SARAF Phase I linac in 2012

Transcription

1 Home Search Collections Journals About Contact us My IOPscience SARAF Phase I linac in 2012 This content has been downloaded from IOPscience. Please scroll down to see the full text. ( View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: This content was downloaded on 05/06/2014 at 15:21 Please note that terms and conditions apply.

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: December 21, 2013 REVISED: February 24, 2014 ACCEPTED: April 5, 2014 PUBLISHED: May 23, 2014 TECHNICAL REPORT SARAF Phase I linac in 2012 L. Weissman, 1 D. Berkovits, A. Arenshtam, Y. Ben-Aliz, Y. Buzaglo, O. Dudovitch, Y. Eisen, I. Eliahu, G. Feinberg, I. Fishman, I. Gavish, I. Gertz, A. Grin, S. Halfon, D. Har-Even, Y.F. Haruvy, D. Hirschmann, T. Hirsh, Z. Horovitz, B. Kaizer, D. Kijel, A. Kreisel, G. Lempert, Y. Luner, E. Reinfeld, J. Rodnizki, D. Rubin, Y. Shapiro, G. Shimel, A. Shor, I. Silverman, M. Wolf and E. Zemach Soreq NRC, Yavne 81800, Israel leo.weissman@gmail.com ABSTRACT: This report outlines the status of beam operations at the SARAF accelerator during Performance of various accelerator subsystems, their limiting factors and the recent improvements are presented. The accumulated experience of proton beam operation is summarized. Future prospects are discussed. KEYWORDS: Accelerator Subsystems and Technologies; Accelerator Applications 1 Corresponding author. c 2014 IOP Publishing Ltd and Sissa Medialab srl doi: / /9/05/t05004

3 Contents 1 Introduction 1 2 Status of accelerator components EIS/LEBT New developments Beam studies RFQ MEBT PSM 12 3 Beam operation Thin foil target experiment Beam studies Pin beam dump target New beam line equipment 24 4 Summary 25 A Statistics of the accelerator related tasks 26 B Statistics of electrical power disturbances 27 C Phase I target room 28 1 Introduction It was realized earlier that many details associated with operation of the SARAF accelerator and the status of its subsystems are known to a limited number of persons and could not be even highlighted in the narrow frame of conference proceedings [1, 2]. The purpose of the previous, the present, and the future reports is to preserve these details for the SARAF team and the broader community. In 2012 the focus of beam operation was on the thin-foil target experiment. Approximately 150 hours of net time on these targets and on the beam dump was collected, including more than 40 hours on one of the thin foil targets. The goal of this experimental program was in part achieved and further progress in this direction is expected at the later stages. In parallel to the beam operation, numerous activities, directed towards improvement of the accelerator performance, took place during the year. Most of those activities and the status of the accelerator subsystems are described in the report. 1

4 Figure 1. The boron-nitrate parts around the exit orifice of the ion source chamber (left). The same view after replacement of the parts (right). Diameter of the larger boron nitride part is 98 mm. Three major factors which hindered beam operation in 2012: lack of technical and engineering resources, lack of target room infrastructure and an anomalous number of electrical power disturbances. More details on these subjects can be found in the appendices. In addition to the major factors above, we had several technical problems associated with the ion source, the cryomodule, the helium liquefier plant and the RFQ-RF amplifier. One of the objectives in 2013 is reaching 500 hours beam on targets. Only continuous improvement of the accelerator and its subsystems, as well as, solving the above mentioned factors will allow us to meet our objectives in Status of accelerator components 2.1 EIS/LEBT New developments Ion source maintenance. Two failures occurred with the ECR ion source during the year. The first was the failure of the magnetron power supply. The unit was sent to the manufacturer for repair and returned to operation. A more serious problem developed by the end of the year when strong instabilities in the ion source performance took place. The ion source was taken apart. Inspection of the ion source showed significant erosion of the boron nitride lining parts due to accumulated plasma effects (figure 1). The parts were replaced by new ones. This maintenance operation was expected after 3000 hours of ECR beam operation (Accel s estimation [4]). The ion source performed well after this operation. It is important to emphasize that this operation was done for the first time by SARAF staff and will probably be repeated in the future according to necessity. Stock of the boron nitride parts was organized in house. 1 General layout of the EIS/LEBT system can be found for example in [5, 6]. 2

5 Figure 2. Falling edge of a 5 kv chopper high voltage (blue trace) is compared with a MSS interlock signal (yellow trace). The falling edge of the MSS interlock was generated at a 3 kv threshold. Monitoring of the chopper high voltage. A prototype of the SPIRAL2 beam slow chopper was introduced into the LEBT beam line in 2011 [5]. The main factor associated with the chopper operation was its integration into the Machine Safety System (MSS). In 2012 we have modified the original electronic unit and introduced a fast monitoring HV system which provides the HV failure signal within a 50 ns period (figure 2). The next step will be introduction of the HV monitoring signal into the MSS FPGA controller. Introduction of the slow chopper to operation is one of important objectives for the near future. New LEBT blocker. A new water cooled beam blocker was introduced at the end of the LEBT section (figure 3(a) and table 1). It replaced the previous blocker that was not cooled and could not sustain the full beam power. There are three possible positions of this element: beam blocker in, beam blocker out and collimator in. Introduction of the new beam blocker allows for the first time operation in the EIS mode, i.e. completely independent operation of the low-energy part of the injector. Operation in this mode significantly facilitates work with the ion source. The largest possible diameter of the collimator is 20 mm. This diameter corresponds to one sigma beam size at the LEBT tune with the best RFQ matching conditions in accordance to the beam dynamics calculations (figure 3(b)). Measuring the beam current (non-suppressed) collected on the collimator provides an additional diagnostic tool for optimizing RFQ transmission. It was interesting to study an effect of application of voltage on the collimator. As it is seen in figure 3(b) introduction of a hundred volts of negative bias improves the transmission through the RFQ. It is not clear whether the effect is due to the additional focusing or due to change in the beam space charge compensation caused by repulsion of the electrons at the RFQ entrance. A new dedicated electron suppressor at the RFQ entrance end flange was designed and orders for fabrication were submitted. New Titanium desorption arc pump. The problem of diffusion of hydrogen along the injector beam line was appreciated earlier. It became evident that the designed pumping capacity in the EIS and the low-energy part of the system was not adequate. A titanium arc pump was installed next 3

6 Figure 3. (a) The new LEBT beam blocker; (b) Position of the beam blocker is compared with the beam dynamics simulations; the collimator diameter is 20 mm; (c) Current measured after the RFQ as a function of the bias applied on the collimator. Diameter of the collimator is 20 mm. Table 1. LEBT blocker/collimator technical data. Parameter Value Total heat load 700 W Max. heat load density 100 W/cm 2 Aperture diameter Set of replaced inserts from 0 (stopper) to 20 mm Body material Oxygen free Copper Insert material 99.99% Pure Aluminum to the ion source for testing. This modification improved the base vacuum in the EIS and LEBT sections and reduced the hydrogen signal at the entrance to the cryomodule by a factor of two. The effect of the arc pump is illustrated in figure 4. The log of a residual gas analyzer (RGA) installed at the MEBT chamber is shown for several hydrogen gas flow values. Plasma of the ion source was not switched on during these measurements. The arc pump discharge took place every 6 minutes at the conditions presented in figure 4. The valve on the arc pump was closed for a few seconds during arc discharge in order to protect the beam line from the evaporated titanium. Closure of the valve corresponds to the spikes in the hydrogen signal in the RGA log (figure 4(a)). More detailed report on the arc pump performance and its influence on the LEBT beam optics will follow Beam studies The beam-optics studies in the LEBT section were performed in 2012 at any available opportunity. The main objectives of these studies were to resolve some long standing questions associated with 4

7 Figure 4. (a) RGA log in the MEBT chamber; (b) values of hydrogen signal measured by the RGA with and without the arc pump for several values of the gas flow in the ion source. intense low-energy beams. Some of these questions are listed below: 1. understanding effects of the beam aperture; 2. understanding neutralization effects by introduction of gas into the beam pipe; 3. effect of beam multi-scattering by residual gas molecules; 4. measuring beam neutralization via comparison of the profile and emittance measurements; 5. effect of secondary electrons on emittance measurements; 6. measuring free electron density in the beam environment [6]; 7. relations between the LEBT beam optics and transmission through RFQ; 8. understanding beam optics of the chopped beam. Most of these issues are not yet resolved due to lack of dedicated beam operation time. Some examples are given below to provide impression on the complexity of the physical phenomena in the LEBT section. Residual gas multi-scattering vs. neutralization. Neutralization of low-energy intense beams by introduction of gas into the vacuum pipe is well-known technique used to reduce the spacecharge effect. A few emittance measurements at poorer beam line vacuum ( 10 5 mbar) did not show improvement of the emittance. On the contrary, a slight increase of the emittance was observed. Emittance measurements with a slits-wire arrangement are time consuming and analysis of the results is complicated. We have tried recently another technique where a series of profiles measurements was performed as a function of current of the second LEBT solenoid (the one before the profiler). The measurements yield parabola-like curves that characterize the beam emittance. The minimum of the parabola corresponds to the smallest possible size of the beam and has to be compared to the RFQ entrance diameter. An example of such measurements of a 3 ma pulsed beam with and without addition of residual argon gas, is shown in figure 5. One can observe in the figure that introduction of the gas results in a larger minimum beam size and, hence, larger emittance. 5

8 Figure 5. Size of a 3 ma beam as a function of the second solenoid current at standard conditions ( mbar, blue curve), and with addition of argon gas in the LEBT vacuum system ( mbar, red curve). Figure 6. The size of the beam as a function of the second solenoid current, for different values of the aperture diameter. It is possible that at the SARAF conditions (a few ma beam current), the beam multi-scattering by the residual gas prevails on the beam neutralization effect and, causes emittance growth. Further studies of influence of the residual gas pressure on the LEBT beam optics will be performed. Effect of the LEBT aperture. The LEBT variable aperture is actively used during beam operation as it allows for variation of the beam intensity. There are a number of questions regarding the aperture beam optics effects. A series of profile measurements as a function of the second solenoid current were performed for different aperture values (figure 6). It is interesting to note that closure of the aperture results in migration of the curve minimum to the higher solenoid current values, which suggests an effective defocusing effect at the aperture closure. This effect is, probably, associated with beam scattering on the aperture. At the moment, we cannot confirm the observed behavior by simulations. Further studies of this effect are in progress. 6

9 Figure 7. (a) RMS normalized emittance value as a function of the suppression coils current; (b) phase space distribution for zero coils current; (c) phase space distribution for 6 A current on the suppression coils. Unexplained emittance measurement results. The results of emittance measurements performed by a slit-wire setup are not always easy for interpretation. In addition to the ion component there is a component of energetic neutrals produced via charge-exchange reactions. Distribution of the neutrals on the phase diagram is different as they are not affected by the second solenoid. Some light emission was observed from the slit region during emittance measurements with an intense DC beam. This observation indicated poor vacuum and, even, local weak plasma in this region during the measurement. A question arose whether the unusual conditions in the slit region distort results of emittance measurements. For example, emission of secondary electrons from the slit graphite surface may cause distortion of the phase space distribution. To verify this question a weak vertical magnetic field was created in the slit plane by Helmholtz coils wound around the slit cross. Application of a current of a few Amps on the coils created a weak vertical field of a few Gauss which did not affect the beam, but was sufficient to suppress the secondary electrons. Measurements of emittance performed with 5.5 ma DC beam with and without the suppression field are shown in figure 7(b,c). As it is seen in the figure, application of the suppression field resulted in significant difference in the periphery region of the phase distribution. The effect is also exhibited in significant reduction of the overall emittance (figure 7(a)). More detailed report on this study can be found in [7]. 2.2 RFQ In 2012 the accelerator operation was dedicated only to proton beams. Some work has been done to improve the RFQ ability to hold the RF power that is needed for acceleration of deuterons. However, we had not ventured an intense RFQ conditioning campaign in order not to risk the routine proton operation. Replacement of the vacuum barrier flanges. During the previous RFQ conditioning campaigns we have experienced vacuum failures of the o-rings serving as vacuum and water barriers of the RFQ cooling channels. We assumed that the problem was associated with penetration of RF field in the region between copper water cooling tubes and the RFQ barrel. A new type of water flange was designed to overcome this problem. The new design ensures good RF contact between the cooling channel and the RFQ barrel. It was achieved by introduction of Be-Cu rings in the volume between the vacuum o-ring seal, vacuum flange and the cooling channel (figure 8). The o-ring seals get squashed during fixation of the flanges leading to uniform strong pressure on the Be-Cu rings 7

10 Figure 8. The water and vacuum barriers of the RFQ cooling channels: original design (left), the new design (right). and, hence, to good electrical contact between the flanges and the cooling channels. In addition, penetrating channels across the new flanges facilitate leak tests of the vacuum seals and allows for prevention of the situation of virtual leaks in the volume between the vacuum and water barriers. All the water flanges were replaced during the spring maintenance period. So far we did not have a chance to perform tests in an intense conditioning campaign of the new water flanges and see whether this solves the experienced vacuum problems. Optical survey of the RFQ rod structure. It was suggested that possible singularities in the RFQ rod structure alignment are the reason for the experienced difficulties in the RFQ conditioning. A campaign for performing an optical survey of the RFQ rod structure was carried out during the 2012 spring maintenance period. The survey was based on: 1. establishing of a fiducial axis parallel to the rod structure using an optical telescope and a set of specially designed optical targets; 2. moving another optical targets along the structure and measuring their coordinates with respect to the fiducial axis (figure 9). The targets were designed to be placed on the bridge parts of the rods. The method could be useful only if the bridge parts were made precisely by the RFQ manufacturer (NTG). No information on manufacturing precision of the rod electrodes was available. The measurements showed significant scattering especially in the x-direction (figure 10). It was not clear whether the scattering was due to poor machining precision of the bridge parts or it was associated with real alignment. It is important to emphasize that the measurements showed a high degree of repeatability. For example, disassembly and consequent reestablishing of the optical axis led to identical results. Thus, although the method cannot be used for the present rod structure, it could be successfully implemented in alignment of the future rod structures, provided that the parts used for installation of the optical targets are produced under stringent precision demands. Ordering new modified RFQ rods is one of the possible future directions. Mechanical survey of the RFQ rod structure. In addition, we have performed a mechanical survey where the distance between external backplane surfaces of the vertical and horizontal rods were measured with a micrometer as a function of its position along the structure. Similar measurements were done two years ago. These measurements as well can yield useful information regarding the rod alignment only for the case of high manufacturing precision of the back plane surfaces. The summary of the results is presented in figure 11. As in the case of the optical survey, 8

11 Figure 9. Schematic view and pictures of the alignment procedure. Figure 10. Results of survey for the x-coordinates. Definition of the direction and the type of bridges is shown (LT, LB, RT, RB stands for Left Top, Left Bottom, Right Top and Right Bottom, respectively). Stem # 40 corresponds to the RFQ entrance. much larger deviations were observed in the horizontal (x) plane with respect to the vertical one. The deviations are of different characters than those in figure 10, indicating that the latter most likely are associated with manufacturing problems. It is worthwhile to note that smaller horizontal distance between the back planes of the first stems is due to intentional shift performed in the 2010 alignment campaign. Some irregularities in the horizontal trend are observed in the rods junction regions (stems 27 and 14). At this stage, it was decided not to take any action to improve the irregularities. Both optical and mechanical surveys campaigns did not yield any conclusion regarding the alignment. However, the experience could prove to be very useful in the future if new rods are manufactured under stringent quality control. 9

12 Figure 11. The results of the mechanical survey. Stem # 40 corresponds to the entrance to the RFQ. Diameter of the collimator is 15 mm. In addition to the mentioned above activities design of RFQ new end flanges have been performed. The new flanges will have better RF contact with the barrel and the base plate. In addition, the upstream flange will have an electrode for suppression of the electrons at the entrance to the RFQ. At this moment, the design of the new end flanges is completed; their production and installation is planned in The above listed and next future planned RFQ improvements raise some optimism regarding the prospective conditioning campaign. 2.3 MEBT New MEBT collimator and beam blocker. The major change in the MEBT was the introduction of a new water cooled beam blocker and a collimator (figure 12 and table 2). This was done via modification of the existing wire profilers in the second MEBT diagnostic chamber. The beam blocker and the collimator are situated in the same plane so only one of them can be introduced at a time. The new beam blocker allows for beam operation of the linac injector in a mode decoupled from the rest of the accelerator. Beam collected on the beam blocker is not suppressed; nevertheless this signal could be used for RFQ transmission studies as the secondary emission coefficient could be reasonably estimated. The collimator is used as a beam scraper. Use of the new collimator reduced the vacuum effects in the cryomodule (see below), indicating that it prevents interaction of the beam tails with cryogenic surfaces. Beam signal collected on the collimator proved to be an important diagnostics tool during operation. Hydrogen signal in the MEBT chamber. The problem of hydrogen diffusion along the linac injector and possible effects of its penetration to the cryomodule became a subject of concern. We 10

13 Table 2. MEBT blocker/collimator technical data. Parameter Value Total heat load 1000 W Max. local heat load 100 W/cm 2 Aperture diameter Set of replaceable inserts up to 15 mm Body material Oxygen free Copper Insert material 99.99% pure Aluminum Figure 12. New MEBT water cooled blocker and collimator based on the existing wire profilers. installed a new residual gas analyzer (RGA) on the first MEBT chamber. The position of the device is not optimal; nevertheless it proved to be very useful. Dramatic increase of hydrogen partial pressure in the MEBT region was observed at the start of the ion source gas line. Furthermore, a reduction of the hydrogen pressure was observed while opening the gate valve to the cryomodule (figure 13(a)). This indicates that hydrogen gas effectively diffuses via the linac injector and is efficiently pumped by the cryomodule. Hydrogen partial pressure was reduced along the whole injector (including the MEBT) by a factor of two after introduction of the titanium arc pump in the ion source region (figure 4). As another step, a 50 l/s ion pump installed at the second MEBT chamber was replaced by a 250 l/s turbo pump (estimated conductance of the port is only 70 l/s). This change alone reduced the base pressure in this region by 35 40%. The effect of opening of the PSM valve on the hydrogen partial pressure was reduced by an order of magnitude (figure 13(b)). Data presented in figure 13(a,b) were taken at 0.25 sccm/min gas flow into the ion source gas line, the plasma in the ion source was not switched on. Use of the MEBT collimator during beam operation leads to increase of hydrogen partial pressure in the MEBT chambers due to diffusion of the hydrogen implanted into the collimator surface 11

14 Figure 13. Log of the RGA installed at the first MEBT chamber. Effect on hydrogen partial pressure at opening of the EIS and PSM valves are shown for the original configuration (a) and after replacement of the 50 l/s ion pump by the 250 l/s turbo one (b). Figure 14. The RGA log in the MEBT chamber during beam operation. Dramatic increase of the hydrogen signal is due to a fraction of the proton beam stopped by the MEBT collimator. (figure 14). Hydrogen content in the MEBT vacuum depends on the beam intensity and tune. Part of this hydrogen diffuses into the cryomodule and produces contamination of its superconductive surfaces. Introduction of the new turbo pump only partially improved the situation but did not solve the problem. At the moment, a new MEBT chamber is under design. The main feature of the new chamber will be a getter pump with high pumping capacity which will be inserted in the chamber volume. Such a solution will improve pumping capacity in the MEBT chamber by an order of magnitude and will protect the cryomodule surfaces. 2.4 PSM Damage and repair of the superconducting solenoid leads. During the spring maintenance warming up, a significant helium leak was discovered in the cryomodule. The origin of the leak was on the PSM top flange, in the base of the current leads of the third superconducting solenoid. Examination of the leads showed signatures of heating and melting of the plastic insulators (figure 15, left). These insulators serve for sealing the helium bath volume from the atmosphere. Most likely the incident took place during operation with deuteron beams when the solenoid current was 12

15 Figure 15. The damaged insulators of the solenoids current leads (left). The sealing assembly on a mock-up test bench(right). twice higher than the values used for proton operation. The hollow leads are cooled by flow of the helium gas evaporated from the liquid helium bath. The helium flow had to be increased correspondingly for the higher solenoid currents (6 l/min instead of nominal 3 l/min). This fact was not reflected any documentation left by the manufacture company (RI, former ACCEL). Replacement of the melted insulators without taking apart the whole solenoid leads structure presented a challenging technical task that took several weeks to accomplish. A special assembly was designed and installed in order to solve the problem (figure 15, right). The assembly consisted of several plastic parts pressed together while applying uniform pressure on the helium tubes and the base flange. Sealing was done with a layer of silicon hermetic. In the future, helium gas flow will be increased every time deuteron beam is operated. At the moment, this adjustment can be done only manually. Keeping helium flow at the higher level all the time results in accumulation of ice on the top of the cryomodule central flange. Automatic He-gas flow control as a function of solenoids currents is under consideration. Installation of a number of temperature probes on the solenoid leads is in progress. Degradation and replacement of the piezo tuners. Degradation of the piezo tuners ranges was observed for the first time in After the tuners replacement the degradation phenomenon took place again in 2011, making operation of the accelerator impossible for most of the tunes. During the 2012 spring maintenance period the tuners were replaced again. After consultation with the manufacturer (Piezomechanics GmbH) it was decided to try another model of the device (Pst1000), which operates at a higher voltage and has superior mechanical strength. Originally, we planned to install only one Pst1000 tuner and observe its performance. However, during warm tests a couple of old low voltage tuners have failed. Therefore, it was decided to replace all six tuners. It has to be emphasized that the operation of piezo replacement was performed for the first time by SARAF personnel. During installation we have noticed rather large scattering in the tuner lengths. Length of each device was corrected by spacers to ensure good mechanical contact 13

16 Figure 16. Left. An improvised test bench for testing of the piezo tuners. Right. Performance of the piezo tuners before (2011, low voltage devices) and after replacement (2012 high voltage devices). with its housing and their performance was tested on a bench (figure 16). The tuners performed well after installation and the cryomodule cooling down. Their tuning ranges are monitored on a monthly basis since May of 2012 (figure 16). A tuner needs to have at least a 1000 Hz dynamic tuning range, far wider than the cavity bandwidth (130 Hz), in order to be useful. No significant deterioration was observed within the measurement uncertainty. Warming of the RF couplers. In 2012 the cavities were operated with the 4 kw RF power amplifiers built in house [8]. Operation has proven to be more stable, since the higher available RF power, compared with the original 2.4 kw supplies, provides better compensation capability against detuning. In fact, replacement of the piezo tuners together with migration to the 4 kw RF amplifiers were the major factors which improved performance of the linac in After improving the tuners and the RF amplifiers, warming of the RF couplers has become the main problem limiting the accelerator operation at high fields. The phenomenon was recognized earlier but has not been resolved yet. Thus, we were compelled to reduce the field of some cavities (2 nd, 3 rd and 6 th ) in order to keep the external coupler s temperature below 120 K during long operation. An example of the operation at the 3.9 MeV proton tune is shown in table 3 and figure 17. As it is seen in the figure, the temperatures of some couplers reach 130 K during 12 hours of operation. It is noteworthy that no significant warming was observed at the 5 th cavity coupler. This cavity could be operated at a nominal voltage value, 830 kv, just before the onset of x-ray emission. Cooling of the couplers is performed by their connecting to the cryostat thermal shield via thermal bridges made from copper braid. A change of the thermal shield temperature by a few degrees results in eventual change of temperature of the couplers, but this process takes as long as hours (figure 18). It is obvious that this cooling of the couplers cooling is not adequate. At 14

17 Figure 17. Warming up of the couplers during operation at the 3.9 MeV proton tune. Table 3. The highest HWR voltages used for long operation and their limiting factors. Nominal HWR voltage value is 840 kv. HWR Voltage Eacc Phase limiting factor # (kv) (MV/m) (deg) Used for bunching Coupler warming Coupler warming Coupler warming X-ray emission threshold Coupler warming the moment, improvement of the couplers cooling is under design. One possible mechanism of the coupler warming is associated with multipacting in the coupler region. Daily variation of the couplers warming up rates, probably, support this assumption. It is possible that application of a voltage bias on the inner coupler electrode will affect the multipacting process and, hence, the coupler warming up rate. We are building and testing a special DC breaker, which will allow us to apply DC voltage on the couplers together with the RF power. The first preliminary experience was positive, although, we could not apply intense RF current via the DC breaker prototype for a long time. New improved modification of the DC breaker is underway. Solving the problem of the couplers warming will allow one to operate the cavities at the field values corresponding to beam energies of 4.5 and 6 MeV, for protons and deuterons, respectively, improving scientific opportunities during phase-i of SARAF operation. PSM vacuum phenomena. Anomalous number of electrical disturbances during the year (appendix B) affected the cryogenic system and the cryomodule. However, it also provided new in- 15

18 Figure 18. Left: the coupler cooling arrangement. Right: thermal reaction of the couplers to a drastic change of the shield temperature. Figure 19. Examples of two different desorption curves during natural warm up of the module. The module is pumped by two ion pumps during warm up. sights on the cryomodule. The module was warmed up at its natural warming rate several times and its warming up desorption curves were measured and compared. It is interesting to note that desorption curves differ for each particular warm up. The main difference between desorption processes takes place at the beginning of warm up at the cavities temperatures of K which correspond to desorption of hydrogen. Differences in the desorption curves are due to different beam operation history and, hence, due to the different hydrogen buildup on the cavities superconducting surface. It is interesting to note that at the later stage of the warm up process (the cavities temperatures above 40 K) a phase transition like event takes place. It is exhibited in a change of warming up rate of the cavities, tuners and couplers, temporary decrease of the thermal shield temperature and dramatic increase of the pressure in the cryostat insulation volume (figure 20). The effect is, probably, related to the massive desorption of frozen contaminants from the thermal shield surfaces into the insulation volume, which results in temporary cool down of the shield and modification of the warming up rates. Increase of warming up rates is, most likely, due to onset of a convection heat exchange process in the insulation volume. On one occasion, the cavities were warmed up drastically to K due to a human mistake 16

19 Figure 20. Change of the natural warming up rates due to massive desorption in the insulation volume. The insulation volume was not pumped during the warm up process. occurred during the cryogenics recovery process. Vacuum pressure in the module jumped a few orders of magnitude during the incident (figure 21). This was probably due to massive desorption of hydrogen that was built up at the entrance of the module during beam operation. Such massive desorption and consequent absorption may result in redistribution of the hydrogen contaminants over more sensitive cryogenic surfaces of the cavities, which would lead to deterioration in their performance. No deterioration in the cavities performance (at typical, less than nominal, accelerating fields) was seen after the event. It is possible that the negative effect could be seen at the higher RF field values. According to the standard cooling down procedure, the cryomodule is cooled down from the K temperature range to the liquid helium temperature in one hour. Such fast cooling is done in order to prevent the so called Q-disease. On one occasion, the system was left in the evening in some intermediate state at a temperature above 130 K. This was done with the purpose to perform fast cooling in the next morning. However, during the night the cryogenic system parameters drifted and the module temperature decreased. Most of the night the module was in the temperature ranges susceptible to the Q-disease. No significant change in the cavities performance was observed after this particular cool down. Again the cavities were operated at the modest field values (table 3). The beam induced effects of the PSM vacuum were always the subject of concern as they might have indicated a problem of the beam optics. The general impression is that those effects were smaller in 2012 due to introduction of the MEBT beam scraper. It was noticed, though, that beam influence on the PSM vacuum is always low at the beginning of operation after a warming up/cooling down cycle. Beam related effects increased as beam operation time was accumulated. Subsequent warming up/cooling down cycle resets the intensity of the effect. An example of dramatic change of the vacuum behavior during beam operation is shown in figure 22 (see below). 17

20 Figure 21. An incident during cryogenics recovery that depicts hydrogen desorption and consequent absorption by the cold surfaces. 3 Beam operation General comment. The accelerator was used for beam studies or for delivering beams for experiments at any available opportunity. So far the most intense period of beam operation consisted of nine working days of continuous (excluding a weekend) operation. This included four days of collection of 0.25 ma, 3.6 MeV CW beam generating 4 W/mm 2 heat flux on a thin foil target, and five days of tests of beam diagnostics from GANIL with ma, 2.2 MeV CW and pulsed beams. Five cavities were used for the former beam and only three for the latter. In this period of operation we approached for the first time our vision of SARAF as a user facility. Our goal for the next years is to achieve a similar mode of continuous and intense beam operation for much longer periods and, eventually, for most of a year. Some features discussed above can be observed in the corresponding operation logs (figure 22). The warming up of the couplers during operation and their cooling overnight is shown in the top of figure 22. It is interesting to note that the warming rates for some couplers vary daily in spite of the fact that the same fields have been applied. The cryomodule vacuum pressure behavior is presented in the bottom of the figure. As it is seen in the figure the beam induced vacuum effect was very moderate until the occurrence of the event highlighted in the insert. After this event the beam induced effects became much stronger until the time of reset by the next warming up/cooling down cycle. 3.1 Thin foil target experiment As mentioned above, beam operation in 2012 was focused on the thin foil target experiment. This experiment is the first step in the future program towards production of radiopharmaceutical iso- 18

21 Figure 22. Operation logs during the nine days of continuous operation. Top. Behavior of the RF coupler temperatures. Bottom. Behavior of the cyomodule module vacuum pressure. Change in the cryomodule vacuum behavior during beam operation is highlighted in the insert. topes at SARAF. More detailed description of the experiment can be found in [9]. Thin foil targets ( 30 microns thick stainless steel) are very sensitive and in the past we experienced a few failures [3]. Significant progress was achieved this year; 3.6 MeV proton beams at the intensity of up to 300 µa were kept on targets for a period of many tens of hours. This was achieved owning to the following two factors: development and improvement of the target design and improvement of the beam diagnostics system and the tuning procedures. A quartz viewer, a thin tantalum foil viewer and two wire profilers were used for tuning of pilot pulsed beams to ensure proper beam location and density distribution on the target surface. A typical beam tuning procedure consisted of: 1. tuning of a pilot beam on the defined position using the quartz and tantalum viewers, 2. further optimization with pulsed beams using signals read from the insulated collimator, chamber body and target flange, 3. measuring beam profiles with wire scanners, and 4. calculating the beam power distribution on the target surface (figure 23(a)). Afterwards the beam was switched to CW mode at the lowest possible current and slowly ramped up. The temperature in the target region, the collected beam currents on the electrodes and neutron dose rates were monitored during operation. The temperatures were monitored using thermocouple probes and an optical pyrometer. At the first stages an infrared thermal camera was used for tuning and monitoring the beam. Later on, visual monitoring with standard CCD cameras has proved to be very useful (figure 23(b,c)). 19

22 Figure 23. (a) Calculation of beam distribution on the target based on profile measurements; picture of the thin target with a current of 50 µa; (b) and 120 µa; (c). Pattern on the foil at the higher current is due to convection in the NaK cooling liquid. The calculated beam distribution was used to estimate the highest current corresponding to the required heat flux at the peak of the distribution. The highest heat flux at which operation was stable was 4 W/mm 2. The experiment demand was to have a beam with as much as possible total power on target, filling the entire target area and a limited power on the collimator located upstream the target. The last quadrupole doublet of the beam line was, for all practical purpose, not in use. The main limiting factor was the foil surface maximum temperature that was kept below 500 o C. At the beam power density higher than 4 W/mm 2, sharp temperature increase of the target surface and of the NaK cooling liquid was observed. Further increase of the power density and total current implies additional improvement of the cooling mechanisms (convection of the liquid metal and thermal conductivity). This work is in progress. An example of a typical run for thin target irradiation is shown in figure 24. The target was irradiated for more than 12 hours. The current on the target was kept stable around 275 µa (unsuppressed). The secondary electrons from the target were collected on the insulated chamber. High current on the collimator is due to specifics of the tune (see above). Vacuum pressure in the pretarget section was kept in the 10 7 mbar scale most of the time, except for occasional spikes. Three trips occurred over the irradiation session; an RFQ trip and two trips of the target ready chain. The stability of operation has to be improved further with the goal of having continuous operation over longer periods. The cumulative irradiation time of this particular target was more than 50 hours, which approaches to the limit of the radiation damage in the target foil - one displacement per each target atom (DPA). Few days after the end of irradiation the target was emptied from its NaK coolant and the foil was tested under high gas pressure (at room temperature). The foil did not rupture and held 5 atm, which demonstrated the target s ability to sustain such beam doses. 20

23 Figure 24. A log of a thin target irradiation test. In 2012, the thin foil target experiment was the main driving force for improvement of the accelerator performance. Significant improvement in the accelerator performance and important operational experience were achieved. 3.2 Beam studies Machine operation of intense beams, especially on high heat flux targets, raises some questions regarding the accelerator beam tune and its robustness. The most important questions are: 1. whether the accelerator and beam line tune performed with a low-intensity pulsed beam is valid for intense CW beam and 2. whether the beam optics changes significantly while the beam intensity is ramped up by two orders of magnitude. The latter question is especially important for us due to the fact that beam intensity is ramped by variation of the LEBT optic parameters: variation of the aperture size and focusing of the first solenoid. In general, the cavities are tuned using two different methods: measuring beam energy by using time-of-flight (TOF) signals from two phase probes and by direct measurement of beam particles Rutherford Backscattered Scattered (RBS) on a thin gold foil. The measurement is done using a collimated Si particle detector. The former method is nondestructive, while the latter is destructive and susceptible to the dead time problems. Therefore, intensity of a beam pulse is different by two-three orders of magnitude for the two cases. We have verified several times that the two methods lead to identical accelerator tunes, and, hence, to first order, the accelerator tune obtained with low-intensity beam is valid for the higher beam intensity. An example of such cross-check is shown in figure 25 where the results of the phasing the sixth cavity performed by the two methods are compared. The results differ by kev at beam 21

24 Figure 25. Phasing of the 6 th cavity using RBS and TOF techniques. The RBS gold target is supported by a carbon foil. Thus both RBS peaks can be used for energy measurement. energy 3.6 MeV. It is worthwhile to mention that the two methods are completely independent; calibration of the particle detector is performed by a spectroscopic alpha source. Several tests were performed to study the robustness of the beam optics during the intensity ramp up. Qualitative behavior of the beam spot was studied with a quartz viewer installed just upstream of the target. We have verified that, to first order, the beam spot position does not change with variation of the LEBT aperture size or change of the first LEBT solenoid focusing current (figure 26). The dependence of the beam spot position on the other parameters such as the LEBT dipole current, the last LEBT solenoid, the LEBT steerers and RFQ forward power was also studied. Typically the quartz viewer is used for pulsed beams at very low duty cycle of 0.01% in order to avoid burning of the quartz surface. Use of quartz or tantalum foil viewers provides only qualitative information. Very limited beam intensity could be applied on the viewers which significantly limits the range of the parameters values. In 2012 we performed beam tests with a nondestructive residual gas monitor (RGM), shipped from GANIL/France in the framework of the SPIRAL2PP FP7 program. The device allows for nondestructive measurement of the beam profile by collecting residual gas ions produced by the beam. A detailed report on the test of the GANIL RGM can be found in [10]. The objectives of these tests were twofold; 1. to verify stability and robustness of beam optics of CW proton beam while its intensity is varied by two orders of magnitude, and 2. to test the performance of the RGM profiler in a wide beam intensity range. The device showed robust performance as a function of beam intensity and duty cycle. Thus, it could be also used as an intensity monitor. Some problems associated with residual gas ion collection from the periphery of the active volume were identified. The obtained information will be used for further device improvements. 22

25 Figure 26. The beam spot position as a function of the LEBT parameters used for beam intensity ramp. Top: as function of aperture diameter and Bottom: as function of the first solenoid current. Figure 27. Beam centroid (a) and beam size (b) as a function of the CW beam intensity ramped up by varying the LEBT aperture diameter. We have performed tests with CW beams that were similar to those performed with weak pulsed beams and the quartz viewer (figure 26). In these measurements we did observe significant increase of the beam size with increase of the beam intensity while varying the LEBT parameters. The highest CW beam intensity was of about 1 ma; to our knowledge residual gas monitors were never used at such intense CW proton beams. The size of the beam increased by more than 20% at the highest intensity, while the position of the beam centroid was quite stable (figure 27). Increase of the beam size at the higher beam intensity is the subject of concern, especially in the experiments with high heat flux targets. Introduction of the slow LEBT beam chopper may allow for a better intensity ramping procedure in the future. 3.3 Pin beam dump target The limitation of the existing SARAF beam dump (tungsten brazed on a water cooled copper block) was discussed in the earlier report [3]. Typically the tungsten surface gets blistered after several hours of intense beam operation. One of the ways to overcome this problem was production of a 23

26 Figure 28. Visual observation of the 3.6 MeV CW beam on the pin dump prototype. beam dump from porous heavy metal material. High porosity of heavy metal will result in efficient hydrogen diffusion and prevent formation of blistering. However, higher residual activation of the heavy metal is expected at higher beam energy. Such a beam dump was prepared in 2012 and will be tested in An alternative new concept of beam dump was proposed. The beam dump consists of grid of tungsten pins inserted in the water cooled matrix. The main heat removal mechanism is radiation of the pins heated to high temperature. The irradiated heat is absorbed by the water cooled environment. This concept satisfies the main requirements of the beam dump: high heat acceptance capacity, low prompt and residual radiation, and non-susceptibility to the blistering phenomenon. In addition, such a beam dump has some beam diagnostic properties. We have prepared and tested the first prototype and performed tests with 3.6 MeV and 2 MeV CW proton beams. The beam intensity was limited to 1 kw due to lack of efficient water cooling of the pipe section adjacent to the pin dump. Glowing of the pins could be observed with a standard CCD camera at beam power higher than 100 W. Observation of the initial glowing allows for tuning of the beam on the pin target. At the higher beam intensity, the pins on the periphery are heated up via absorption of radiation and practically the whole pin assembly starts glowing (figure 28). The highest beam density achieved for the 2 MeV beam was higher than 1 kw/cm 2. Very low prompt gamma and neutron radiation and no residual radiation were observed in the tests. The pins were inspected after the test and did not exhibit any blistering or surface erosion. A detailed report on this test can be found in [11]. At the moment an improved version of the pin beam dump with superior water cooling is being built. 3.4 New beam line equipment New beam diagnostic station. A new beam diagnostic station before the target is being assembled (figure 29(a)). The concept of the station is based on the experience accumulated during the recent year of beam operation. The station includes two wire profilers and quartz and tantalum foil viewers for monitoring pilot beams. In addition, a current transformer from Bergoz is installed for nondestructive beam monitoring. It is possible that this station will become a prototype for diagnostic stations at future SARAF beam lines. General use station. A general purpose experimental chamber was installed at a beam line section before the beam dump. The chamber is a six-way cross with additional four smaller size ports. 24

27 Figure 29. (a) New beam diagnostic chamber; (b) New general use chamber with GANIL equipment. Large number of various ports provides an opportunity of simultaneous use of different equipment with considerable flexibility. A linear motion manipulator with a load-lock chamber was installed later on. The equipment enables irradiation tests of various samples with weak proton and deuteron beams. The first use of the station was the test of the GANIL RGM profiler (figures 27 and 29(b)). We plan to launch an experimental program for measuring cross-sections of deuteron reactions. In addition, we are preparing an experiment for irradiation with low density proton beams. Beam Position Monitor (BPM). A prototype of the GANIL BPM was installed just before of the SARAF diagnostic plate (figure 30(a)) and was tested with a pulsed 3.5 MeV beam. Signals from the BPM electrodes were displayed directly on a fast scope (figure 30(b)). It was possible to deflect the beam with a steerer placed before the BPM section. The dependence of the BPM signals on the steerer current was measured and compared to the results of the profile measurements performed with the wire scanners positioned almost 2 meters downstream. The results of the measurements are shown in figure 30(c) and (d). The signals behavior is consistent with steering in the x-direction. This work was performed in the framework of the SPIRAL2PP FP7 program. Beam Loss Monitor (BLM). We are currently at the final stage of preparation of a Beam Loss Monitor (BLM) network. The network includes a number of Geiger Muller (GM) detectors with preamplifiers and electronic units that provide pulses with rate proportional to the radiation intensity. Five of these detectors will be placed in front of the RFQ viewports and twelve others will be distributed along the cryomodule and the selected position of the beam lines. Implementation of the BLM network will be a significant step in improving beam tune possibilities. 4 Summary In 2012 the beam operations were dedicated to thin foil targets tests and significant progress was achieved in this direction. Improvements and modifications were introduced in practically all the components of the SARAF linac. 25

28 Figure 30. (a) The SPIRAL II BMP prototype installed at SARAF; (b) signals collected from the BPM electrodes; dependence of the BPM signals (c) as a function of steerer current is compared with profile measurements (d). We have demonstrated again that even at the present stage, the SARAF facility has potential of becoming a user facility with intense beams, viable scheduling, and high beam availability. However, this goal cannot be achieved without ensuring more adequate technical and engineering support, better electrical and target room infrastructure and further improvement of the accelerator subsystems and development of beam diagnostics. Improvements in all these directions are in progress. A Statistics of the accelerator related tasks The lack of engineering and technical resources at SARAF was emphasized in many occasions. In order to get better insight to this situation, we started to follow statistics of the accelerator related tasks (figure below). The numbers of pending tasks, tasks in progress and the solved tasks are shown as a function of time. The tasks include fixing of failures, maintenance, and improvement of accelerator subsystems. The tasks are not weighted in this simple statistical tool: the task which requires few hours of work is counted with the same weight as the one that requires major resource and time allocation. Nevertheless, the information is very useful. It is clear from figure 31 that the number of tasks associated with such a complicated system as an accelerator grows at the constant rate. An important parameter which reflects the performance of the SARAF team is the ratio of solved to total tasks ( performance parameter ). The performance parameter is around 30% only, indicating 26

29 Figure 31. Top: statistics of the accelerator related tasks. Bottom: evolution of the performance parameter. need for more man power dedicated to these issues. A statistical tool allows for analysis of the weakest points. For example, a cut of the tasks related to the accelerator control system yields a performance parameter value of 20% only. In the following years we should strive to improve the performance parameter and sustain its modest positive trend. B Statistics of electrical power disturbances Distribution of electric power incidents over the 2011 and 2012 years is compared in figure 32. In 2012 we experienced anomalously high frequency of electrical power disruptions creating significant problems with the cryogenics system and, hence, hindered accelerator operation. Rate of electrical failures dropped dramatically by the end of the year to the normal moderate rate (less than one per month). The Israeli Electrical Company did not provide any explanation of this behavior. The high rate of the disruptions prompted work on the improvement of the SARAF electrical power network. The proposal for installation of a powerful 750 kw UPS system was prepared. The installation of the system is planned to take place in the second half of

30 Figure 32. Distribution of electrical power disruptions in 2012 compared with that in C Figure 33. Preliminary design of the target room and the new beam line. Phase I target room At the moment, the main logistical problem of beam operation is that the experiments are being set up in the accelerator hall. Thus, running the accelerator prevents the users from working on their experimental setups, and, vice versa, work on the setup hinders the accelerator operation. In addition, the concrete shield of the accelerator corridor was built for low beam losses and does not allow full deuteron beam intensity operation. Construction of a new target room was proposed in order to decouple the experiments from the accelerator. The target room is planned in the area adjacent to the accelerator hall, which is currently filled by sand (figure 33). Entrance into the room will be possible from the accelerator hall, as well as from outside. A special beam line is being designed in order to deliver the beam to the target room. The beam line will be a continuation of the first 45 degree bend. In the target room the beam line will be split using a dipole magnet. Thus, two experiments could be installed in the target area. 28