

Syratchev, I V

Taborelli, M

Wuensch, Walter

CERN Document Server

CERN – EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH             CLIC-NOTE-699     HIGH-GRADIENT TEST OF A TUNGSTEN-IRIS X-BAND ACCELERATOR STRUCTURE AT NLCTA  S. Döbert, A. Grudiev, S. Heikkinen, J.A. Rodriguez, I.V. Syratchev, M. Taborelli, W. Wuensch CERN, Geneva, Switzerland  C. Adolphsen, L. Laurent SLAC, Menlo Park, US  Abstract  The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of molybdenum and tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with short pulses (16 ns) in the CLIC Test Facility at CERN. At 30 GHz these structures reached gradients of 150 MV/m for tungsten and 195 MV/m for molybdenum. These experiments were designed to provide data on the dependence of rf breakdown on pulse length and frequency. This paper reports in particular on the high-gradient test of the tungsten-iris structure. At the shortest possible pulse length of 22 ns a gradient of 125 MV/m was reached at X-band, 20 % lower than the 150 MV/m measured at 30 GHz in the CLIC Test Facility. The pulse length dependence and the dependence of the break down rate as a function of gradient were measured in detail.   The results are compared to data obtained from the molybdenum-iris experiment at X-band which took place earlier as well as to 30 GHz data.    Presented at LINAC06, Knoxville, TN, US, August 21-25, 2006   Geneva, Switzerland November 2006 CERN-OPEN-2006-07220/11/2006HIGH-GRADIENT TEST OF A TUNGSTEN-IRIS X-BAND ACCELERATOR STRUCTURE AT NLCTA S. Döbert, A. Grudiev, S. Heikkinen, J. A. Rodriquez, I.V. Syratchev, M. Taborelli, W. Wuensch,  CERN, Geneva, Switzerland C. Adolphsen, L. Laurent, SLAC, Menlo Park, USA  Abstract  The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of molybdenum and tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with short pulses (16 ns) in the CLIC Test Facility at CERN. At 30 GHz these structures reached gradients of 150 MV/m for tungsten and 195 MV/m for molybdenum. These experiments were designed to provide data on the dependence of rf breakdown on pulse length and frequency. This paper reports in particular on the high-gradient test of the tungsten-iris structure. At the shortest possible pulse length of 22 ns a gradient of 125 MV/m was reached at X-band, 20 % lower than the 150 MV/m measured at 30 GHz in the CLIC Test Facility. The pulse length dependence and the dependence of the break down rate as a function of gradient were measured in detail.   The results are compared to data obtained from the molybdenum-iris experiment at X-band which took place earlier as well as to 30 GHz data. INTRODUCTION The high-gradient test of an X-band accelerating structure equipped with irises made of tungsten in the Next Linear Collider Test Accelerator (NLCTA) [1] at SLAC, is the second experiment of this kind. An identical structure with molybdenum inserts was tested previously and the results are reported in [2]. This experiment completes a series of high gradient test using very similar structures with different materials at two different frequencies. The series started with tests at 30 GHz performed in CTF II using identical structures made of copper, tungsten and molybdenum. These experiments achieved record gradients (copper: 110 MV/m, tungsten: 150 MV/m, molybdenum:  195 MV/m) at the short pulse length of 16 ns available in CTF II [3]. Inspired from this success the structure was scaled to X-band to be tested in NLCTA to provide data at a longer pulse length and insight into frequency dependence of rf breakdown. The design principle can be seen in figure 1, a washer like iris is inserted into a copper disk forming together one cell of the accelerator with the new material in the region of high electrical surface fields and copper in the region of high magnetic fields. These cells are then simply clamped together and installed in a vacuum tank for high power  Figure 1: Tungsten-iris inserted into a copper disk forming together a cell of the rf structure.  testing. A reasonable good electrical contact is established by clamping the softer copper onto the much harder    tungsten. The first X-band structure was built with molybdenum irises. As reported in [2] the result from CTF II could not be confirmed furthermore the structure conditioned very slowly and reached only a gradient of 85 MV/m at 30 ns after 700 hours of processing.  The second structure equipped with tungsten inserts is otherwise identical to the molybdenum version. The parameters are summarized in table 1. The irises were made out of the same high purity (99.95%) tungsten as for the 30 GHz version using the same supplier. The structure was assembled at CERN and shipped under vacuum to SLAC where it was installed into the NLCTA beam line without an insitu bake out.  Table 1: Structure Parameters Frequency 11.424 GHz Number of cells 30+2 matching cells Phase advance per cell 2π/3 Beam aperture  9.19 mm (constant) Group velocity, vg/c 4.6 % Fill time 20 ns E surface / E axis 2.2 Input Power for 100 MV/m Peak Gradient (first cell) 175 MW EXPERIMENTAL RESULTS 0 100 200 300 400050100150200250300350400450Pulse length (ns)Maximum surface field (MV/m)Cu 30 GHzW 30 GHzMo 30 GHzCu 11 GHzMo 11 GHzW 11 GHzThe tungsten-iris structure was high-power tested a total of almost 800 hours. The initial condition lasted about 450 hours and the rest of the time was used for measuring breakdown rates and pulse length dependence. The structure was conditioned at 60 Hz using the automated rf conditioning system developed for NLC. The structure was conditioned with rf pulses up to 260 MW of X-band power ranging in pulse length from 22 to 200 ns. The conditioning procedure was very similar to the previous tested molybdenum structure, details can be found therefore in [2]. The conditioning history is shown in figure 2. The gradient on the vertical axis is the accelerating field in the first cell of the constant impedance structure. 30 ns 50 ns 70/90/60 ns Figure 2: Conditioning history of the tungsten-iris structure in NLCTA.   The structure conditioned reasonably well with short pulses of 30 ns but the conditioning slowed down considerably while extending the pulse length. Breakdowns occurred fairly uniformly distributed over the length of the structure according to timing analysis of the rf pulses, which is somewhat surprising for the constant impedance structure with about 40% surface field variation over the length of the structure. The pulse length dependence at a constant trip rate was studied in detail revealing somewhat stronger dependence as measured previously with the molybdenum structure or with NLC-type copper structures [4]. The maximum surface field obtained as a function of pulse length is compared in figure 3 for the clamped tungsten and molybdenum structure and the fully brazed NLC structures. While the copper and molybdenum structure show a reduction of the obtainable surface field proportional to Es ~ τ -1/6 the tungsten accelerator shows quarter-root (Es ~ τ -1/4) dependence fitting the full pulse length range. The data would be as well consistent with the τ -1/6 behaviour and a step at 30 ns. The results obtained at 30 GHz for the different materials are also included as single points at 16 ns. The results fit very well for copper indicating no significant frequency  Figure 3: Pulse length dependence of the maximal surface field for different structures. dependence.  They are also consistent for tungsten where at 11 GHz a gradient of 125 MV/m was achieved at a pulse length of 22 ns. This gradient corresponds to the limit of the available rf power (~260 MW) in the test station in NLCTA. The result for Molybdenum on the other hand falls significantly short of what was achieved at 30 GHz.  A comparison of the breakdown probability as a function of accelerating gradient is shown in figure 4. for different structures. It is eye catching that the slopes for copper are roughly twice as steep as for refractory metals independent from frequency. If this is a material property or due to construction or preparation techniques is an important question to answer. For a linear collider a breakdown probability below 10-6 is needed therefore the useable gradient for time being is still better for copper than for molybdenum or tungsten. The curve for copper 11 GHz consist out of a measured data point at 50 ns using a NLC prototype accelerator but the slope indicated by the line was measured at 400 ns [4].  40 60 80 100 120 14010-810-610-410-2100Breakdown ProbabilityPeak gradientMo 60 ns 30 GHzCu 70 ns 30 GHzW 70 ns 11 GHzMo 50 ns 11 GHzCu 50 ns 11 GHz Figure 4: Break down probability as a function of gradient, for different structures. CONCLUSIONS The high power test of the tungsten-iris structure at X-band didn’t show as spectacular performance as at 30 GHz. The gradient achieved is only slightly higher at short pulse as for comparable NLC-type structures made out of copper. In addition at acceptable breakdown rates the gradient is lower and the pulse length dependence less favourable for the tungsten structure compared to copper. The obtained results are nevertheless consistent with what was achieved at 30 GHz taking into account the maximum power available at short pulses. Extrapolating to 16 ns using the measured pulse length dependence results in a gradient of 135 MV/m which is within 10 % of the 150 MV/m achieved with tungsten at 30 GHz. Post mortem analysis of the tungsten irises, made at SLAC, revealed a similar picture as for the 30 GHz version (see figure 5.). The first high gradient iris (iris 2) has clear traces of heavy rf activity and local melting. The signature of heavy conditioning diminishes as one goes along the structure. On iris 15 for example the machining marks still dominate the SEM picture. The first iris is a matching iris with a bigger diameter and a much lower surface field therefore the amount of local melting is much less than on iris 2. The findings of the post mortem inspection indicate that most of the breakdowns occurred in the first third of the structure which is somewhat contradictory to the online determination of the breakdown location using the rf signals. Carbon and aluminium particles were found on the surfaces which could remain from the polishing process applied to machine the tungsten irises to the required precision. It is possible that these particles are sources of breakdowns although there are no obvious visible correlations. Micro cracks were observed as well on the conditioned surface confirming the observations on tungsten at 30 GHz at CERN.   Figure 5: SEM pictures from the coupling iris (Iris 1) the first high gradient Iris (Iris 2) and an iris from the middle of the accelerator. The fact that results are consistent for copper and tungsten comparing 11GHz and 30 GHz at short pulse length suggests the conclusion that the inconsistently bad result for molybdenum is not systematic. It is therefore likely that something went wrong during the construction and preparation of the molybdenum structure. The molybdenum structure was sent to SLAC under vacuum but arrived vented therefore a hot nitrogen bake was performed in situ after installation to improve the vacuum. The tungsten structure instead arrived still under vacuum and was tested without any bake out. It could be that this difference is responsible for the very different performance.  It turns out that refractory metals can be conditioned to high gradients even in excess of copper results but need much more conditioning time and have a higher breakdown rate compared to traditional copper structures. If it turns out that the shallower slope is a property of refractory metals than it might be that one can’t capitalize on a higher gradient achievable during conditioning. Therefore it is important to find out if the different slopes are a characteristic of the bulk material or due to surface preparation or assembly technique. An origin for the different slope could be the clamping technique used to build the structure. Decolourisations were found around the clamping joints during the post mortem analysis Experiments to answer these questions are already under way at CERN and SLAC.  SLAC launched a campaign to prepare “copper like” surfaces for tungsten and molybdenum and test their high gradient behaviour. The CLIC group is currently testing a new structure design (HDS [5]) which has no clamping contact in areas with rf currents. This new type of structure will be tested with different materials at 30 GHz at CERN using CTF3 as well as scaled versions at 11 GHz in NLCTA at SLAC continuing the fruitful collaboration on these high gradient issues.   REFERENCES [1] C. Adolphsen, “Normal-Conducting RF Structure Test Facilities and Results”, Proc. Particle Accelerator Conference 2003, Portland, Oregon, USA (2003), 668 [2] S. Döbert et al. “High Gradient Test of a clamped, Molybednum Iris, X-band Accelerator at NLCTA”, SLAC-PUB-10551, 2004[3] H. Braun et al. “A Demonstration of High Gradient Acceleration”, Proc. Particle Accelerator Conference 2003, Portland, Oregon, USA (2003), 495 [4] S. Döbert et al. “High Gradient Performance of NLC/GLC X-band Accelerating Structures”, Proc. Particle Accelerator Conference 2005, Knoxville, Tennessee, USA (2005), 372 [5] A. Grudiev et al, “A Newly Designed and Optimized CLIC Main Linac Accelerating structure”, CLIC-Note 601, (2004)  

High-Gradient Test of a Tungsten-Iris X-Band Accelerator Structure at NLCTA

High-Gradient Test of a Tungsten-Iris X-Band Accelerator Structure at NLCTA

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