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The thermal behaviour of the tritium source in KATRIN
Cryogenics 55–56 (2013) 5–11
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
The thermal behaviour of the tritium source in KATRIN S. Grohmann a,b,⇑, T. Bode c, M. Hötzel d, H. Schön b, M. Süßer b, T. Wahl e a
Karlsruhe Institute of Technology (KIT), Institute for Technical Thermodynamics and Refrigeration (ITTK), Kaiserstr. 12, 76131 Karlsruhe, Germany Karlsruhe Institute of Technology (KIT), Institute for Technical Physics (ITEP), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c Technical University Munich, Physics Department E15, Garching, Germany d Karlsruhe Institute of Technology (KIT), Institute for Experimental Nuclear Physics (IEKP), Kaiserstr. 12, 76131 Karlsruhe, Germany e Karlsruhe Institute of Technology (KIT), Institute for Nuclear Physics (IKP), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b
a r t i c l e
i n f o
Article history: Received 5 October 2012 Received in revised form 21 December 2012 Accepted 2 January 2013 Available online 15 February 2013 Keywords: Neon Thermosiphon Temperature stability Measurement Vapour pressure sensors
a b s t r a c t The tritium source in the Karlsruhe Tritium Neutrino Experiment (KATRIN) will deliver 1011 b decay electrons per second, in order to determine the mass of the electron antineutrino through analysing the tritium b spectrum. The source is built of a 10 m long beam tube of 90 mm inner diameter, which is operated at 30 K. Gaseous tritium is injected through a central injection chamber and diffuses towards the tube ends, where it is pumped by large turbomolecular pumps and further processed in a closed tritium loop. In order to achieve the KATRIN sensitivity of 0.2 eV/c2, the decay rate in the source (and hence the tritium density profile) must be stable to a level of ±0.1%. As the density profile is influenced by the beam tube conductance, both the temperature stability and the temperature homogeneity must be within a range of ±0.03 K at 30 K. A thermosiphon with saturated neon was developed for this purpose, with horizontal evaporator tubes connected all along the 10 m beam tube. The system behaviour was tested in a 12 m long test cryostat, containing the original beam tube with the adjacent pumping chambers, as well as the cooling circuits and the thermal shields. The so-called ‘‘Demonstrator’’ was operated in the Tritium Laboratory Karlsruhe (TLK) being connected to the cryogenic infrastructure of KATRIN. The temperature stability was found a factor 20 better than specified, achieving a standard deviation of only 1.5 mK/h, which corresponds to DT/T = 5 105 h1 relative stability at 30 K. The ±0.03 K temperature homogeneity along the 10 m beam tube was not yet reached, because of an increased heat load through the pump ports. The repeatability of the temperature measurement with vapour pressure sensors was within ± 0.004 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The Karlsruhe Tritium Neutrino Experiment KATRIN will measure the effective mass of the electron antineutrino with a sensitivity of 0.2 eV/c2, based on the precise measurement of the tritium (T2) b spectrum in a region close to the endpoint at 18.6 keV [1]. Out of 1011b decay electrons generated in the source per second, only a fraction of 4 109 falls in the last 30 eV to be investigated by KATRIN. The integral tail of the spectrum will be scanned point-bypoint with an electrostatic spectrometer at changing retarding potentials, counting electrons that pass the spectrometer with a count rate down to 10 mHz. The decay rate in the source, i.e. the number of electrons per time unit sent to the spectrometer, must hence be stable to a level of 103 in order to fit the single measure⇑ Corresponding author at: Karlsruhe Institute of Technology (KIT), Institute for Technical Thermodynamics and Refrigeration (ITTK), Kaiserstr. 12, 76131 Karlsruhe, Germany. Tel.: +49 72160842332. E-mail addresses: [email protected] (S. Grohmann), [email protected] (T. Bode), [email protected] (M. Hötzel), [email protected] (H. Schön), [email protected] (M. Süßer), [email protected] (T. Wahl). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.01.001
ments to a common baseline spectrum. To achieve its sensitivity, KATRIN will run in measurement cycles of 60 days, until 1000 days of standard measurement operation have been accumulated. The source consists of a 10 m long beam tube with 90 mm inner diameter (called WGTS-tube), which is operated at 30 K and surrounded by a series of 3.6 T superconducting solenoids. Gaseous tritium (T2) is injected through a central injection chamber at a pressure of 3.35 103 mbar, and pumped at either tube end (Fig. 1). The decay rate is predominantly determined by the number of T2 molecules in the source, i. e. the so-called column density, which signifies the T2 density profile integrated along the length of the WGTS-tube. It is determined by the injection rate, the beam tube temperature, the pump rate and the T2 purity. The WGTS-tube temperature must therefore be uniform and stable within ±0.03 K during standard operation to keep a constant decay rate. In a calibration mode with krypton, the source is operated at 120 K with ±0.12 K required temperature stability and uniformity. The WGTS-tube is cooled by evaporating neon, which is reliquefied in a condenser using gaseous helium as shown in the thermosiphon circuit of Fig. 2. The liquefied neon flows from the condenser through a trap in two parallel two-phase tubes that
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S. Grohmann et al. / Cryogenics 55–56 (2013) 5–11
DPS1-F
WGTS-tube T2 injection
Solenoid
Beamtube
DPS1-R Pump ports
WGTS beam line 10 m DN250
16 m
Flange at room-temperature for connecting a turbomolecular pump Ne condenser
Connection to external Ne buffer Heat switch connection
Thermal radiation shield
He outlet
LN2 cooling tubes
He inlet
Ne return line
Inner shield between beam tube and magnet bores
90 mm
He cooling tubes Connection to DPS1 beam tube
WGTS-tube
Pumping chamber
Two-phase tube with capacitive level sensor
Liquid level, heater
Ne supply line
Fig. 1. 3D view of the WGTS-tube and a fourfold pump port on one tube end [2].
Helium return (27 K)
Neon condenser Shield (32 K)
Helium supply (25 K)
Beam tube
Magnet bore (4.5 K)
Neon return Sensor mounts at +90° Sensor mount at +45° Evaporator tubes
Beam tube (30 K) Fig. 2. Principle of the neon thermosiphon for the KATRIN tritium source (WGTS-tube on photo rotated by +90°). Each sensor mount holds both a Pt500 and a vapour pressure sensor. The sensor mounts at +90° reach on top of the beam tube below the neon return. The sensor mount at +45° shows an installed vapour pressure sensor, while the Pt500 was not yet installed in the adjacent hole.
are connected to the WGTS-tube over its entire length. The entire development of this cooling system is described in a trilogy of papers. In Part I [2], the mechanical design, the thermal environment and the nature of heat sources were explained. The thermohydraulic design of the neon thermosiphon circuit was discussed with its operational limits, and the temperature stability was anticipated by the numeric solution of a differential equation model. Part II
[3] dealt with the 30 K temperature measuring system, consisting of 24 Pt500 sensors and 24 vapour pressure sensors for in situ calibration, which were distributed in pairs along the 10 m beam tube. The measuring resolution and individual effects contributing to the standard uncertainty of the temperature measurements were discussed. This final Part III contains the experimental results of the full-scale ‘‘Demonstrator’’ test. After the presentation of
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cool-down data, results for controlling the operating temperature level are shown. This is followed by temperature stability measurements over periods of 4 h and 1 week, and by measurements of the temperature profile. Finally, the implications for the physics of KATRIN are discussed.
Temperature (K)
Results from cooling down the cryostat are shown in Figs. 3 and 4. The cryostat was first pre-cooled by thermal radiation over the outer LN2 shield. Two 2-stage GM cryocoolers were then switched on for cooling down a large aluminium cold mass to a final temperature of 27 K. The cold mass replaced the superconducting magnets in the test, providing the mechanical support and a comparable thermal background for the WGTS-tube. Fig. 3 shows the cool-down of the pumping chambers with gaseous helium. The concentric thermal radiation shields inside the pumping channels Fig. 1, which were supposed to operate at 80 K, reached only temperatures of 230 K. This was caused by the parallel connection of their LN2 supply tubes with the outer LN2 shield circuit, despite the larger flow impedance. The issue will be solved in the final system by a separate LN2 supply. The performance of those intermediate bellow coolers is important in order to absorb P90% of the heat load on the LN2 level. With the increased heat load, temperatures of 34 K were reached in the pumping chambers. The neon thermosiphon was switched on by adding neon to the evacuated circuit; the cooling curve is shown in Fig. 4. The circuit response was stable right from the beginning without any need for tuning. When two-phase conditions were reached, the liquid level was increased by adding pressurised neon from a cylinder to the circuit. The effect of filling could be recognised as peaks in the temperature curve, caused by the saturation temperature following the increased filling pressure TNe,sat = f(pNe) and the subsequent condensation and return to the previous equilibrium. The two peaks in Fig. 4 were zoomed in Fig. 5, showing temperature measurements of three Pt500 sensors distributed over the rear-section of the WGTS-tube. The uniform response of the sensors, which were
First neon condensation
120
Re-filling of neon circuit
90
Warm-up 60
Stationary operation 30
0 12.12.2010
16.12.2010
20.12.2010
Time Fig. 4. Cool-down of the tritium source to 30 K through the neon thermosiphon.
40
37
Temperature (K)
2. Cool-down of the tritium source
150
34
31
28
250 25 Intermediate bellow cooler
15.12.2010
16.12.2010
17.12.2010
Time
Temperature (K)
200 Fig. 5. Verification of two-phase cooling conditions in the rear-section of the beam tube. Sensor distances z from the central injection chamber and angles / from the horizontal median plane: (1) z = 0.10 m, / = +90°, (2) z = 4.70 m, / = 90°, (3) z = 2.15 m, / = 90°.
150 Pump port
separated by several metres, confirmed the presence of stable two-phase conditions over the entire length.
GHe-circuit 100 Start of He-cooling
Final equilibrium temperature 34 K
50
07.12.10
08.12.10
09.12.10
10.12.10
11.12.10
Time Fig. 3. Temperatures during cool-down of the pumping chambers (data for rearend pumping chamber).
3. Control of the temperature level The control of the WGTS-tube temperature level relied on the thermal characteristics of the neon condenser. It was designed for a logarithmic mean temperature difference of DTm = 4 K at a nominal heat load of Q_ ¼ 4 W, yielding a saturation temperature _ He ¼ 1 g=s and of TNe,sat = 30 K at helium inlet conditions of m T 0He ¼ 25 K. With a constant thermal resistance R over a temperature range of several kelvin, where the condenser material properties are practically constant, the heat load Q_ and the mean temperature difference DTm have a linear correlation:
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1 Q_ ¼ DT m : R
30.30
ð1Þ
The parasitic heat load on the WGTS-tube could not be calculated precisely, but was estimated in the Watt range (see [2]). Electrical compensation heaters were placed inside the two-phase tubes for adjusting the overall heat load in the circuit, and so enabling the control of the saturation temperature. Fig. 6 shows experimental results for controlling the WGTStube temperature by the compensation heater power. Theoretically, the temperature could be set to any desired value in the two-phase region of neon, starting from Tmin 27.5 K and limited by the maximum operating pressure of 2 MPa. In practice, however, the temperature will be kept at 30 K in order to operate the neon circuit at a moderate overpressure.
Saturation temperature (K)
30.25
Average temperature: Max. peak-to-peak: Standard deviation: Standard uncertainty:
Tm = 30.173 K ΔTmax = ±0.0075 K σT = ±0.0016 K u = 0.004 K
30.20
TSat = f ( pSat )
30.15
4. Temperature stability of the tritium source 30.10 06:00
07:00
08:00
09:00
10:00
Time (hh:mm) Fig. 7. Stability of the saturation temperature in the two-phase tubes (from pressure in the neon cooling circuit).
30.30
Pt500 sensor
30.25
Temperature (K)
The temperature stabilization of the WGTS-tube relied on a passive system. The behaviour was influenced by essentially static heat sources on the one hand, and by temperature and flow fluctuations in the heat sink on the other hand. The behaviour of the heat sink (i.e. the primary helium cooling circuit) was investigated in prior experiments, yielding inlet fluctuations of DT 0He ¼ 0:3 K _ He ¼ 5% [2]. The flattening of those fluctuations by two and Dm orders of magnitude should be achieved by the combined effects of thermal capacitance and transient heat conduction in the neon condenser. The design principles were thoroughly discussed in [2]. Beside the parasitic heat loads through thermal radiation and thermal conduction, the compensation heaters mentioned above had to fulfil equivalent stability requirements. A power supply of type HMP 4040 [4] was selected for those heaters. The device test showed fluctuation noise of 0.35 mV at 12.9 V and 3.75 lA at 0.076 A, respectively, yielding a combined standard uncertainty of 5.5 105 W. Experimental results of the temperature stability over a period of 4 h are presented in Figs. 7 and 8, where the grey areas depict the KATRIN specification of ±30 mK/h. Fig. 7 shows saturation temperatures in the two-phase tubes Tsat = f(psat), calculated from
30.20
30.15
Average temperature: Max. peak-to-peak: Standard deviation: Standard uncertainty:
Tm = 30.243 K ΔTmax = ±0.006 K σT = ±0.0014 K u = 0.090 K
31 30.10 06:00
Saturation temperature (K)
Measurements Linear fit
28 3
4
09:00
10:00
Fig. 8. Temperature stability of the WGTS-tube measured with a Pt500 sensor. Sensor position z = 4.5 m, / = +90°.
29
2
08:00
Time (hh:mm)
30
1
07:00
5
6
7
Heater power (W) Fig. 6. Control of the WGTS-tube temperature level by the compensation heater power. The uncertainty on the temperature measurement was composed of statistical fluctuations of ±1 mK and systematic uncertainties of ±4 mK as stated in [3]. The uncertainty on the applied heater power was deduced from the measurement (below ±1 mW) and from the specifications given in [4], yielding ±21 mW.
pressure measurements in the neon cooling circuit taken every second1. The standard deviation rT over 4 h was only ±1.6 mK, which was a factor 20 below the requirement. This corresponds to a relative stability of DT/T = 5 105 h1 at 30 K. The results confirmed the expected condenser performance as well as the accuracy of the analytic and numeric models published in [2], where a temperature stability of 2 mK was projected. Fig. 8 contains data of Pt500 measurements on top of the WGTStube, recorded in the same period. The standard deviation of rT = ±1.4 mK was slightly reduced due to transient heat conduction in the beam tube wall. The temperature offset between Figs. 7 and 8 does not indicate the actual radial temperature gradient in the tube wall at this location, because the standard uncertainty
1
The two-phase tubes also functioned as large vapour pressure sensors.
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Saturation temperature (K)
30.180
30.175
30.170 07:45
07:46
07:47
07:48
07:49
07:50
Time (hh:mm) Fig. 9. Resolution of the saturation temperature measurement.
Fig. 11. Stability of the saturation temperature over 1 week of stationary operation.
u = ±0.09 K of the uncalibrated Pt500 measurement must be considered (see [3] for details on measurement uncertainties). Fig. 9 focuses on the resolution of the saturation temperature measurement, zooming a 5 min interval out of Fig. 7. The resolution of the vapour pressure measurement was well below 1 mK, which allowed to identify the physical temperature fluctuation on a sub-mK level. On the other side, the Pt500 measurement had a resolution of 1 mK (Fig. 10). This method has a sufficient resolution for the continuous monitoring of the WGTS-tube temperature fluctuations during the 60 days measurement cycles of KATRIN. The results proved the overall functionality of the temperature measuring concept already presented in [3]. Finally, the outstanding stability of the WGTS-tube temperature is shown in Fig. 11, where the KATRIN requirement of ±30 mK per hour was even satisfied over 1 week of operation. 5. Temperature homogeneity of the tritium source In the WGTS-tube, a homogeneous temperature distribution is required to ensure equal starting conditions for electrons emitted 30.250
at different positions in the source. Different temperatures in the source lead to different thermal velocities of the emitting T2 molecules and therefore different Doppler broadened b spectra, inducing a systematic uncertainty to the measurement. A temperature homogeneity of ±30 mK was therefore required over 95% of the tube length. This implies that larger gradients are only permitted in the last 25 cm on either tube end. Due to the cryostat design, the predominant parasitic heat load on the WGTS-tube is thermal radiation through the pump ports. Its magnitude determines the temperature gradients in the WGTStube ends (see [2]). Turbomolecular pumps (TMPs) will be connected vertically to the pumping channels shown in Fig. 1 via 45° elbows, so that a major fraction of the photons emitted from the TMP rotor blades should be absorbed by the concentric thermal radiation shields. During the Demonstrator test, homogeneity measurements were carried out with blackened blind flanges replacing the TMPs. Because of the elevated shield temperatures of 230 K (Fig. 3) compared to the design value of 80 K, the effectiveness of the intermediate bellow coolers was roughly reduced by a factor of
F Q;intermediate _
2934 2304 2934 804
¼ 0:62:
ð2Þ
Temperature (K)
On the pumping chambers and on the WGTS-tube both at 30 K, on the other hand, the higher shield temperatures implied an increased heat load by roughly factor
F Q;30K _
30.245 Resolution ≤ 1 mK
30.240 07:45
07:46
07:47
07:48
07:49
Time (hh:mm) Fig. 10. Resolution of the Pt500 temperature measurement.
7:50
2304 304 804 304
¼ 70:
ð3Þ
The actual results of temperature homogeneity shown in Fig. 12 can therefore be substantially improved in the final system. With intermediate bellow coolers operating at 80 K and 1=F Q_ ;30K applied as correction to Fig. 12, the specified homogeneity might be reached. The homogeneity measurements were carried out sequentially, calculating the differences between the 24 vapour pressure sensors and the saturation temperature in the two-phase tubes. The repeatability of the vapour pressure measurement was verified by conducting the same measurements with unchanged neon fillings of the vapour pressure sensors after 2 weeks. Additional measurements were carried out after adding a small amount of neon to
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Temperature difference (K)
1.0 0.8
Position of central injection chamber
0.6 Rear-section
0.4
Front-section
0.2 0 -5
-4
-3
-2
-1
0
1
2
3
4
5
Sensor distances from the injection chamber (m) Fig. 12. Measured temperature homogeneity along the 10 m beam tube (Angular positions: h 90° up; s 270° down; and M ±45°).
all vapour pressure sensors, and finally after evacuating and changing the sensor fillings completely. All results were within ±4 mK with regard to the reference data in Fig. 12, confirming the anticipated uncertainty of the vapour pressure measurement discussed in [3]. Fig. 12 further shows that the measured temperature gradients decreased from the front-section towards the rear-section, despite the symmetric design. The additional gradients in the front-section were caused by conductive heat leaks along the vapour pressure sensor capillaries and the Pt500 sensor leads, which were all installed from the front-side. The shortest capillaries so induced the largest heat load on the sensor and vice versa. The thermal anchoring of the capillaries and leads already discussed in [3] was not yet installed in the Demonstrator. Although it will be implemented in the final system upgrade, the entire beam tube will be turned over to shift the inherently larger temperature gradients to the rear-section. There, temperature gradients are less severe as electrons from the rear-section contribute less to the measured b spectrum than those from the front-section, because of inelastic scattering on their way through the source. The results in Fig. 12 were obtained with the blind flanges at room-temperature. However, the magnitude of thermal radiation that reaches the 30 K level strongly depends on operating the TMPs in the stray field of the superconducting solenoids. Investigations in [5] showed that rotor temperatures can exceed 90 °C in perpendicular magnetic fields above 4 mT, for example. Exemplary homogeneity measurements were therefore carried out with the blind flanges heated up to 100 °C. This increased the maximum temperature gradient in the front-section from 0.85 K (Fig. 12) to more than 3 K, depending also on the flow rates in the helium cooling tubes. The results illustrated that both the 30 K heat load and the temperature homogeneity will, above all, be determined by the quality of magnetic shielding around the turbomolecular pumps (T4 dependence of radiative heat). This will be considered in the magnetic shielding design with stray fields well below 4 mT.
6. Implications for the physics of KATRIN The measured temperature stability – better by a factor of 20 than the required stability of ±0.1% – reduces critical fluctuations of the column density in the WGTS. Together with other improvements to stabilize the gas injection pressure and tritium purity and to monitor the activity of the source reported in [6], this leads to improved stability and monitoring of the T2 column density. This allows for a smaller systematic uncertainty on the effective neutrino mass measured in KATRIN. Although the measured longitudinal temperature gradients were larger than specified, the resulting temperature profile will
not affect the sensitivity of KATRIN. The temperature profile influences the T2 density profile and therefore the column density in the source, which is a critical parameter. But as long as the temperature profile is stable, the column density remains unchanged. Monitoring the temperature profile continuously with the Pt500 measurement ensures that changes of the column density can be detected. Additionally, the column density is determined every 2 h in dedicated calibration measurements and monitored continuously by activity monitors presented in [6]. This again reduces the strict requirements on the temperature homogeneity, so that the measured temperature gradients up to 0.85 K are acceptable. Furthermore, near the ends of the beam tube where the gradients occur, the density of tritium molecules is low compared to the central part at 30 K. Therefore, only a small fraction of molecules experiences higher temperatures and their emitted b electron spectrum experiences a stronger Doppler broadening. Dedicated simulations showed that the effect of an increased Doppler broadening is negligible and does not reduce the sensitivity of KATRIN, as long as the temperature gradients are stable. Nevertheless, a homogeneous source is preferable to provide equal starting conditions for electrons emitted from different parts of the source.
7. Summary and outlook A thermosiphon cooling system for the tritium source of KATRIN was developed and tested successfully. The thermosiphon was operated with neon at 30 K, which evaporated in 10 m long two-phase cooling tubes connected to the source. The neon vapour was reliquefied in a condenser using supercritical helium as a heat sink. The constant evaporating temperature in the cooling tubes provided a homogeneous temperature profile. The temperature stabilization relied on a passive design, where temperature fluctuations from the supercritical helium circuit were flattened by both the capacitance and the resistance of the neon condenser. The operation of the thermosiphon was very smooth. A temperature stability of c. ±1.5 mK/h was measured at 30 K, corresponding to a relative stability of DT/T = 5 105 h1. This value was a factor 20 better than the KATRIN requirement. The measured temperature homogeneity was 60.3 K at the rear-end and 60.85 K at the front end of the beam tube. Both values were larger than the KATRIN specification of ±30 mK, caused by the malfunction of LN2-cooled thermal radiation shields inside the pump ducts and hence increased heat loads, and by insufficient heat sinking of the temperature sensor leads. An estimation showed that the ±30 mK limit could be reached by correcting those issues in the final system. It was pointed out, however, that the heat load and the temperature profiles will eventually be defined by the magnetic shielding of the turbomolecular pumps, as the stray fields of the
S. Grohmann et al. / Cryogenics 55–56 (2013) 5–11
superconducting solenoids induce eddy currents in the rotors, which for example, increase their temperatures to 90 °C at 4 mT. The measured temperature stability and other improvements in the tritium loop lead to the required stability and monitoring of the tritium column density, so that the detection limit of KATRIN can be reached or even be improved. The effect of increased temperature gradients near the tube ends was found negligible, as long as the gradients are stable. The 12 m long test cryostat was dismantled after completion of the Demonstrator tests and the construction of the final WGTS source cryostat has started. It will include additional 2 1 m long beam tubes with additional twofold pumping chambers on either end, as well as the superconducting magnet system and the various cooling circuits.
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References [1] J. Angrik et al., KATRIN Design Report, FZKA Scientific Report 7090, Forschungszentrum Karlsruhe GmbH, 2004. [2] Grohmann S. Stability analyses of the beam tube cooling system in the KATRIN source cryostat. Cryogenics 2009;49:413–20. http://dx.doi.org/10.1016/ j.cryogenics.2009.06.001. [3] Grohmann S et al. Precise temperature measurement at 30 K in the KATRIN source cryostat. Cryogenics 2011;51(8):438–45. http://dx.doi.org/10.1016/ j.cryogenics.2011.05.001. [4] HAMEG Instruments GmbH, Power Supply HMP 4030, HMP 4040 – Manual (2010). [5] Wolf J et al. Investigation of turbo-molecular pumps in strong magnetic fields. Vacuum 2011;86(4):361–9. http://dx.doi.org/10.1016/j.vacuum.2011.07.063. [6] Babutzka M et al. Monitoring of the operating parameters of the KATRIN Windowless Gaseous Tritium Source. New Journal of Physics 2012;14(10):103046. http://dx.doi.org/10.1088/1367-2630/14/10/103046.
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
The thermal behaviour of the tritium source in KATRIN S. Grohmann a,b,⇑, T. Bode c, M. Hötzel d, H. Schön b, M. Süßer b, T. Wahl e a
Karlsruhe Institute of Technology (KIT), Institute for Technical Thermodynamics and Refrigeration (ITTK), Kaiserstr. 12, 76131 Karlsruhe, Germany Karlsruhe Institute of Technology (KIT), Institute for Technical Physics (ITEP), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c Technical University Munich, Physics Department E15, Garching, Germany d Karlsruhe Institute of Technology (KIT), Institute for Experimental Nuclear Physics (IEKP), Kaiserstr. 12, 76131 Karlsruhe, Germany e Karlsruhe Institute of Technology (KIT), Institute for Nuclear Physics (IKP), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b
a r t i c l e
i n f o
Article history: Received 5 October 2012 Received in revised form 21 December 2012 Accepted 2 January 2013 Available online 15 February 2013 Keywords: Neon Thermosiphon Temperature stability Measurement Vapour pressure sensors
a b s t r a c t The tritium source in the Karlsruhe Tritium Neutrino Experiment (KATRIN) will deliver 1011 b decay electrons per second, in order to determine the mass of the electron antineutrino through analysing the tritium b spectrum. The source is built of a 10 m long beam tube of 90 mm inner diameter, which is operated at 30 K. Gaseous tritium is injected through a central injection chamber and diffuses towards the tube ends, where it is pumped by large turbomolecular pumps and further processed in a closed tritium loop. In order to achieve the KATRIN sensitivity of 0.2 eV/c2, the decay rate in the source (and hence the tritium density profile) must be stable to a level of ±0.1%. As the density profile is influenced by the beam tube conductance, both the temperature stability and the temperature homogeneity must be within a range of ±0.03 K at 30 K. A thermosiphon with saturated neon was developed for this purpose, with horizontal evaporator tubes connected all along the 10 m beam tube. The system behaviour was tested in a 12 m long test cryostat, containing the original beam tube with the adjacent pumping chambers, as well as the cooling circuits and the thermal shields. The so-called ‘‘Demonstrator’’ was operated in the Tritium Laboratory Karlsruhe (TLK) being connected to the cryogenic infrastructure of KATRIN. The temperature stability was found a factor 20 better than specified, achieving a standard deviation of only 1.5 mK/h, which corresponds to DT/T = 5 105 h1 relative stability at 30 K. The ±0.03 K temperature homogeneity along the 10 m beam tube was not yet reached, because of an increased heat load through the pump ports. The repeatability of the temperature measurement with vapour pressure sensors was within ± 0.004 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The Karlsruhe Tritium Neutrino Experiment KATRIN will measure the effective mass of the electron antineutrino with a sensitivity of 0.2 eV/c2, based on the precise measurement of the tritium (T2) b spectrum in a region close to the endpoint at 18.6 keV [1]. Out of 1011b decay electrons generated in the source per second, only a fraction of 4 109 falls in the last 30 eV to be investigated by KATRIN. The integral tail of the spectrum will be scanned point-bypoint with an electrostatic spectrometer at changing retarding potentials, counting electrons that pass the spectrometer with a count rate down to 10 mHz. The decay rate in the source, i.e. the number of electrons per time unit sent to the spectrometer, must hence be stable to a level of 103 in order to fit the single measure⇑ Corresponding author at: Karlsruhe Institute of Technology (KIT), Institute for Technical Thermodynamics and Refrigeration (ITTK), Kaiserstr. 12, 76131 Karlsruhe, Germany. Tel.: +49 72160842332. E-mail addresses: [email protected] (S. Grohmann), [email protected] (T. Bode), [email protected] (M. Hötzel), [email protected] (H. Schön), [email protected] (M. Süßer), [email protected] (T. Wahl). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.01.001
ments to a common baseline spectrum. To achieve its sensitivity, KATRIN will run in measurement cycles of 60 days, until 1000 days of standard measurement operation have been accumulated. The source consists of a 10 m long beam tube with 90 mm inner diameter (called WGTS-tube), which is operated at 30 K and surrounded by a series of 3.6 T superconducting solenoids. Gaseous tritium (T2) is injected through a central injection chamber at a pressure of 3.35 103 mbar, and pumped at either tube end (Fig. 1). The decay rate is predominantly determined by the number of T2 molecules in the source, i. e. the so-called column density, which signifies the T2 density profile integrated along the length of the WGTS-tube. It is determined by the injection rate, the beam tube temperature, the pump rate and the T2 purity. The WGTS-tube temperature must therefore be uniform and stable within ±0.03 K during standard operation to keep a constant decay rate. In a calibration mode with krypton, the source is operated at 120 K with ±0.12 K required temperature stability and uniformity. The WGTS-tube is cooled by evaporating neon, which is reliquefied in a condenser using gaseous helium as shown in the thermosiphon circuit of Fig. 2. The liquefied neon flows from the condenser through a trap in two parallel two-phase tubes that
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DPS1-F
WGTS-tube T2 injection
Solenoid
Beamtube
DPS1-R Pump ports
WGTS beam line 10 m DN250
16 m
Flange at room-temperature for connecting a turbomolecular pump Ne condenser
Connection to external Ne buffer Heat switch connection
Thermal radiation shield
He outlet
LN2 cooling tubes
He inlet
Ne return line
Inner shield between beam tube and magnet bores
90 mm
He cooling tubes Connection to DPS1 beam tube
WGTS-tube
Pumping chamber
Two-phase tube with capacitive level sensor
Liquid level, heater
Ne supply line
Fig. 1. 3D view of the WGTS-tube and a fourfold pump port on one tube end [2].
Helium return (27 K)
Neon condenser Shield (32 K)
Helium supply (25 K)
Beam tube
Magnet bore (4.5 K)
Neon return Sensor mounts at +90° Sensor mount at +45° Evaporator tubes
Beam tube (30 K) Fig. 2. Principle of the neon thermosiphon for the KATRIN tritium source (WGTS-tube on photo rotated by +90°). Each sensor mount holds both a Pt500 and a vapour pressure sensor. The sensor mounts at +90° reach on top of the beam tube below the neon return. The sensor mount at +45° shows an installed vapour pressure sensor, while the Pt500 was not yet installed in the adjacent hole.
are connected to the WGTS-tube over its entire length. The entire development of this cooling system is described in a trilogy of papers. In Part I [2], the mechanical design, the thermal environment and the nature of heat sources were explained. The thermohydraulic design of the neon thermosiphon circuit was discussed with its operational limits, and the temperature stability was anticipated by the numeric solution of a differential equation model. Part II
[3] dealt with the 30 K temperature measuring system, consisting of 24 Pt500 sensors and 24 vapour pressure sensors for in situ calibration, which were distributed in pairs along the 10 m beam tube. The measuring resolution and individual effects contributing to the standard uncertainty of the temperature measurements were discussed. This final Part III contains the experimental results of the full-scale ‘‘Demonstrator’’ test. After the presentation of
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cool-down data, results for controlling the operating temperature level are shown. This is followed by temperature stability measurements over periods of 4 h and 1 week, and by measurements of the temperature profile. Finally, the implications for the physics of KATRIN are discussed.
Temperature (K)
Results from cooling down the cryostat are shown in Figs. 3 and 4. The cryostat was first pre-cooled by thermal radiation over the outer LN2 shield. Two 2-stage GM cryocoolers were then switched on for cooling down a large aluminium cold mass to a final temperature of 27 K. The cold mass replaced the superconducting magnets in the test, providing the mechanical support and a comparable thermal background for the WGTS-tube. Fig. 3 shows the cool-down of the pumping chambers with gaseous helium. The concentric thermal radiation shields inside the pumping channels Fig. 1, which were supposed to operate at 80 K, reached only temperatures of 230 K. This was caused by the parallel connection of their LN2 supply tubes with the outer LN2 shield circuit, despite the larger flow impedance. The issue will be solved in the final system by a separate LN2 supply. The performance of those intermediate bellow coolers is important in order to absorb P90% of the heat load on the LN2 level. With the increased heat load, temperatures of 34 K were reached in the pumping chambers. The neon thermosiphon was switched on by adding neon to the evacuated circuit; the cooling curve is shown in Fig. 4. The circuit response was stable right from the beginning without any need for tuning. When two-phase conditions were reached, the liquid level was increased by adding pressurised neon from a cylinder to the circuit. The effect of filling could be recognised as peaks in the temperature curve, caused by the saturation temperature following the increased filling pressure TNe,sat = f(pNe) and the subsequent condensation and return to the previous equilibrium. The two peaks in Fig. 4 were zoomed in Fig. 5, showing temperature measurements of three Pt500 sensors distributed over the rear-section of the WGTS-tube. The uniform response of the sensors, which were
First neon condensation
120
Re-filling of neon circuit
90
Warm-up 60
Stationary operation 30
0 12.12.2010
16.12.2010
20.12.2010
Time Fig. 4. Cool-down of the tritium source to 30 K through the neon thermosiphon.
40
37
Temperature (K)
2. Cool-down of the tritium source
150
34
31
28
250 25 Intermediate bellow cooler
15.12.2010
16.12.2010
17.12.2010
Time
Temperature (K)
200 Fig. 5. Verification of two-phase cooling conditions in the rear-section of the beam tube. Sensor distances z from the central injection chamber and angles / from the horizontal median plane: (1) z = 0.10 m, / = +90°, (2) z = 4.70 m, / = 90°, (3) z = 2.15 m, / = 90°.
150 Pump port
separated by several metres, confirmed the presence of stable two-phase conditions over the entire length.
GHe-circuit 100 Start of He-cooling
Final equilibrium temperature 34 K
50
07.12.10
08.12.10
09.12.10
10.12.10
11.12.10
Time Fig. 3. Temperatures during cool-down of the pumping chambers (data for rearend pumping chamber).
3. Control of the temperature level The control of the WGTS-tube temperature level relied on the thermal characteristics of the neon condenser. It was designed for a logarithmic mean temperature difference of DTm = 4 K at a nominal heat load of Q_ ¼ 4 W, yielding a saturation temperature _ He ¼ 1 g=s and of TNe,sat = 30 K at helium inlet conditions of m T 0He ¼ 25 K. With a constant thermal resistance R over a temperature range of several kelvin, where the condenser material properties are practically constant, the heat load Q_ and the mean temperature difference DTm have a linear correlation:
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1 Q_ ¼ DT m : R
30.30
ð1Þ
The parasitic heat load on the WGTS-tube could not be calculated precisely, but was estimated in the Watt range (see [2]). Electrical compensation heaters were placed inside the two-phase tubes for adjusting the overall heat load in the circuit, and so enabling the control of the saturation temperature. Fig. 6 shows experimental results for controlling the WGTStube temperature by the compensation heater power. Theoretically, the temperature could be set to any desired value in the two-phase region of neon, starting from Tmin 27.5 K and limited by the maximum operating pressure of 2 MPa. In practice, however, the temperature will be kept at 30 K in order to operate the neon circuit at a moderate overpressure.
Saturation temperature (K)
30.25
Average temperature: Max. peak-to-peak: Standard deviation: Standard uncertainty:
Tm = 30.173 K ΔTmax = ±0.0075 K σT = ±0.0016 K u = 0.004 K
30.20
TSat = f ( pSat )
30.15
4. Temperature stability of the tritium source 30.10 06:00
07:00
08:00
09:00
10:00
Time (hh:mm) Fig. 7. Stability of the saturation temperature in the two-phase tubes (from pressure in the neon cooling circuit).
30.30
Pt500 sensor
30.25
Temperature (K)
The temperature stabilization of the WGTS-tube relied on a passive system. The behaviour was influenced by essentially static heat sources on the one hand, and by temperature and flow fluctuations in the heat sink on the other hand. The behaviour of the heat sink (i.e. the primary helium cooling circuit) was investigated in prior experiments, yielding inlet fluctuations of DT 0He ¼ 0:3 K _ He ¼ 5% [2]. The flattening of those fluctuations by two and Dm orders of magnitude should be achieved by the combined effects of thermal capacitance and transient heat conduction in the neon condenser. The design principles were thoroughly discussed in [2]. Beside the parasitic heat loads through thermal radiation and thermal conduction, the compensation heaters mentioned above had to fulfil equivalent stability requirements. A power supply of type HMP 4040 [4] was selected for those heaters. The device test showed fluctuation noise of 0.35 mV at 12.9 V and 3.75 lA at 0.076 A, respectively, yielding a combined standard uncertainty of 5.5 105 W. Experimental results of the temperature stability over a period of 4 h are presented in Figs. 7 and 8, where the grey areas depict the KATRIN specification of ±30 mK/h. Fig. 7 shows saturation temperatures in the two-phase tubes Tsat = f(psat), calculated from
30.20
30.15
Average temperature: Max. peak-to-peak: Standard deviation: Standard uncertainty:
Tm = 30.243 K ΔTmax = ±0.006 K σT = ±0.0014 K u = 0.090 K
31 30.10 06:00
Saturation temperature (K)
Measurements Linear fit
28 3
4
09:00
10:00
Fig. 8. Temperature stability of the WGTS-tube measured with a Pt500 sensor. Sensor position z = 4.5 m, / = +90°.
29
2
08:00
Time (hh:mm)
30
1
07:00
5
6
7
Heater power (W) Fig. 6. Control of the WGTS-tube temperature level by the compensation heater power. The uncertainty on the temperature measurement was composed of statistical fluctuations of ±1 mK and systematic uncertainties of ±4 mK as stated in [3]. The uncertainty on the applied heater power was deduced from the measurement (below ±1 mW) and from the specifications given in [4], yielding ±21 mW.
pressure measurements in the neon cooling circuit taken every second1. The standard deviation rT over 4 h was only ±1.6 mK, which was a factor 20 below the requirement. This corresponds to a relative stability of DT/T = 5 105 h1 at 30 K. The results confirmed the expected condenser performance as well as the accuracy of the analytic and numeric models published in [2], where a temperature stability of 2 mK was projected. Fig. 8 contains data of Pt500 measurements on top of the WGTStube, recorded in the same period. The standard deviation of rT = ±1.4 mK was slightly reduced due to transient heat conduction in the beam tube wall. The temperature offset between Figs. 7 and 8 does not indicate the actual radial temperature gradient in the tube wall at this location, because the standard uncertainty
1
The two-phase tubes also functioned as large vapour pressure sensors.
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Saturation temperature (K)
30.180
30.175
30.170 07:45
07:46
07:47
07:48
07:49
07:50
Time (hh:mm) Fig. 9. Resolution of the saturation temperature measurement.
Fig. 11. Stability of the saturation temperature over 1 week of stationary operation.
u = ±0.09 K of the uncalibrated Pt500 measurement must be considered (see [3] for details on measurement uncertainties). Fig. 9 focuses on the resolution of the saturation temperature measurement, zooming a 5 min interval out of Fig. 7. The resolution of the vapour pressure measurement was well below 1 mK, which allowed to identify the physical temperature fluctuation on a sub-mK level. On the other side, the Pt500 measurement had a resolution of 1 mK (Fig. 10). This method has a sufficient resolution for the continuous monitoring of the WGTS-tube temperature fluctuations during the 60 days measurement cycles of KATRIN. The results proved the overall functionality of the temperature measuring concept already presented in [3]. Finally, the outstanding stability of the WGTS-tube temperature is shown in Fig. 11, where the KATRIN requirement of ±30 mK per hour was even satisfied over 1 week of operation. 5. Temperature homogeneity of the tritium source In the WGTS-tube, a homogeneous temperature distribution is required to ensure equal starting conditions for electrons emitted 30.250
at different positions in the source. Different temperatures in the source lead to different thermal velocities of the emitting T2 molecules and therefore different Doppler broadened b spectra, inducing a systematic uncertainty to the measurement. A temperature homogeneity of ±30 mK was therefore required over 95% of the tube length. This implies that larger gradients are only permitted in the last 25 cm on either tube end. Due to the cryostat design, the predominant parasitic heat load on the WGTS-tube is thermal radiation through the pump ports. Its magnitude determines the temperature gradients in the WGTStube ends (see [2]). Turbomolecular pumps (TMPs) will be connected vertically to the pumping channels shown in Fig. 1 via 45° elbows, so that a major fraction of the photons emitted from the TMP rotor blades should be absorbed by the concentric thermal radiation shields. During the Demonstrator test, homogeneity measurements were carried out with blackened blind flanges replacing the TMPs. Because of the elevated shield temperatures of 230 K (Fig. 3) compared to the design value of 80 K, the effectiveness of the intermediate bellow coolers was roughly reduced by a factor of
F Q;intermediate _
2934 2304 2934 804
¼ 0:62:
ð2Þ
Temperature (K)
On the pumping chambers and on the WGTS-tube both at 30 K, on the other hand, the higher shield temperatures implied an increased heat load by roughly factor
F Q;30K _
30.245 Resolution ≤ 1 mK
30.240 07:45
07:46
07:47
07:48
07:49
Time (hh:mm) Fig. 10. Resolution of the Pt500 temperature measurement.
7:50
2304 304 804 304
¼ 70:
ð3Þ
The actual results of temperature homogeneity shown in Fig. 12 can therefore be substantially improved in the final system. With intermediate bellow coolers operating at 80 K and 1=F Q_ ;30K applied as correction to Fig. 12, the specified homogeneity might be reached. The homogeneity measurements were carried out sequentially, calculating the differences between the 24 vapour pressure sensors and the saturation temperature in the two-phase tubes. The repeatability of the vapour pressure measurement was verified by conducting the same measurements with unchanged neon fillings of the vapour pressure sensors after 2 weeks. Additional measurements were carried out after adding a small amount of neon to
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Temperature difference (K)
1.0 0.8
Position of central injection chamber
0.6 Rear-section
0.4
Front-section
0.2 0 -5
-4
-3
-2
-1
0
1
2
3
4
5
Sensor distances from the injection chamber (m) Fig. 12. Measured temperature homogeneity along the 10 m beam tube (Angular positions: h 90° up; s 270° down; and M ±45°).
all vapour pressure sensors, and finally after evacuating and changing the sensor fillings completely. All results were within ±4 mK with regard to the reference data in Fig. 12, confirming the anticipated uncertainty of the vapour pressure measurement discussed in [3]. Fig. 12 further shows that the measured temperature gradients decreased from the front-section towards the rear-section, despite the symmetric design. The additional gradients in the front-section were caused by conductive heat leaks along the vapour pressure sensor capillaries and the Pt500 sensor leads, which were all installed from the front-side. The shortest capillaries so induced the largest heat load on the sensor and vice versa. The thermal anchoring of the capillaries and leads already discussed in [3] was not yet installed in the Demonstrator. Although it will be implemented in the final system upgrade, the entire beam tube will be turned over to shift the inherently larger temperature gradients to the rear-section. There, temperature gradients are less severe as electrons from the rear-section contribute less to the measured b spectrum than those from the front-section, because of inelastic scattering on their way through the source. The results in Fig. 12 were obtained with the blind flanges at room-temperature. However, the magnitude of thermal radiation that reaches the 30 K level strongly depends on operating the TMPs in the stray field of the superconducting solenoids. Investigations in [5] showed that rotor temperatures can exceed 90 °C in perpendicular magnetic fields above 4 mT, for example. Exemplary homogeneity measurements were therefore carried out with the blind flanges heated up to 100 °C. This increased the maximum temperature gradient in the front-section from 0.85 K (Fig. 12) to more than 3 K, depending also on the flow rates in the helium cooling tubes. The results illustrated that both the 30 K heat load and the temperature homogeneity will, above all, be determined by the quality of magnetic shielding around the turbomolecular pumps (T4 dependence of radiative heat). This will be considered in the magnetic shielding design with stray fields well below 4 mT.
6. Implications for the physics of KATRIN The measured temperature stability – better by a factor of 20 than the required stability of ±0.1% – reduces critical fluctuations of the column density in the WGTS. Together with other improvements to stabilize the gas injection pressure and tritium purity and to monitor the activity of the source reported in [6], this leads to improved stability and monitoring of the T2 column density. This allows for a smaller systematic uncertainty on the effective neutrino mass measured in KATRIN. Although the measured longitudinal temperature gradients were larger than specified, the resulting temperature profile will
not affect the sensitivity of KATRIN. The temperature profile influences the T2 density profile and therefore the column density in the source, which is a critical parameter. But as long as the temperature profile is stable, the column density remains unchanged. Monitoring the temperature profile continuously with the Pt500 measurement ensures that changes of the column density can be detected. Additionally, the column density is determined every 2 h in dedicated calibration measurements and monitored continuously by activity monitors presented in [6]. This again reduces the strict requirements on the temperature homogeneity, so that the measured temperature gradients up to 0.85 K are acceptable. Furthermore, near the ends of the beam tube where the gradients occur, the density of tritium molecules is low compared to the central part at 30 K. Therefore, only a small fraction of molecules experiences higher temperatures and their emitted b electron spectrum experiences a stronger Doppler broadening. Dedicated simulations showed that the effect of an increased Doppler broadening is negligible and does not reduce the sensitivity of KATRIN, as long as the temperature gradients are stable. Nevertheless, a homogeneous source is preferable to provide equal starting conditions for electrons emitted from different parts of the source.
7. Summary and outlook A thermosiphon cooling system for the tritium source of KATRIN was developed and tested successfully. The thermosiphon was operated with neon at 30 K, which evaporated in 10 m long two-phase cooling tubes connected to the source. The neon vapour was reliquefied in a condenser using supercritical helium as a heat sink. The constant evaporating temperature in the cooling tubes provided a homogeneous temperature profile. The temperature stabilization relied on a passive design, where temperature fluctuations from the supercritical helium circuit were flattened by both the capacitance and the resistance of the neon condenser. The operation of the thermosiphon was very smooth. A temperature stability of c. ±1.5 mK/h was measured at 30 K, corresponding to a relative stability of DT/T = 5 105 h1. This value was a factor 20 better than the KATRIN requirement. The measured temperature homogeneity was 60.3 K at the rear-end and 60.85 K at the front end of the beam tube. Both values were larger than the KATRIN specification of ±30 mK, caused by the malfunction of LN2-cooled thermal radiation shields inside the pump ducts and hence increased heat loads, and by insufficient heat sinking of the temperature sensor leads. An estimation showed that the ±30 mK limit could be reached by correcting those issues in the final system. It was pointed out, however, that the heat load and the temperature profiles will eventually be defined by the magnetic shielding of the turbomolecular pumps, as the stray fields of the
S. Grohmann et al. / Cryogenics 55–56 (2013) 5–11
superconducting solenoids induce eddy currents in the rotors, which for example, increase their temperatures to 90 °C at 4 mT. The measured temperature stability and other improvements in the tritium loop lead to the required stability and monitoring of the tritium column density, so that the detection limit of KATRIN can be reached or even be improved. The effect of increased temperature gradients near the tube ends was found negligible, as long as the gradients are stable. The 12 m long test cryostat was dismantled after completion of the Demonstrator tests and the construction of the final WGTS source cryostat has started. It will include additional 2 1 m long beam tubes with additional twofold pumping chambers on either end, as well as the superconducting magnet system and the various cooling circuits.
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References [1] J. Angrik et al., KATRIN Design Report, FZKA Scientific Report 7090, Forschungszentrum Karlsruhe GmbH, 2004. [2] Grohmann S. Stability analyses of the beam tube cooling system in the KATRIN source cryostat. Cryogenics 2009;49:413–20. http://dx.doi.org/10.1016/ j.cryogenics.2009.06.001. [3] Grohmann S et al. Precise temperature measurement at 30 K in the KATRIN source cryostat. Cryogenics 2011;51(8):438–45. http://dx.doi.org/10.1016/ j.cryogenics.2011.05.001. [4] HAMEG Instruments GmbH, Power Supply HMP 4030, HMP 4040 – Manual (2010). [5] Wolf J et al. Investigation of turbo-molecular pumps in strong magnetic fields. Vacuum 2011;86(4):361–9. http://dx.doi.org/10.1016/j.vacuum.2011.07.063. [6] Babutzka M et al. Monitoring of the operating parameters of the KATRIN Windowless Gaseous Tritium Source. New Journal of Physics 2012;14(10):103046. http://dx.doi.org/10.1088/1367-2630/14/10/103046.