K. C. Wu

An Experimental Study of Catastrophic Loss of Vacuum for RHIC Dipole in MAGCOOL

Experimental studies of catastrophic loss of insulating vacuum for RHIC dipoles to air and to helium have been performed in the Brookhaven MAGCOOL magnet test facility. A RHIC dipole was cooled to 4.5 Kelvin and powered to its 5000 Ampere design current prior to the test. Ambient air or helium was then introduced into the enclosure to simulate the vacuum loss conditions. The response of the cryogenic system, including the insulating vacuum, pressure, temperature and flow rate for the cooling helium, temperatures on the vacuum tank and heat shield are given as a function of time. The heat load is calculated from the sum of the heat into the magnet and the adjacent helium cooling pipes. The results indicate that for this system the heat load from a sudden loss of vacuum is of the order of 10 kW [0.086 W/cm2] for the helium test and 20 kW [0.17 W/cm2] for the air test. These are approximately one order of magnitude larger than the magnet quench heat load, and thus require special consideration of a pressure relief system for the helium cooling circuit.

RHIC 25 kW Refrigerator and Distribution System, Construction, Testing, and Initial Operating Experience

The installation and testing of most of the Relativistic Heavy Ion Collider (RHIC) cryogenic system is complete. The RHIC cryogenic system consisting of the modified ISABELLE refrigerator, and other new cryogenic components, has been completed from the 25 kW refrigerator through one sixth of the 3.8 km circumference RHIC accelerator up to the 4:00 valve box. This paper covers the progress to date with respect to construction and installation of the helium distribution system, including the vacuum jacketed piping and valve boxes, recoolers, and controls. In addition, the 25 kW refrigerator has been operated on three seChapaute occasions over the past two years. These first experiences with the operation of the refrigerator and warm compressor system, along with some test results will be discussed here.

Operational Tests of the BNL 24.8 kW, 3.8 K Helium Refrigerator

The BNL 24.8 kW refrigeration system is completely installed and major portions of the acceptance tests have been completed. So far, the equipment tested has performed at or above design levels. The room temperature helium compressor station has been completely tested and accepted. The two-stage oil injected screw compressor system exhibited an isothermal efficiency of 57% while delivering a helium flow in excess of 4400 g/s. Data on the performance of the make-up gas cryogenic purifier is also given.

Status, Operation and Performance of the RHIC Helium Refrigerator

The Relativistic Heavy Ion Collider RHIC helium refrigerator was modified from the refrigerator originally constructed fifteen years ago for the ISABELLE accelerator. The modifications include a new cold vacuum compressor and relocating the cold circulating compressor to the cryogenic distribution valve boxes, which are located near the cryogenic feed to the collider rings. Prior to start up, an extensive effort was spent on improving conditions of the compressor system, turbine skids, valves, instrumentation and control apChapautus. In February, 1996, the RHIC refrigerator was successfully cooled to liquid helium temperature with 10 kW of heat input at 4.5 K, 53 kW at 60 K and 44 g/s of liquefaction, using approximately two-thirds of the installed compressor capacity. In July, 1996, the refrigerator was used to cool a 350 meter vacuum jacketed transfer line and two cryogenic distribution valve boxes. In February, 1997, the refrigerator was used to cool the first RHIC sextant for over one month of operation. At the end of the first sextant test, an electric heat input of 25 kW at approximately 5.5 K was introduced to simulate a full refrigeration capacity mode. The capacity and reliability of the refrigerator have been demonstrated. The performance of the refrigerator including turbines, heat exchangers, cold vacuum compressor and circulating compressor is given.

Thermal Characteristics of the Magcool Cryogenic System After Quenches of Rhic Dipoles

Thermal characteristics of the MAGCOOL cryogenic system after both natural and heater-induced quenches of the RHIC dipoles have been investigated. For natural quench currents around 6700 amperes, the pressure in the cooling loop exceeded the 15 atmosphere (15.2 MPa) setting of the quench relief valve causing helium to vent through the valve. The initial pressure rise rate was approximately 18 atm/min and the loop pressure increased from 5 to 15 atm in about 30 seconds. To ensure that no helium was vented from the MAGCOOL cooling loop in order to minimize uncertainties in the thermal measurements and better understand the heating/cooling process, the RHIC magnet was then quenched at lower currents between 2000 and 5000 amperes using a strip heater on the magnet coils. A series of tests with the cooling loop connected to a large volume surge tank was also performed to show the dependence of loop pressure on the loop volume. The peak pressure and temperature in the magnet cooling loop were found to be linearly proportional to the energy released for a given loop volume. The time lag of temperature along the cooling loop during quench recovery was determined. Very good agreement between total cooling provided and the magnetic stored energy was found for each of the several values of quench current.

Peak Loop Pressure and Temperature and Comparison of Total Cooling Provided to the Energy Released after Low Current Quenches of SSC Dipoles in Magcool Cryogenic System

The performance of the MAGCOOL cryogenic system after low current strip heater quenches of the SSC dipoles has been investigated. For quench currents between 2000 and 3500 amperes, the loop pressure after a quench does not exceed the set relief pressure. The peak pressure and temperature in the magnet cooling loop were found to be linearly proportional to the energy released during a quench. Excellent agreement between total cooling provided and the magnetic stored energy was found for each of the several values of quench current. The results indicate that the magnetic stored energy is the key parameter; the measurements are accurate and the methodology is appropriate and one that could be applied to similar cryogenic systems.

RHIC Accelerator Commissioning, Cryogenic Tests and Initial Operating Experience of the 25 kW Refrigerator and Distribution System

The installation, initial cooldown, and cryogenic testing of the RHIC accelerator magnets and cryogenic system are complete. This paper covers the final phase of cryogenic equipment installation, performance of the system during the first accelerator cooldown, and difficulties encountered with cooling and commissioning. The 25 kW refrigerator, although operated for limited time periods in the past, has been running continuously for months. Operational experiences of the refrigerator and warm compressor systems will be discussed.

Ring Warmup and Helium Recovery for the RHIC Accelerator: Thermal Analysis of the Warmup of a Superconducting Magnet String

RHIC design criteria require that a faulty string be warmed rapidly to a serviceable temperature. Integral electric heaters warm the superconducting magnets. The magnets are very sensitive to temperature peaks and local gradients. Helium gas is circulated during warmup so that all points are warmed at nearly equal rates. An optimized string warmup procedure gives a continuous, uniform temperature profile, that is flat and rises steadily to room temperature at a rate which allows manageable recovery of the liquid helium content of the magnets. The ideal is not easily obtained. Various warmup schemes are studied. Limitations on the process, including those of the RHIC helium recovery system are discussed. A finite difference code (Fortran) is used to model a magnet string with flowing helium and electric heat generation. Temperature dependent properties of metal and gas including variable density are accounted for. Predictions are compared to physical test data.

Performance and Operating Experience of the RHIC First Sextant Test

Installation and testing of the first sextant of RHIC magnets has been completed. The tests consisted of temperature cycling, quenching magnets, and measuring important engineering parameters of a string of magnets totaling a length of over 500 meters. This represents one sixth of the 3.8 kilometer circumference of the RHIC machine and serves as a basis for extrapolation of parameters for full machine operation. This paper gives an overview of the entire system, a detailed discussion of startup problems, operations and cryogenic system reliability over the period of the test and details of some of the important cryogenic operating parameters.

Testing of a cryogenic recooler heat exchanger at Brookhaven National Laboratory

Brookhaven National Laboratory has tested a recooler heat exchanger intended to be used in the cryogenic system of the Relativistic Heavy Ion Collider. The unit is required to transfer 225 Watts from a supercritical helium stream flowing at 100 g/s to a helium bath boiling at 4.25 K. Measurements made with heat loads of 50 to over 450 Watts on the unit indicate its cooling capacity is greater than 400 Watts, as expected, and it will be suitable for use in the RHIC ring. Presented are the modifications made to BNL’s MAGCOOL test facility that were necessary for testing, test procedure, and recooler performance.