1. Process Simulation for the LCLS-II Cryogenic Systems
V Ravindranath1, H. Bai1, V. Heloin1, E. Fauve1, D. Pflueckhahn1, T. Peterson1, D. Arenius2, M. Bevins2, C. Scanlon2, R. Than3, G. Hays1, M. Ross SLAC(1), Jefferson Lab(2) & Brookhaven National Lab(3)
Abstract—The Linac Coherent Light Source II (LCLS-II), a 4 GeV continuous-wave (CW) superconducting electron linear accelerator, is to be constructed in the existing two mile Linac facility at the SLAC National Accelerator Laboratory. The first light from the new facility is scheduled to be in The LCLS-II Linac consists of thirty-five, 1.3 GHz and two, 3.9 GHz superconducting cryomodules. The Linac cryomodules require cryogenic cooling for the super-conducting niobium cavities at 2.0 K, low temperature thermal intercept at K, and a thermal shield at K. The equivalent 4.5 K refrigeration capacity needed for the Linac operations range from a minimum of 11 kW to a maximum of 24 kW. Two cryogenic plants with 18 kW of equivalent 4.5 K refrigeration capacity will be used for supporting the Linac cryogenic cooling requirements. These cryogenic plants are based on the Jefferson Lab’s CHL-II cryogenic plant which uses the “Floating Pressure” design to support a wide variation in the cooling load. In this paper we describe the cryogenic process for the integrated LCLS-II cryogenic system with the help of a simulation for a 4.5 K cryoplant in combination with a 2 K cold compressor box, and the Linac cryomodules.
1. Introduction
2. LCLS-II Cryo-system Process Flow Diagram
3. Cryo-System Process Model
LCLS-II 4K Cold Box (based on CHL-II cryoplant process cycle) designed for supporting a wide variation in load.
For a single 4.5 K Cold Box
Mode 1 (Maximum Capacity) : Cooling capacity equivalent to 18 kW of Refrigeration at 4K
Mode 7 (50% of Maximum Capacity): 8.3 kW equivalent 4K Refrigeration
Maximum 4.5K Liquefaction Capacity: 140 g/s (for cryomodule cool down)
A. Process Modeling Approach
The entire process model is split in small control volumes.
At the boundary of the control volumes boundary conditions-temperature and pressure are prescribed as “independent variables”
Mass Flows within each control volume starting from the “Cold End” (LINAC→Warm Compressor System) are computed using “Steady State Steady Flow Process” assumptions.
B. For off-design conditions/turn down mode:
The compressor (HP) discharge pressure/turbine inlet pressure is varied while ensuring that the computed turbine mass flow for each control volume satisfies the design turbine flow coefficients.
The suction pressure of the HP and MP stages are solved given the mass flow from the cold box solution.
Heat exchanger UA are estimated based on the reference design UA and the reduction in the mass flow rate. The temperature boundary conditions are adjusted to match with the estimated UA.
Heat exchangers cooling curves are checked to ensure 2nd Law is not violated [i.e., no temperature reversal & crossing curves, inside the heat exchanger].
LN2 Dewar
Cooling Tower
GHe Storage Tanks
Compressor Room 2
Cold Box Room
Compressor Room 1
Two CDS Transfer lines
Electrical Yard
Klystron Gallery
Figure 6: Mode 1, Maximum Capacity Process TS Diagram
Figure 1: Cryoplant Building
First Light
One Cryoplant
Two Cryomodule Strings
U-Tube Connections
14 MV/m Gradient*
2.0≤ Qo ≤3.0
Gradient may be as high as 16 MV/m depending on final Qo
Normal Beam Operations
Two Cryoplants
Two Cryomodule Strings
U-Tube Connections Removed
Gradient - 16 MV/m
2.0 ≤ Qo ≤ 3.0
Figure 7: Mode 7, 50% Turn Down Process TS Diagram
Figure 5: LCLS-II Integrated Cryo-System Process Flow Model
Table 4: Summary of 2K Operating Conditions at Cold Compressor Suction
Table 1: 4.5 K Cold Box Operating Modes
MODE 1
MODE 2
MODE 3
MODE 4
MODE 5
MODE 6
MODE 7
MODE 8
Max Cap
Nom. Cap
Max. Liq.
Max. Ref
Max. Fill
4K Standby
50% of Mode 1
50% of Mode 6
Heat Load
Warm Shield [kW]
15.2
10.1
7.5 5
Cold Intercept [kW]
1.3
0.9
0.7
0.45
Refrigeration [kW] NA 9
0.5
0.25
Liquefaction [g/s] 15
140 45
Sub-Atm [kW]
4.0
3.14 2
Exergy [kW]
1199
844
1075
837
1215
169
553 84
4.5K Eq. Ref. Power [kW] 18
12.7
16.2
18.3
2.55
8.34
1.27
Figure 2: The simplified flow diagram shows the cryoplant configuration for the two commissioning phases of the LCLS-II Linac cryomodules, (a) First Light Configuration with a single cryoplant providing cooling capacity of both Linac strings: upstream & downstream and (b) Normal Beam Operation in which each Linac string supported by a dedicated cryoplant.
(a)
(b)
Table 2: 2K Cold Compressor Specification-High Flow (CC1-CC5)
Modes
Mass Flow
CC1 Suc. Press.
CC1 Suc. Temp.
CC Dis. Press
CC Dis. Temp
[g/s]
[mbar]
[K]
[bar]
[k]
Max
215 27
3.5
1.2
< 30
Nom
157 28
3.6
Min
150
Table 2: 2K Cold Compressor Specification-Low Flow (CC2-CC6)
Modes
Mass Flow
CC1 Suc. Press.
CC1 Suc. Temp.
CC Dis. Press
CC Dis. Temp
[g/s]
[mbar]
[K]
[bar]
[k]
Max
132 27
3.5
1.2
< 30
Nom
120
Min
100 28
3.6
Figure 8: 2K Process Model Example (E=16 Mv/m, Q0 = 2.7x1010, Upstream LINAC)
Summary:
An integrated process model was developed for the LCLS-II cryogenic system using Microsoft Excel VBA tool.
The process modeling capability served as a critical tool in defining the performance boundary for various components of the cryogenic system such as the 4.5K Cold Box and the 2K Cold Box.
The process model developed for the LCLS-II cryogenic system was helpful in estimating the system performance under a wide variation in the Linac Heat Load (50% turn down).
The process model results were also valuable in evaluating and cross checking vendor provided calculations.
Figure 3: LCLS-II Linac Heat Load comprises of (1) Warm 40K, (2) 5K and (3) Cavities& 2K. The 2K heat load accounts for 80% of the total load on the cryoplant
Figure 4: Shows the variation in the Linac heat load in terms of the 4.5K Equivalent Refrigeration Power versus Cavity Qo for (a) First Light and (b) Normal Beam Operation. The Equivalent 4.5 K refrigeration capacity needed for the Linac operations range from a minimum of 11 kW to a maximum of 24 kW